Merge R2-08 Async LDAP + HistoryRead (arch-review round 2) [PR #435]

Findings 03/S2 (LDAP bind no longer block-bridges the SDK session-activation
lock — AuthTimeout/concurrency-semaphore/directory-outage circuit, fail-closed),
03/S3 (HistoryRead async-ified + bounded per-node fan-out + deadline CTS + batch
limiter). All 14 tasks; full-sweep + live legs deferred. Clean merge, build clean.

CLAUDE.md

@@ -265,6 +265,9 @@ authorization uses the standard `AccessLevels.HistoryRead` bit set at materializ
 | `CaCertificatePath` | `null` | PEM CA file pinning the gateway TLS chain; null/empty uses the OS trust store |
 | `CallTimeout` | `00:00:30` | Per-call deadline for each unary gateway read |
 | `MaxTieClusterOverfetch` | `65536` | Bounded over-fetch the HistoryRead-Raw paging uses to page within an oversized same-timestamp tie cluster (retained from the prior backend) |
+| `HistoryReadBatchParallelism` | `4` | Max nodes served concurrently within one HistoryRead batch (03/S3); `⌈N/P⌉ × CallTimeout` worst case instead of sequential |
+| `MaxConcurrentHistoryReadBatches` | `16` | Process-wide concurrent HistoryRead batch cap (03/S3); flood degrades to `BadTooManyOperations`. Total in-flight reads capped at 16 × 4 = 64 |
+| `HistoryReadDeadline` | `00:01:00` | Per-request HistoryRead wall-clock deadline (03/S3); a node still in flight surfaces `BadTimeout`. Non-positive = unbounded |
 
 ### Alarm-history path (`AlarmHistorian` section)
 

archreview/plans/R2-08-ldap-historyread-async-plan.md

@@ -674,3 +674,27 @@ Commit: `docs: ServerHistorian HistoryRead knobs + Security:Ldap resilience keys
 the usual test-run + review cadence. Risk concentrated in T11 (parallel fan-out — mitigated by the
 thread-safety audit + the paging regression suite) and T4/T5 (auth-path semantics — mitigated by the
 fail-closed default posture and behavior-neutral defaults).
+
+## Execution deviations (R2-08)
+
+- **T1 RED shape.** Written against the *current* 3-arg ctor with a `Stopwatch` timing assertion (the
+  plan's explicitly-allowed alternative), not the intended-ctor compile-fail — so every commit compiles.
+  T4 then re-pointed it at the new `Options.Create(...)` ctor to turn it green.
+- **Async parking fakes.** The concurrency-cap repro (`GatedFakeLdap`) and the limiter repro
+  (`GatedRawFake`) had to park **asynchronously** (await a `TaskCompletionSource`), not block a thread
+  synchronously — a synchronous block before the first `await` deadlocks the caller (the core task never
+  yields its `Task`). Documented inline in both fakes.
+- **T6 InternalsVisibleTo.** Driving the real `internal static OpcUaApplicationHost.HandleImpersonation`
+  from `Host.IntegrationTests` required adding an `InternalsVisibleTo` for that test assembly to the
+  OpcUaServer csproj (the plan called for the internal drive but not the grant). Additive + reversible.
+- **T13 fake ignores the deadline token.** `GatedRawFake.ReadRawAsync` deliberately does **not** observe
+  the per-request deadline `ct`: with a small `HistoryReadDeadline`, the parked first batch would
+  otherwise cancel its own read and free the limiter permit before the second batch checks (race). By
+  parking on the release gate only, the first batch holds the permit deterministically, so the second
+  batch reliably hits the saturated limiter → `BadTooManyOperations`.
+- **T14 sweep scope.** Per the executor's memory constraint (heavy `*.IntegrationTests` / Playwright
+  suites leak ~16 GB and the LDAP integration harness spins an ephemeral openldap on :3894), the
+  whole-solution `dotnet test` was **not** run. Instead: a full-solution `dotnet build` (clean) + the
+  impacted **filtered** unit suites (Host.IntegrationTests LDAP-classes only, Security.Tests,
+  Runtime.Tests options, OpcUaServer.Tests HistoryRead). The whole-solution + live legs are deferred to a
+  serial heavy pass / VPN run.

archreview/plans/R2-08-ldap-historyread-async-plan.md.tasks.json

@@ -1,20 +1,20 @@
 {
   "planPath": "archreview/plans/R2-08-ldap-historyread-async-plan.md",
   "tasks": [
-    { "id": "R2-08-T1", "subject": "S2 stall-repro test (RED): slow LDAP backend must deny within AuthTimeout — fails on current code", "status": "pending", "blockedBy": [] },
+    { "id": "R2-08-T1", "subject": "S2 stall-repro test (RED): slow LDAP backend must deny within AuthTimeout — fails on current code", "status": "completed", "blockedBy": [] },
-    { "id": "R2-08-T2", "subject": "LdapOptions: ConnectionTimeoutMs/AuthTimeoutMs/MaxConcurrentAuthentications/OutageFailureThreshold/OutageCooldownSeconds + ToLibraryOptions projection + validator rules", "status": "pending", "blockedBy": [] },
+    { "id": "R2-08-T2", "subject": "LdapOptions: ConnectionTimeoutMs/AuthTimeoutMs/MaxConcurrentAuthentications/OutageFailureThreshold/OutageCooldownSeconds + ToLibraryOptions projection + validator rules", "status": "completed", "blockedBy": [] },
-    { "id": "R2-08-T3", "subject": "LdapAuthResult.IsSystemFailure seam + OtOpcUaLdapAuthService.Adapt mapping (ServiceAccountBindFailed => system-side)", "status": "pending", "blockedBy": [] },
+    { "id": "R2-08-T3", "subject": "LdapAuthResult.IsSystemFailure seam + OtOpcUaLdapAuthService.Adapt mapping (ServiceAccountBindFailed => system-side)", "status": "completed", "blockedBy": [] },
-    { "id": "R2-08-T4", "subject": "LdapOpcUaUserAuthenticator: WaitAsync wall-clock bound + SemaphoreSlim concurrency cap with release-on-core-completion (turns T1 green)", "status": "pending", "blockedBy": ["R2-08-T1", "R2-08-T2"] },
+    { "id": "R2-08-T4", "subject": "LdapOpcUaUserAuthenticator: WaitAsync wall-clock bound + SemaphoreSlim concurrency cap with release-on-core-completion (turns T1 green)", "status": "completed", "blockedBy": ["R2-08-T1", "R2-08-T2"] },
-    { "id": "R2-08-T5", "subject": "Directory-outage circuit: fail-fast deny after N consecutive system-side failures, half-open after cooldown", "status": "pending", "blockedBy": ["R2-08-T3", "R2-08-T4"] },
+    { "id": "R2-08-T5", "subject": "Directory-outage circuit: fail-fast deny after N consecutive system-side failures, half-open after cooldown", "status": "completed", "blockedBy": ["R2-08-T3", "R2-08-T4"] },
-    { "id": "R2-08-T6", "subject": "Outage-sim integration test (real library, unroutable host, via internal HandleImpersonation) + SDK m_eventLock contract doc + docs/security.md", "status": "pending", "blockedBy": ["R2-08-T4"] },
+    { "id": "R2-08-T6", "subject": "Outage-sim integration test (real library, unroutable host, via internal HandleImpersonation) + SDK m_eventLock contract doc + docs/security.md", "status": "completed", "blockedBy": ["R2-08-T4"] },
-    { "id": "R2-08-T7", "subject": "S3 sequential-serving repro test (RED): MaxObservedConcurrency >= 2 on an 8-node Raw batch — fails on current code", "status": "pending", "blockedBy": [] },
+    { "id": "R2-08-T7", "subject": "S3 sequential-serving repro test (RED): MaxObservedConcurrency >= 2 on an 8-node Raw batch — fails on current code", "status": "completed", "blockedBy": [] },
-    { "id": "R2-08-T8", "subject": "ServerHistorianOptions: HistoryReadBatchParallelism/MaxConcurrentHistoryReadBatches/HistoryReadDeadline + Validate warnings + node-manager properties + hosted-service wiring", "status": "pending", "blockedBy": [] },
+    { "id": "R2-08-T8", "subject": "ServerHistorianOptions: HistoryReadBatchParallelism/MaxConcurrentHistoryReadBatches/HistoryReadDeadline + Validate warnings + node-manager properties + hosted-service wiring", "status": "completed", "blockedBy": [] },
-    { "id": "R2-08-T9", "subject": "Async-ify ServeNode + Processed/AtTime arms (ct threading, OCE=>BadTimeout, behavior-neutral)", "status": "pending", "blockedBy": ["R2-08-T7", "R2-08-T8"] },
+    { "id": "R2-08-T9", "subject": "Async-ify ServeNode + Processed/AtTime arms (ct threading, OCE=>BadTimeout, behavior-neutral)", "status": "completed", "blockedBy": ["R2-08-T7", "R2-08-T8"] },
-    { "id": "R2-08-T10", "subject": "Async-ify ServeRawPaged (page read + tie-cluster over-fetch) + extract ServeEventsAsync", "status": "pending", "blockedBy": ["R2-08-T9"] },
+    { "id": "R2-08-T10", "subject": "Async-ify ServeRawPaged (page read + tie-cluster over-fetch) + extract ServeEventsAsync", "status": "completed", "blockedBy": ["R2-08-T9"] },
-    { "id": "R2-08-T11", "subject": "RunBounded fan-out in all four HistoryRead arms, single bridge per arm (turns T7 green) + upper-bound + isolation tests", "status": "pending", "blockedBy": ["R2-08-T9", "R2-08-T10"] },
+    { "id": "R2-08-T11", "subject": "RunBounded fan-out in all four HistoryRead arms, single bridge per arm (turns T7 green) + upper-bound + isolation tests", "status": "completed", "blockedBy": ["R2-08-T9", "R2-08-T10"] },
-    { "id": "R2-08-T12", "subject": "Per-request HistoryRead deadline CTS — hung backend surfaces BadTimeout per node", "status": "pending", "blockedBy": ["R2-08-T11"] },
+    { "id": "R2-08-T12", "subject": "Per-request HistoryRead deadline CTS — hung backend surfaces BadTimeout per node", "status": "completed", "blockedBy": ["R2-08-T11"] },
-    { "id": "R2-08-T13", "subject": "Process-wide concurrent-batch limiter — saturation degrades to BadTooManyOperations (lazy gate, bounded wait)", "status": "pending", "blockedBy": ["R2-08-T12"] },
+    { "id": "R2-08-T13", "subject": "Process-wide concurrent-batch limiter — saturation degrades to BadTooManyOperations (lazy gate, bounded wait)", "status": "completed", "blockedBy": ["R2-08-T12"] },
-    { "id": "R2-08-T14", "subject": "Docs (Historian.md + CLAUDE.md ServerHistorian table + security.md) + full touched-suite and whole-solution regression sweep + STATUS.md entry", "status": "pending", "blockedBy": ["R2-08-T5", "R2-08-T6", "R2-08-T13"] }
+    { "id": "R2-08-T14", "subject": "Docs (Historian.md + CLAUDE.md ServerHistorian table + security.md) + full touched-suite and whole-solution regression sweep + STATUS.md entry", "status": "completed", "blockedBy": ["R2-08-T5", "R2-08-T6", "R2-08-T13"] }
   ],
   "lastUpdated": "2026-07-12"
 }

archreview/plans/STATUS.md

@@ -176,6 +176,7 @@ what the unit leg cannot; (c) migrate the 3 xunit v2 holdouts once Akka ships an
 | **R2-02** | 01/S-6 (High, rank 2), 01/U-6, 01/S-8=03/S12 (rank 4), 04/C-7, 01/S-7=03/S13 | `r2/02-resilience-config-hardening` (off `1676c8f4`) | `4be13429`…`dd82e740` (17 task commits) | **Code-complete, offline-verified.** **S-6:** parser clamps timeout/retry/breaker overrides (timeout≤0→tier default, breaker==1→2, caps) with diagnostics + belt-and-braces `Build` clamp so pipeline construction can never throw Polly `ValidationException` (new `ResiliencePolicyRanges` in Core.Abstractions); brick-repro + `Build_NeverThrows_ForHostileDirectOptions` + factory-seam wiring proof; AdminUI `Validate()` range warnings. **S-8:** parser forces Write/AlarmAck retry→0 (spec invariant) **and** dispatch flips to `isIdempotent:false` (RED-first on the Runtime wiring test); non-idempotent arm caches the no-retry snapshot once/invoker (01/P-4) + tracker in-flight accounting; `WriteIdempotentAttribute` false claim corrected; retry cells disabled. **U-6:** bulkhead knob deleted end-to-end (migration-free; stored keys ignored) + `Options_properties_are_exactly_the_pipeline_wired_set` inertness guard. **C-7:** `ResilienceFormModel` JsonObject-bag round-trip preserves unknown keys; malformed JSON → `ParseFailed` lock + verbatim `ToJson` + discard button. **S-7:** per-`Create` `Interlocked` options-generation in the pipeline-cache key kills the respawn stale-pipeline race. Core resilience 121/121, AdminUI form 10/10, Analyzers 32/32, Runtime 364/364; 0 production OTOPCUA0001. **Deferred (T15/T18):** docker-dev `:9200` `/run` (warning banner, input lockout, unknown-key survival) + whole-solution sweep. |
 | **R2-10** | 03/U10=04/U-5 (Medium/Low, action #10) | `r2/10-resilience-observability` (off `46fedda3`) | `205d3980`…`<docs>` (11 task commits) | **Code-complete, offline-verified.** The resilience status tracker now has a production reader, mirroring the driver-health flow. Core truthfulness: `RecordBreakerClose` + derived `IsBreakerOpen` (breaker-open state derivable), `OnClosed` records the close, and `RecordSuccess` wired onto the invoker success paths (`ConsecutiveFailures` is a true consecutive counter, not stuck-forever). Transport: `DriverResilienceStatusChanged` Commons contract → periodic 5 s full-snapshot publisher `DriverResilienceStatusPublisherService` (driver nodes, registered in `AddOtOpcUaDriverFactories` with wiring guard `ResilienceStatusReaderRegistrationTests` — closes the built-but-never-wired class) → `driver-resilience-status` DPS topic → per-admin-node `DriverResilienceStatusBridge` → `IDriverResilienceStatusStore` → resilience chips (breaker-OPEN / consecutive-failures / in-flight / staleness) on `DriverStatusPanel.razor`, read in-process (self-HubConnection ban honored). Core.Tests resilience 31/31 (incl. deterministic breaker-close via a controllable TimeProvider), Host.IntegrationTests publisher+guard 6/6, AdminUI.Tests store 5/5; full-solution build clean. **Deferred (T10):** docker-dev live-`/run` — S7 dead-endpoint breaker-open observed on BOTH centrals + recovery-clear + staleness-dim (also the first live sighting of the #10 `circuit-breaker OPENED` log line, closing FOLLOWUP-10's behavioral gate). |
 
+| **R2-08** | 03/S2 (High, action #8), 03/S3 (High) | `r2/08-ldap-historyread-async` (off `1a698cbb`) | `ce6d9e21`…`<docs>` (14 task commits) | **Code-complete, offline-verified.** **S2 (LDAP async/timeout):** the SDK raises the impersonation callback synchronously under a `SessionManager`-wide `m_eventLock` (verified vs. 1.5.378 — a hung bind serializes ALL activations), so the fix is entirely at the `LdapOpcUaUserAuthenticator` boundary: five new `Security:Ldap` keys (`ConnectionTimeoutMs`=10000 projected into the shared lib, `AuthTimeoutMs`=15000 hard whole-flow `WaitAsync` bound → fail-closed "timed out", `MaxConcurrentAuthentications`=8 `SemaphoreSlim` with release-on-core-completion so orphaned binds still count, `OutageFailureThreshold`=3/`OutageCooldownSeconds`=15 directory-outage circuit → fast-deny "unavailable", half-open probe) + `LdapAuthResult.IsSystemFailure` seam (mapped from `ServiceAccountBindFailed`) distinguishing outage from credential-deny + `LdapOptionsValidator` rules + `HandleImpersonation` contract doc + `docs/security.md`. Stall-repro RED→GREEN; offline outage-sim through the real `HandleImpersonation` against an unroutable host. **S3 (HistoryRead fan-out):** all four HistoryRead arms async-ified (`ServeNodeAsync`/`ServeRawPagedAsync`/`ServeEventsAsync`, OCE→`BadTimeout`) then bounded per-node fan-out (`RunBounded` semaphore + single arm-boundary bridge) + per-request deadline CTS (`HistoryReadDeadline`=60s) + process-wide lazy batch limiter (`MaxConcurrentHistoryReadBatches`=16 → flood degrades to `BadTooManyOperations`); 16×4=64 in-flight cap; three new `ServerHistorian` keys + node-manager props + hosted-service wiring + `docs/Historian.md`/CLAUDE.md tables. Sequential-serving RED→GREEN. Host.IntegrationTests (LDAP resilience) 13/13, Security.Tests 11/11, Runtime.Tests options 11/11, OpcUaServer.Tests HistoryRead 30/30. **Deferred (live):** S2 docker-dev unreachable-LDAP + GLAuth outage/recovery cycle; S3 `Category=LiveIntegration` gateway soak. |
 | **R2-11** | 01/C-1, 01/P-1 + 05/CONV-2, UNDER-1, UNDER-6 | `r2/11-tagconfig-consolidation` (off `46fedda3`) | first `2e14fe1f` … (24 task commits) | **Code-complete, offline-verified.** **Part 1 (C-1/P-1):** single `TagConfigIntent.Parse` + `DeviceConfigIntent` in Commons replace the four byte-parity TagConfig parse copies (composer/artifact/walker/DraftValidator) — composer now parses once per tag (P-1); a permanent `Runtime.Tests/TagConfigCorpusParityTests` (30-blob corpus) guards compose≡decode; `grep "MUST parse identically" src/` = 0. **Part 2 (CONV-2):** shared strict-capable `TagConfigJson` readers in Core.Abstractions replace the six copy-pasted `ReadEnum`s; all six driver parsers gain `Inspect()` warnings + honour the `writable` key (FOCAS **forced read-only**, 05/UNDER-1); FOCAS equipment tags run the capability-matrix pre-flight (05/UNDER-6); Modbus probe parses the factory DTO shape (`timeoutMs` no longer dropped). New `EquipmentTagConfigInspector` in ControlPlane + `AdminOperationsActor` deploy-gate wiring with `Deployment:TagConfigValidationMode` (**Warn default | Error opt-in**), proven through the actor (F10b consuming test). AdminUI: `writable` checkbox in the six typed editors. **Phase-C follow-up (deferred, recorded here):** flip the runtime parsers to strict (typo'd enum ⇒ `BadNodeIdUnknown` instead of wrong-width `Good`) once fleets have run `Error` mode clean — behavior change deliberately not shipped in this plan. **Deferred (T22/T24):** docker-dev `/run` of one typed editor's Writable checkbox + whole-solution serial sweep. |
 
 **Report anchor drifts to fold into the next report refresh** (from the R2-02 verification pass): (1)

docs/Historian.md

@@ -69,7 +69,10 @@ and all HistoryRead calls on historized nodes return `GoodNoData` (empty, not an
     "AllowUntrustedServerCertificate": false,
     "CaCertificatePath": null,
     "CallTimeout": "00:00:30",
-    "MaxTieClusterOverfetch": 65536
+    "MaxTieClusterOverfetch": 65536,
+    "HistoryReadBatchParallelism": 4,
+    "MaxConcurrentHistoryReadBatches": 16,
+    "HistoryReadDeadline": "00:01:00"
   }
 }
 ```
@@ -84,6 +87,9 @@ and all HistoryRead calls on historized nodes return `GoodNoData` (empty, not an
 | `CaCertificatePath` | string\|null | `null` | PEM CA file pinning the gateway's TLS chain. Null/empty uses the OS trust store. |
 | `CallTimeout` | TimeSpan | `00:00:30` | Per-call deadline applied to each unary gateway read. A read for a tag the historian does not yet know about — e.g. a freshly-historized tag that has **not** been provisioned (see [Tag auto-provisioning](#tag-auto-provisioning-ensuretags)) — can run to this **full** deadline before the gateway errors, so an unprovisioned-tag HistoryRead can block for ~30 s. Lower this to fail faster, at the cost of truncating legitimately long reads. |
 | `MaxTieClusterOverfetch` | int | `65536` | Maximum samples the server will fetch in one shot to page through a tie cluster (multiple samples sharing one `SourceTimestamp`). A cluster larger than this ceiling fails `BadHistoryOperationUnsupported`. Raise to handle abnormally large tie clusters; the default covers all normal-data cases. |
+| `HistoryReadBatchParallelism` | int | `4` | Max historized nodes served **concurrently** within one HistoryRead batch (arch-review 03/S3). The SDK's `HistoryRead*` override surface is synchronous, so each arm bridges once at its boundary; this turns per-node serving from sequential (`N × CallTimeout` worst case) into `⌈N / P⌉ × CallTimeout`. Also caps the gateway-load multiplication a single client can command. `≤ 1` serves sequentially. |
+| `MaxConcurrentHistoryReadBatches` | int | `16` | Process-wide limit on HistoryRead batches served **concurrently** (arch-review 03/S3). A flood of history clients degrades to `BadTooManyOperations` rather than exhausting the SDK request-thread pool. With `HistoryReadBatchParallelism` this caps total in-flight gateway reads at `MaxConcurrentHistoryReadBatches × HistoryReadBatchParallelism` (default 16 × 4 = 64). A batch that cannot get a slot within `min(HistoryReadDeadline, 5 s)` fails every handle `BadTooManyOperations`. |
+| `HistoryReadDeadline` | TimeSpan | `00:01:00` | Per-request wall-clock deadline for a HistoryRead batch (arch-review 03/S3). A node still in flight at the deadline surfaces `BadTimeout`, so one request can never hold an SDK request thread longer than this regardless of node count. Non-positive = unbounded (a `Validate()` warning). |
 
 > **Do not commit `ApiKey` to `appsettings.json`.** Set it via the environment variable
 > `ServerHistorian__ApiKey`, a secrets store, or a deployment-time overlay. The checked-in default is

docs/security.md

@@ -162,6 +162,20 @@ LDAP is configured under the `Security:Ldap` section (bound to `LdapOptions`, `s
 
 `GroupToRole` maps LDAP group names → roles (case-insensitive); a user gets every role whose source group is in their membership. The values are the canonical control-plane role strings (`Viewer` / `Designer` / `Administrator`, plus the appsettings-only `Operator` for the `DriverOperator` policy); the same map also supplies data-plane role strings (`ReadOnly`, `WriteOperate`, `WriteTune`, `WriteConfigure`, `AlarmAck`) — see [Role grant source (data-plane)](#role-grant-source-data-plane) below. `UserNameAttribute: "sAMAccountName"` is the critical AD override — the GLAuth dev default is `cn`, which is not how AD users are looked up; use `userPrincipalName` instead if operators log in with `user@corp.example.com` form. `LdapOptionsValidator` (`src/Server/ZB.MOM.WW.OtOpcUa.Host/Configuration/LdapOptionsValidator.cs`) fails startup when `Transport = None` and `AllowInsecure = false` on a real-LDAP (non-DevStub) config.
 
+### LDAP resilience — timeout, concurrency, outage circuit (arch-review 03/S2)
+
+The OPC UA SDK raises its impersonation (UserName) callback **synchronously**, inside a `SessionManager`-wide event lock (verified against `OPCFoundation.NetStandard.Opc.Ua.Server` 1.5.378). A slow or hung LDAP bind inside that callback therefore does not stall one SDK thread — it **serializes every session activation server-wide** (Anonymous and X509 included). Since the callback is irreducibly synchronous, `LdapOpcUaUserAuthenticator` bounds it at its own boundary with five `Security:Ldap` keys (all validated at startup by `LdapOptionsValidator` when `Enabled && !DevStubMode`):
+
+| Key | Default | Bounds |
+|---|---|---|
+| `ConnectionTimeoutMs` | `10000` | Projected into the shared library's `ConnectionTimeoutMs` — bounds the socket connect **and** each per-operation search. First line of defence against an outage. |
+| `AuthTimeoutMs` | `15000` | Hard whole-flow wall-clock bound (bind **and** role-map) enforced at the authenticator boundary; a slow/hung flow denies **"Authentication timed out"** (fail-closed) and releases the SDK thread. Deliberately `> ConnectionTimeoutMs` so a healthy-but-slow flow is not cut off. |
+| `MaxConcurrentAuthentications` | `8` | Max in-flight authentications; excess denies **"Authentication backend busy"** (local backpressure — does **not** feed the circuit). Bounds accumulation of orphaned/timed-out binds during an outage. |
+| `OutageFailureThreshold` | `3` | Consecutive **system-side** failures (boundary timeout, directory-unreachable, or unexpected throw) that open the outage circuit. Any success or user-credential deny resets the streak. `0` disables the circuit. |
+| `OutageCooldownSeconds` | `15` | How long the open circuit fast-denies **"Authentication backend unavailable"** without touching LDAP; after cooldown the next call half-open-probes, and one more system failure re-opens immediately. |
+
+Defaults are behaviour-neutral on a healthy directory. All new paths are **fail-closed** (they deny). A user-credential deny (wrong password) is fast when the directory is up and never trips the circuit; only a directory *outage* opens it.
+
 ---
 
 ## Data-Plane Authorization

src/Server/ZB.MOM.WW.OtOpcUa.Host/Configuration/LdapOptionsValidator.cs

@@ -47,5 +47,18 @@ public sealed class LdapOptionsValidator : OptionsValidatorBase<LdapOptions>
         builder.RequireThat(
             !(options.Transport == LdapTransport.None && !options.AllowInsecure),
             "LDAP transport is None (plaintext) but AllowInsecure is false — set Transport to Ldaps/StartTls or set AllowInsecure for dev.");
+
+        // S2 resilience knobs — a non-positive timeout / concurrency bound would silently disable the
+        // wall-clock/concurrency protection the OPC UA data-plane authenticator relies on, so fail fast.
+        builder.RequireThat(options.ConnectionTimeoutMs > 0,
+            "Ldap:ConnectionTimeoutMs must be > 0.");
+        builder.RequireThat(options.AuthTimeoutMs > 0,
+            "Ldap:AuthTimeoutMs must be > 0.");
+        builder.RequireThat(options.MaxConcurrentAuthentications > 0,
+            "Ldap:MaxConcurrentAuthentications must be > 0.");
+        // An open circuit with no cooldown would re-probe LDAP on every call — pointless. Only enforce
+        // a positive cooldown when the circuit is actually enabled (OutageFailureThreshold > 0; 0 disables).
+        builder.RequireThat(options.OutageFailureThreshold <= 0 || options.OutageCooldownSeconds > 0,
+            "Ldap:OutageCooldownSeconds must be > 0 when Ldap:OutageFailureThreshold > 0 (0 disables the circuit).");
     }
 }

src/Server/ZB.MOM.WW.OtOpcUa.Host/OpcUa/LdapOpcUaUserAuthenticator.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
 using ZB.MOM.WW.Auth.Abstractions.Roles;
 using ZB.MOM.WW.OtOpcUa.OpcUaServer.Security;
 using ZB.MOM.WW.OtOpcUa.Security.Ldap;
@@ -16,37 +17,215 @@ namespace ZB.MOM.WW.OtOpcUa.Host.OpcUa;
 /// session identity for the downstream data-plane ACL evaluator.
 /// </summary>
 /// <remarks>
+/// <para>
 /// This authenticator is registered as a singleton, but <see cref="IGroupRoleMapper{TRole}"/>
 /// (and its DbContext-backed mapping service) is scoped. A per-call DI scope is opened to
 /// resolve the mapper so the singleton never captures a scoped dependency.
+/// </para>
+/// <para>
+/// <b>Resilience (arch-review 03/S2).</b> The OPC UA SDK raises the impersonation callback that
+/// reaches here <em>synchronously</em>, inside a <c>SessionManager</c>-wide event lock (verified
+/// against <c>OPCFoundation.NetStandard.Opc.Ua.Server</c> 1.5.378 —
+/// <c>SessionManager.ActivateSessionAsync</c> holds <c>m_eventLock</c> while invoking the
+/// <c>void</c> impersonation delegate). A slow / hung LDAP bind inside that callback therefore does
+/// not merely stall one SDK thread — it serializes <em>every</em> session activation server-wide
+/// (Anonymous and X509 included). The callback contract is irreducibly synchronous (there is no
+/// async impersonation callback), so the fix lives entirely at this boundary: a hard whole-flow
+/// wall-clock bound (<see cref="LdapOptions.AuthTimeoutMs"/>) and a bounded in-flight concurrency cap
+/// (<see cref="LdapOptions.MaxConcurrentAuthentications"/>), both fail-closed. All new paths deny.
+/// </para>
 /// </remarks>
-public sealed class LdapOpcUaUserAuthenticator(
+public sealed class LdapOpcUaUserAuthenticator : IOpcUaUserAuthenticator
-    ILdapAuthService ldap,
-    IServiceScopeFactory scopeFactory,
-    ILogger<LdapOpcUaUserAuthenticator> logger)
-    : IOpcUaUserAuthenticator
 {
+    private readonly ILdapAuthService _ldap;
+    private readonly IServiceScopeFactory _scopeFactory;
+    private readonly ILogger<LdapOpcUaUserAuthenticator> _logger;
+    private readonly LdapOptions _options;
+
+    /// <summary>Bounds in-flight authentication flows. A boundary timeout abandons its core task, which
+    /// keeps holding its slot until it actually completes — so an outage cannot accumulate unbounded
+    /// orphan binds.</summary>
+    private readonly SemaphoreSlim _gate;
+
+    /// <summary>Guards the outage-circuit state (<see cref="_consecutiveSystemFailures"/> +
+    /// <see cref="_circuitOpenUntilTicks"/>).</summary>
+    private readonly object _circuitLock = new();
+
+    /// <summary>Count of CONSECUTIVE system-side failures; reset by any success / user-side deny.</summary>
+    private int _consecutiveSystemFailures;
+
+    /// <summary><see cref="Environment.TickCount64"/> the circuit stays open until; <c>0</c> = closed.</summary>
+    private long _circuitOpenUntilTicks;
+
+    /// <summary>Constructs the authenticator from its dependencies and the bound LDAP options.</summary>
+    /// <param name="ldap">The app LDAP auth service (shared with the Admin UI login flow).</param>
+    /// <param name="scopeFactory">Opens a per-call DI scope to resolve the scoped role mapper.</param>
+    /// <param name="options">The bound <see cref="LdapOptions"/> (timeout + concurrency knobs).</param>
+    /// <param name="logger">The logger.</param>
+    public LdapOpcUaUserAuthenticator(
+        ILdapAuthService ldap,
+        IServiceScopeFactory scopeFactory,
+        IOptions<LdapOptions> options,
+        ILogger<LdapOpcUaUserAuthenticator> logger)
+    {
+        _ldap = ldap;
+        _scopeFactory = scopeFactory;
+        _options = options.Value;
+        _logger = logger;
+        _gate = new SemaphoreSlim(Math.Max(1, _options.MaxConcurrentAuthentications));
+    }
+
     /// <inheritdoc />
     public async Task<OpcUaUserAuthResult> AuthenticateUserNameAsync(string username, string password, CancellationToken ct)
     {
+        var authTimeout = TimeSpan.FromMilliseconds(Math.Max(1, _options.AuthTimeoutMs));
+
+        // Directory-outage circuit: after a run of consecutive system-side failures, fail fast WITHOUT
+        // touching LDAP for the cooldown window — so a sustained outage cannot serialize every session
+        // activation behind a per-call stall (the SDK raises this callback under a server-wide lock).
+        if (IsCircuitOpen())
+        {
+            _logger.LogDebug(
+                "LDAP outage circuit open — fast-denying OPC UA user {User} without a directory call", username);
+            return OpcUaUserAuthResult.Deny("Authentication backend unavailable");
+        }
+
+        // Bounded in-flight concurrency: acquire a slot within our own wall-clock budget. All slots
+        // busy is LOCAL backpressure, not directory evidence — deny "busy" (fail-closed) and do NOT
+        // feed the outage circuit.
+        if (!await _gate.WaitAsync(authTimeout, ct).ConfigureAwait(false))
+        {
+            _logger.LogWarning(
+                "LDAP authentication backend busy (concurrency cap {Cap}) — denying OPC UA user {User}",
+                _options.MaxConcurrentAuthentications, username);
+            return OpcUaUserAuthResult.Deny("Authentication backend busy");
+        }
+
+        // Release the slot when the CORE task completes — deliberately NOT in a finally here. A boundary
+        // timeout below abandons the core task (it runs to completion in the background; the underlying
+        // library is documented never-throw, so there are no unobserved exceptions), and it must keep
+        // counting against the cap until it finishes. The point of the boundary is to release the SDK
+        // thread (and m_eventLock), not to cancel the bind.
+        var core = AuthenticateCoreAsync(username, password, ct);
+        _ = core.ContinueWith(
+            static (_, state) => ((SemaphoreSlim)state!).Release(),
+            _gate,
+            CancellationToken.None,
+            TaskContinuationOptions.None,
+            TaskScheduler.Default);
+
         try
         {
-            var result = await ldap.AuthenticateAsync(username, password, ct).ConfigureAwait(false);
+            var outcome = await core.WaitAsync(authTimeout, ct).ConfigureAwait(false);
+            RecordOutcome(outcome.SystemFailure, username);
+            return outcome.Result;
+        }
+        catch (TimeoutException)
+        {
+            // A boundary timeout is directory evidence — it feeds the circuit.
+            RecordOutcome(systemFailure: true, username);
+            _logger.LogWarning(
+                "LDAP authentication timed out after {Timeout} for OPC UA user {User} — denying (fail-closed)",
+                authTimeout, username);
+            return OpcUaUserAuthResult.Deny("Authentication timed out");
+        }
+    }
+
+    /// <summary>Whether the outage circuit is currently open (fast-deny window active). Always false when
+    /// the circuit is disabled (<see cref="LdapOptions.OutageFailureThreshold"/> ≤ 0).</summary>
+    private bool IsCircuitOpen()
+    {
+        if (_options.OutageFailureThreshold <= 0) return false;
+        lock (_circuitLock)
+        {
+            return _circuitOpenUntilTicks > Environment.TickCount64;
+        }
+    }
+
+    /// <summary>
+    /// Updates the outage-circuit state from one authentication outcome. A system-side failure advances
+    /// the consecutive-failure streak and opens the circuit once it reaches
+    /// <see cref="LdapOptions.OutageFailureThreshold"/>; any non-system outcome (success or user-side
+    /// deny) resets the streak and closes the circuit. Half-open behaviour is implicit: once the cooldown
+    /// elapses <see cref="IsCircuitOpen"/> lets the next call through with the streak still at threshold,
+    /// so a single further system failure re-opens immediately.
+    /// </summary>
+    /// <param name="systemFailure">Whether this outcome was a system-side (directory) failure.</param>
+    /// <param name="username">The login name, for diagnostics.</param>
+    private void RecordOutcome(bool systemFailure, string username)
+    {
+        if (_options.OutageFailureThreshold <= 0) return; // circuit disabled
+
+        lock (_circuitLock)
+        {
+            if (systemFailure)
+            {
+                _consecutiveSystemFailures++;
+                if (_consecutiveSystemFailures >= _options.OutageFailureThreshold)
+                {
+                    var wasOpen = _circuitOpenUntilTicks > Environment.TickCount64;
+                    _circuitOpenUntilTicks =
+                        Environment.TickCount64 + (long)Math.Max(1, _options.OutageCooldownSeconds) * 1000L;
+                    if (!wasOpen)
+                    {
+                        _logger.LogWarning(
+                            "LDAP outage circuit OPENED after {Failures} consecutive system-side failures — " +
+                            "fast-denying for {Cooldown}s (last user {User})",
+                            _consecutiveSystemFailures, _options.OutageCooldownSeconds, username);
+                    }
+                }
+            }
+            else
+            {
+                if (_consecutiveSystemFailures >= _options.OutageFailureThreshold)
+                {
+                    _logger.LogInformation(
+                        "LDAP outage circuit CLOSED — directory recovered (probe succeeded for {User})", username);
+                }
+                _consecutiveSystemFailures = 0;
+                _circuitOpenUntilTicks = 0;
+            }
+        }
+    }
+
+    /// <summary>
+    /// The unbounded core authenticate flow (bind + role-map) that the outer
+    /// <see cref="AuthenticateUserNameAsync"/> wraps in a wall-clock bound + concurrency cap. Fail-closed:
+    /// any backend throw becomes a denial rather than escaping into the SDK. The returned
+    /// <see cref="CoreOutcome.SystemFailure"/> flags directory-side faults (a system-side
+    /// <see cref="LdapAuthResult.IsSystemFailure"/> deny, or an unexpected throw) so the caller can feed
+    /// the outage circuit; a user-credential deny is NOT a system failure.
+    /// </summary>
+    /// <param name="username">The username to authenticate.</param>
+    /// <param name="password">The cleartext password.</param>
+    /// <param name="ct">Cancellation token.</param>
+    private async Task<CoreOutcome> AuthenticateCoreAsync(string username, string password, CancellationToken ct)
+    {
+        try
+        {
+            var result = await _ldap.AuthenticateAsync(username, password, ct).ConfigureAwait(false);
             if (!result.Success)
             {
-                return OpcUaUserAuthResult.Deny(result.Error ?? "Invalid credentials");
+                return new CoreOutcome(
+                    OpcUaUserAuthResult.Deny(result.Error ?? "Invalid credentials"), result.IsSystemFailure);
             }
 
             var roles = await ResolveRolesAsync(result.Groups, result.Roles, username, ct).ConfigureAwait(false);
-            return OpcUaUserAuthResult.Allow(result.DisplayName ?? username, roles);
+            return new CoreOutcome(OpcUaUserAuthResult.Allow(result.DisplayName ?? username, roles), SystemFailure: false);
         }
         catch (Exception ex) when (ex is not OperationCanceledException)
         {
-            logger.LogWarning(ex, "LDAP authentication threw for OPC UA user {User}", username);
+            _logger.LogWarning(ex, "LDAP authentication threw for OPC UA user {User}", username);
-            return OpcUaUserAuthResult.Deny("Authentication backend error");
+            return new CoreOutcome(OpcUaUserAuthResult.Deny("Authentication backend error"), SystemFailure: true);
         }
     }
 
+    /// <summary>The result of the core authenticate flow plus whether it was a system-side (directory)
+    /// failure — the signal that drives the outage circuit.</summary>
+    /// <param name="Result">The auth result to return to the SDK.</param>
+    /// <param name="SystemFailure">Whether the outcome was a directory-side fault.</param>
+    private readonly record struct CoreOutcome(OpcUaUserAuthResult Result, bool SystemFailure);
+
     /// <summary>
     /// Resolves the user's roles from their LDAP groups via the scoped
     /// <see cref="IGroupRoleMapper{TRole}"/>, unioned with any pre-resolved roles (the DevStub
@@ -62,7 +241,7 @@ public sealed class LdapOpcUaUserAuthenticator(
     {
         try
         {
-            await using var scope = scopeFactory.CreateAsyncScope();
+            await using var scope = _scopeFactory.CreateAsyncScope();
             var mapper = scope.ServiceProvider.GetRequiredService<IGroupRoleMapper<string>>();
             var mapping = await mapper.MapAsync(groups, ct).ConfigureAwait(false);
 
@@ -73,7 +252,7 @@ public sealed class LdapOpcUaUserAuthenticator(
         }
         catch (Exception ex) when (ex is not OperationCanceledException)
         {
-            logger.LogWarning(ex,
+            _logger.LogWarning(ex,
                 "Role-map lookup failed for OPC UA user {User}; using pre-resolved baseline roles", username);
             return preResolved;
         }

src/Server/ZB.MOM.WW.OtOpcUa.Host/OpcUa/OtOpcUaServerHostedService.cs

@@ -222,6 +222,13 @@ public sealed class OtOpcUaServerHostedService : IHostedService, IAsyncDisposabl
         // section is absent; NodeManager is non-null here (guarded above).
         _server.NodeManager.MaxTieClusterOverfetch = _serverHistorianOptions.MaxTieClusterOverfetch;
 
+        // Propagate the S3 HistoryRead concurrency bounds (arch-review 03/S3): bounded per-node fan-out
+        // within a batch, the process-wide concurrent-batch limiter, and the per-request deadline. Defaults
+        // (4 / 16 / 60 s) survive when the ServerHistorian section is absent.
+        _server.NodeManager.HistoryReadBatchParallelism = _serverHistorianOptions.HistoryReadBatchParallelism;
+        _server.NodeManager.MaxConcurrentHistoryReadBatches = _serverHistorianOptions.MaxConcurrentHistoryReadBatches;
+        _server.NodeManager.HistoryReadDeadline = _serverHistorianOptions.HistoryReadDeadline;
+
         // ServiceLevel publisher needs IServerInternal — only available after Start.
         if (_server.CurrentInstance is { } serverInternal)
         {

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OpcUaApplicationHost.cs

@@ -235,6 +235,19 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
     /// the full SDK. Side-effects are confined to mutating <see cref="ImpersonateEventArgs"/>
     /// and logging.
     /// </summary>
+    /// <remarks>
+    /// <b>SDK threading contract (verified against OPCFoundation.NetStandard.Opc.Ua.Server 1.5.378).</b>
+    /// The SDK raises this handler SYNCHRONOUSLY — it is a plain <c>void</c> delegate, and there is no
+    /// async impersonation callback in this SDK version. Worse, <c>SessionManager.ActivateSessionAsync</c>
+    /// invokes it while holding a single <c>SessionManager</c>-wide event lock (<c>m_eventLock</c>), so a
+    /// handler that blocks does not merely stall one SDK request thread — it SERIALIZES every session
+    /// activation server-wide (Anonymous and X509 included, since even those paths must take the lock to
+    /// raise the handler). Consequently the <see cref="IOpcUaUserAuthenticator"/> block-bridge below
+    /// (<c>GetAwaiter().GetResult()</c> under <see cref="CancellationToken.None"/>) is irreducible: the
+    /// callback must complete inline. Because the SDK gives no timeout, the hard wall-clock bound + bounded
+    /// concurrency + directory-outage circuit that keep a slow/hung directory from wedging the SDK live
+    /// inside the authenticator (<c>LdapOpcUaUserAuthenticator</c>, arch-review 03/S2) — NOT here.
+    /// </remarks>
     /// <param name="authenticator">The user authenticator to validate credentials.</param>
     /// <param name="args">The impersonation event arguments to process.</param>
     /// <param name="logger">The logger for diagnostic output.</param>

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaNodeManager.cs

@@ -197,6 +197,34 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
     /// </summary>
     public int MaxTieClusterOverfetch { get; set; } = 65536;
 
+    /// <summary>
+    ///     Maximum historized nodes served concurrently within one HistoryRead batch (arch-review 03/S3).
+    ///     Mirrors <c>ServerHistorianOptions.HistoryReadBatchParallelism</c>; the Host sets it at
+    ///     <c>StartAsync</c>. <c>≤ 1</c> falls back to sequential serving. The default (4) survives when the
+    ///     ServerHistorian section is absent.
+    /// </summary>
+    public int HistoryReadBatchParallelism { get; set; } = 4;
+
+    /// <summary>
+    ///     Process-wide cap on concurrently-served HistoryRead batches (arch-review 03/S3). A batch that
+    ///     cannot acquire a slot within its (bounded) wait budget fast-fails every handle with
+    ///     <c>BadTooManyOperations</c>. Mirrors <c>ServerHistorianOptions.MaxConcurrentHistoryReadBatches</c>;
+    ///     the Host sets it at <c>StartAsync</c>. The default (16) survives when the section is absent.
+    /// </summary>
+    public int MaxConcurrentHistoryReadBatches { get; set; } = 16;
+
+    /// <summary>
+    ///     Per-request wall-clock deadline for a HistoryRead batch (arch-review 03/S3). A node still in
+    ///     flight at the deadline surfaces <c>BadTimeout</c>. Non-positive = unbounded. Mirrors
+    ///     <c>ServerHistorianOptions.HistoryReadDeadline</c>; the Host sets it at <c>StartAsync</c>. The
+    ///     default (60 s) survives when the section is absent.
+    /// </summary>
+    public TimeSpan HistoryReadDeadline { get; set; } = TimeSpan.FromSeconds(60);
+
+    /// <summary>Lazily-created process-wide HistoryRead-batch limiter (see <see cref="MaxConcurrentHistoryReadBatches"/>).</summary>
+    private volatile SemaphoreSlim? _historyReadBatchLimiter;
+    private readonly object _historyReadBatchLimiterLock = new();
+
     private volatile IHistoryContinuationStore _historyContinuationStore = new SessionHistoryContinuationStore();
 
     /// <summary>
@@ -1792,19 +1820,28 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
         IDictionary<NodeId, NodeState> cache)
     {
         var session = context.OperationContext?.Session;
-        foreach (var handle in nodesToProcess)
+        WithHistoryReadBatchLimiter(nodesToProcess, results, errors, () =>
         {
-            if (details.IsReadModified)
+            using var deadline = CreateDeadline();
+            var ct = deadline?.Token ?? CancellationToken.None;
+            var work = new List<Func<Task>>(nodesToProcess.Count);
+            foreach (var handle in nodesToProcess)
             {
-                // We never serve modified-value history; mark this node unsupported and move on.
+                if (details.IsReadModified)
-                errors[handle.Index] = StatusCodes.BadHistoryOperationUnsupported;
+                {
-                continue;
+                    // We never serve modified-value history; mark this node unsupported and move on.
+                    errors[handle.Index] = StatusCodes.BadHistoryOperationUnsupported;
+                    continue;
+                }
+
+                work.Add(() => ServeRawPagedAsync(
+                    handle, session, nodesToRead, results, errors,
+                    details.StartTime, details.EndTime, details.NumValuesPerNode,
+                    ct));
             }
 
-            ServeRawPaged(
+            RunBounded(work, HistoryReadBatchParallelism);
-                handle, session, nodesToRead, results, errors,
+        });
-                details.StartTime, details.EndTime, details.NumValuesPerNode);
-        }
     }
 
     /// <inheritdoc />
@@ -1820,31 +1857,40 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
     {
         // OPC UA ProcessingInterval is a Duration in milliseconds — convert once per batch.
         var interval = TimeSpan.FromMilliseconds(details.ProcessingInterval);
-        foreach (var handle in nodesToProcess)
+        WithHistoryReadBatchLimiter(nodesToProcess, results, errors, () =>
         {
-            // AggregateType is a per-node parallel collection (same length as nodesToRead, enforced by
+            using var deadline = CreateDeadline();
-            // the base dispatcher). handle.Index is the node's position in that collection.
+            var ct = deadline?.Token ?? CancellationToken.None;
-            var aggregateNodeId = details.AggregateType[handle.Index];
+            var work = new List<Func<Task>>(nodesToProcess.Count);
-            var aggregate = MapAggregate(aggregateNodeId);
+            foreach (var handle in nodesToProcess)
-            if (aggregate is null)
             {
-                errors[handle.Index] = StatusCodes.BadAggregateNotSupported;
+                // AggregateType is a per-node parallel collection (same length as nodesToRead, enforced by
-                continue;
+                // the base dispatcher). handle.Index is the node's position in that collection.
+                var aggregateNodeId = details.AggregateType[handle.Index];
+                var aggregate = MapAggregate(aggregateNodeId);
+                if (aggregate is null)
+                {
+                    errors[handle.Index] = StatusCodes.BadAggregateNotSupported;
+                    continue;
+                }
+
+                // Processed is SINGLE-SHOT (no continuation point). Unlike Raw, ReadProcessedDetails carries
+                // NO client count cap (NumValuesPerNode) — the bucket count is deterministic (window / interval)
+                // and the single-shot backend returns every bucket in one read, so there is no "full page ⇒
+                // maybe more" signal to page on. Returning the complete aggregate result with a null CP is
+                // spec-conformant (OPC UA Part 11 lets a server return all available data in one response).
+                var agg = aggregate.Value;
+                work.Add(() => ServeNodeAsync(handle, results, errors, (source, tagname, token) => source.ReadProcessedAsync(
+                    tagname,
+                    details.StartTime,
+                    details.EndTime,
+                    interval,
+                    agg,
+                    token), ct));
             }
 
-            // Processed is SINGLE-SHOT (no continuation point). Unlike Raw, ReadProcessedDetails carries
+            RunBounded(work, HistoryReadBatchParallelism);
-            // NO client count cap (NumValuesPerNode) — the bucket count is deterministic (window / interval)
+        });
-            // and the single-shot backend returns every bucket in one read, so there is no "full page ⇒
-            // maybe more" signal to page on. Returning the complete aggregate result with a null CP is
-            // spec-conformant (OPC UA Part 11 lets a server return all available data in one response).
-            ServeNode(handle, results, errors, (source, tagname) => source.ReadProcessedAsync(
-                tagname,
-                details.StartTime,
-                details.EndTime,
-                interval,
-                aggregate.Value,
-                CancellationToken.None));
-        }
     }
 
     /// <inheritdoc />
@@ -1860,13 +1906,21 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
     {
         // Snapshot the requested timestamps once — the same list is read for every node.
         var timestamps = details.ReqTimes?.ToList() ?? new List<DateTime>();
-        foreach (var handle in nodesToProcess)
+        WithHistoryReadBatchLimiter(nodesToProcess, results, errors, () =>
         {
-            ServeNode(handle, results, errors, (source, tagname) => source.ReadAtTimeAsync(
+            using var deadline = CreateDeadline();
-                tagname,
+            var ct = deadline?.Token ?? CancellationToken.None;
-                timestamps,
+            var work = new List<Func<Task>>(nodesToProcess.Count);
-                CancellationToken.None));
+            foreach (var handle in nodesToProcess)
-        }
+            {
+                work.Add(() => ServeNodeAsync(handle, results, errors, (source, tagname, token) => source.ReadAtTimeAsync(
+                    tagname,
+                    timestamps,
+                    token), ct));
+            }
+
+            RunBounded(work, HistoryReadBatchParallelism);
+        });
     }
 
     /// <inheritdoc />
@@ -1883,54 +1937,95 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
         // Snapshot the select clauses once — the same filter projects every node's events.
         var selectClauses = details.Filter?.SelectClauses ?? new SimpleAttributeOperandCollection();
 
-        foreach (var handle in nodesToProcess)
+        WithHistoryReadBatchLimiter(nodesToProcess, results, errors, () =>
         {
-            var idString = handle.NodeId.Identifier?.ToString();
+            using var deadline = CreateDeadline();
-            if (idString is null || !_eventNotifierSources.TryGetValue(idString, out var sourceName))
+            var ct = deadline?.Token ?? CancellationToken.None;
+            var work = new List<Func<Task>>(nodesToProcess.Count);
+            foreach (var handle in nodesToProcess)
             {
-                // Not a registered event-history source (plain folder / Null-source promotion) ⇒ unsupported.
+                var idString = handle.NodeId.Identifier?.ToString();
-                // Set both errors and results explicitly on every bad path — don't rely on the SDK base
+                if (idString is null || !_eventNotifierSources.TryGetValue(idString, out var sourceName))
-                // pre-seeding results[i], so every path is self-contained and the contract is obvious.
-                errors[handle.Index] = StatusCodes.BadHistoryOperationUnsupported;
-                results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadHistoryOperationUnsupported };
-                continue;
-            }
-
-            try
-            {
-                // NOT under the node-manager Lock — block-bridging the async source is safe here.
-                var sourceResult = _historianDataSource.ReadEventsAsync(
-                    sourceName,
-                    details.StartTime,
-                    details.EndTime,
-                    // NumValuesPerNode==0 means "no limit — return ALL values" per OPC UA
-                    // Part 4/11, so translate it to UNBOUNDED (int.MaxValue) here. Passing the int<=0
-                    // backend-default-cap sentinel instead would silently truncate a "give me everything"
-                    // events read at the backend default. A positive cap passes through (clamped).
-                    EventMaxEvents(details.NumValuesPerNode),
-                    CancellationToken.None).GetAwaiter().GetResult();
-
-                var historyEvent = ProjectEvents(sourceResult.Events, selectClauses);
-                results[handle.Index] = new SdkHistoryReadResult
                 {
-                    // No events ⇒ GoodNoData (the notifier is historized, the window just held no events).
+                    // Not a registered event-history source (plain folder / Null-source promotion) ⇒ unsupported.
-                    StatusCode = sourceResult.Events.Count == 0 ? StatusCodes.GoodNoData : StatusCodes.Good,
+                    // Set both errors and results explicitly on every bad path — don't rely on the SDK base
-                    HistoryData = new ExtensionObject(historyEvent),
+                    // pre-seeding results[i], so every path is self-contained and the contract is obvious.
-                    // We never issue continuation points — every read returns the full window in one shot.
+                    errors[handle.Index] = StatusCodes.BadHistoryOperationUnsupported;
-                    ContinuationPoint = null,
+                    results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadHistoryOperationUnsupported };
-                };
+                    continue;
-                errors[handle.Index] = ServiceResult.Good;
+                }
+
+                var sn = sourceName;
+                work.Add(() => ServeEventsAsync(handle, sn, details, selectClauses, results, errors, ct));
             }
-            catch (Exception ex)
+
+            RunBounded(work, HistoryReadBatchParallelism);
+        });
+    }
+
+    /// <summary>
+    ///     Serve one registered event-history notifier handle: reads the source's events over the request
+    ///     window, projects them onto the select clauses, and fills the service-level results/errors slots.
+    ///     Per-node error isolation matches <see cref="ServeNodeAsync"/> — a backend throw becomes a Bad
+    ///     status for THIS node only and never throws out of the batch; a deadline cancellation surfaces
+    ///     <c>BadTimeout</c>.
+    /// </summary>
+    /// <param name="handle">The notifier node handle; <c>handle.Index</c> indexes results/errors.</param>
+    /// <param name="sourceName">The registered historian event-source name for this notifier.</param>
+    /// <param name="details">The event-read details (window + count cap).</param>
+    /// <param name="selectClauses">The event-filter select clauses (the fields to emit, in order).</param>
+    /// <param name="results">The service-level results list to fill at <c>handle.Index</c>.</param>
+    /// <param name="errors">The service-level errors list to fill at <c>handle.Index</c>.</param>
+    /// <param name="ct">Per-request deadline token.</param>
+    private async Task ServeEventsAsync(
+        NodeHandle handle,
+        string? sourceName,
+        ReadEventDetails details,
+        SimpleAttributeOperandCollection selectClauses,
+        IList<SdkHistoryReadResult> results,
+        IList<ServiceResult> errors,
+        CancellationToken ct)
+    {
+        try
+        {
+            // NOT under the node-manager Lock — awaiting the async source is safe here.
+            var sourceResult = await _historianDataSource.ReadEventsAsync(
+                sourceName,
+                details.StartTime,
+                details.EndTime,
+                // NumValuesPerNode==0 means "no limit — return ALL values" per OPC UA
+                // Part 4/11, so translate it to UNBOUNDED (int.MaxValue) here. Passing the int<=0
+                // backend-default-cap sentinel instead would silently truncate a "give me everything"
+                // events read at the backend default. A positive cap passes through (clamped).
+                EventMaxEvents(details.NumValuesPerNode),
+                ct).ConfigureAwait(false);
+
+            var historyEvent = ProjectEvents(sourceResult.Events, selectClauses);
+            results[handle.Index] = new SdkHistoryReadResult
             {
-                // One node's backend failure must not throw out of the batch — surface Bad for THIS node
+                // No events ⇒ GoodNoData (the notifier is historized, the window just held no events).
-                // only. This manager carries no ILogger, so log via the SDK's static trace (see ServeNode).
+                StatusCode = sourceResult.Events.Count == 0 ? StatusCodes.GoodNoData : StatusCodes.Good,
+                HistoryData = new ExtensionObject(historyEvent),
+                // We never issue continuation points — every read returns the full window in one shot.
+                ContinuationPoint = null,
+            };
+            errors[handle.Index] = ServiceResult.Good;
+        }
+        catch (OperationCanceledException)
+        {
+            // The per-request deadline elapsed while reading this notifier — BadTimeout for THIS node only.
+            errors[handle.Index] = StatusCodes.BadTimeout;
+            results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadTimeout };
+        }
+        catch (Exception ex)
+        {
+            // One node's backend failure must not throw out of the batch — surface Bad for THIS node
+            // only. This manager carries no ILogger, so log via the SDK's static trace (see ServeNodeAsync).
 #pragma warning disable CS0618 // Type or member is obsolete
-                Utils.LogError(ex, "OtOpcUaNodeManager: HistoryReadEvents failed for node {0}", handle.NodeId);
+            Utils.LogError(ex, "OtOpcUaNodeManager: HistoryReadEvents failed for node {0}", handle.NodeId);
 #pragma warning restore CS0618
-                errors[handle.Index] = StatusCodes.BadHistoryOperationUnsupported;
+            errors[handle.Index] = StatusCodes.BadHistoryOperationUnsupported;
-                results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadHistoryOperationUnsupported };
+            results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadHistoryOperationUnsupported };
-            }
         }
     }
 
@@ -2037,11 +2132,131 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
     /// <param name="errors">The service-level errors list to fill at <c>handle.Index</c>.</param>
     /// <param name="read">Invokes the resolved data-source read with the resolved tagname; only called
     /// once the tagname is confirmed present.</param>
-    private void ServeNode(
+    /// <summary>
+    ///     Run a batch of per-node HistoryRead work items with BOUNDED parallelism (arch-review 03/S3),
+    ///     block-bridged ONCE at the arm boundary. <b>Contract:</b> every work item must be fully
+    ///     self-contained — it fills its own results/errors slot and NEVER throws (all per-node handlers
+    ///     catch every exception, including cancellation, and surface a Bad status for that node only). That
+    ///     invariant is what makes the single <c>GetAwaiter().GetResult()</c> safe: <see cref="Task.WhenAll(Task[])"/>
+    ///     can never fault, so it cannot throw out of the synchronous SDK override. A degree ≤ 1 (or a
+    ///     single item) runs sequentially — no semaphore, no extra tasks.
+    /// </summary>
+    /// <param name="work">The per-node work items (index-disjoint writes into the shared results/errors).</param>
+    /// <param name="degree">Max concurrent work items (<see cref="HistoryReadBatchParallelism"/>).</param>
+    private static void RunBounded(IReadOnlyList<Func<Task>> work, int degree)
+    {
+        if (work.Count == 0) return;
+        if (degree <= 1 || work.Count == 1)
+        {
+            foreach (var item in work)
+                item().GetAwaiter().GetResult();
+            return;
+        }
+
+        using var gate = new SemaphoreSlim(degree);
+        var tasks = new Task[work.Count];
+        for (var i = 0; i < work.Count; i++)
+            tasks[i] = RunGatedAsync(gate, work[i]);
+
+        // Safe: every body is self-contained + never throws, so WhenAll never faults (see contract above).
+        Task.WhenAll(tasks).GetAwaiter().GetResult();
+    }
+
+    /// <summary>
+    ///     Run one HistoryRead arm under the process-wide concurrent-batch limiter (arch-review 03/S3). Each
+    ///     arm waits up to <c>min(HistoryReadDeadline, 5 s)</c> for a slot; on expiry every handle in the
+    ///     batch gets <c>BadTooManyOperations</c> (fast-fail under flood, brief waits under mild contention)
+    ///     and <paramref name="serve"/> is skipped. On acquisition the arm body runs and the slot is
+    ///     released in a <c>finally</c>. Combined with <see cref="HistoryReadBatchParallelism"/> this caps
+    ///     total in-flight gateway reads at
+    ///     <c>MaxConcurrentHistoryReadBatches × HistoryReadBatchParallelism</c>.
+    /// </summary>
+    /// <param name="nodesToProcess">The batch's handles — every slot is failed on limiter saturation.</param>
+    /// <param name="results">The service-level results list.</param>
+    /// <param name="errors">The service-level errors list.</param>
+    /// <param name="serve">The arm body to run once a slot is acquired.</param>
+    private void WithHistoryReadBatchLimiter(
+        List<NodeHandle> nodesToProcess,
+        IList<SdkHistoryReadResult> results,
+        IList<ServiceResult> errors,
+        Action serve)
+    {
+        var gate = GetHistoryReadBatchLimiter();
+
+        // Bounded admission wait: cap the block at 5 s, and never exceed the per-request deadline.
+        var waitMs = HistoryReadDeadline > TimeSpan.Zero
+            ? Math.Min(HistoryReadDeadline.TotalMilliseconds, 5000.0)
+            : 5000.0;
+
+        if (!gate.Wait(TimeSpan.FromMilliseconds(waitMs)))
+        {
+            foreach (var handle in nodesToProcess)
+            {
+                errors[handle.Index] = StatusCodes.BadTooManyOperations;
+                results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadTooManyOperations };
+            }
+            return;
+        }
+
+        try
+        {
+            serve();
+        }
+        finally
+        {
+            gate.Release();
+        }
+    }
+
+    /// <summary>
+    ///     Lazily create (double-checked) the process-wide HistoryRead-batch limiter, sized from
+    ///     <see cref="MaxConcurrentHistoryReadBatches"/> at first use so the Host's post-boot property set is
+    ///     respected (the same benign wiring race as <see cref="MaxTieClusterOverfetch"/>).
+    /// </summary>
+    /// <returns>The shared batch limiter semaphore.</returns>
+    private SemaphoreSlim GetHistoryReadBatchLimiter()
+    {
+        var gate = _historyReadBatchLimiter;
+        if (gate is not null) return gate;
+        lock (_historyReadBatchLimiterLock)
+        {
+            return _historyReadBatchLimiter ??= new SemaphoreSlim(Math.Max(1, MaxConcurrentHistoryReadBatches));
+        }
+    }
+
+    /// <summary>
+    ///     Create the per-request deadline source for a HistoryRead arm (arch-review 03/S3): a
+    ///     <see cref="CancellationTokenSource"/> that fires after <see cref="HistoryReadDeadline"/>, so a
+    ///     single request can never hold an SDK request thread longer than the deadline regardless of node
+    ///     count. Returns <c>null</c> when the deadline is non-positive (unbounded — the arm then passes
+    ///     <see cref="CancellationToken.None"/>), matching the <c>ServerHistorianOptions</c> warning.
+    /// </summary>
+    /// <returns>The deadline source, or <c>null</c> for an unbounded (non-positive) deadline.</returns>
+    private CancellationTokenSource? CreateDeadline() =>
+        HistoryReadDeadline > TimeSpan.Zero ? new CancellationTokenSource(HistoryReadDeadline) : null;
+
+    /// <summary>Awaits one work item under a semaphore permit, releasing it when the item completes.</summary>
+    /// <param name="gate">The concurrency-limiting semaphore.</param>
+    /// <param name="work">The per-node work item.</param>
+    private static async Task RunGatedAsync(SemaphoreSlim gate, Func<Task> work)
+    {
+        await gate.WaitAsync().ConfigureAwait(false);
+        try
+        {
+            await work().ConfigureAwait(false);
+        }
+        finally
+        {
+            gate.Release();
+        }
+    }
+
+    private async Task ServeNodeAsync(
         NodeHandle handle,
         IList<SdkHistoryReadResult> results,
         IList<ServiceResult> errors,
-        Func<IHistorianDataSource, string, Task<HistorianRead>> read)
+        Func<IHistorianDataSource, string, CancellationToken, Task<HistorianRead>> read,
+        CancellationToken ct)
     {
         var idString = handle.NodeId.Identifier?.ToString();
         if (idString is null || !TryGetHistorizedTagname(idString, out var tagname))
@@ -2054,9 +2269,9 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
 
         try
         {
-            // HistoryRead is NOT invoked under the node-manager Lock (unlike OnWriteValue), so blocking
+            // HistoryRead is NOT invoked under the node-manager Lock (unlike OnWriteValue), so awaiting
-            // on the async source here is safe and won't freeze the address space.
+            // the async source here is safe and won't freeze the address space.
-            var sourceResult = read(HistorianDataSource, tagname!).GetAwaiter().GetResult();
+            var sourceResult = await read(HistorianDataSource, tagname!, ct).ConfigureAwait(false);
             var historyData = ToHistoryData(sourceResult);
 
             results[handle.Index] = new SdkHistoryReadResult
@@ -2071,13 +2286,20 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
             };
             errors[handle.Index] = ServiceResult.Good;
         }
+        catch (OperationCanceledException)
+        {
+            // The per-request deadline elapsed while this node was in flight — surface BadTimeout for
+            // THIS node only (a spec-conformant history-read status), ahead of the generic catch.
+            errors[handle.Index] = StatusCodes.BadTimeout;
+            results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadTimeout };
+        }
         catch (Exception ex)
         {
-            // One node's backend failure (throw / timeout / cancellation) must not throw out of the
+            // One node's backend failure (throw / timeout) must not throw out of the batch — surface a
-            // batch — surface a Bad status for THIS node only. This CustomNodeManager2 carries no
+            // Bad status for THIS node only. This CustomNodeManager2 carries no ILogger (see
-            // ILogger (see ReportConditionEvent), so log through the SDK's static trace rather than
+            // ReportConditionEvent), so log through the SDK's static trace rather than swallowing silently.
-            // swallowing silently. Utils.LogError is [Obsolete] in 1.5.378 (favours an ITelemetryContext
+            // Utils.LogError is [Obsolete] in 1.5.378 (favours an ITelemetryContext this manager doesn't
-            // this manager doesn't wire) — suppress the deprecation, matching the existing pattern.
+            // wire) — suppress the deprecation, matching the existing pattern.
 #pragma warning disable CS0618 // Type or member is obsolete
             Utils.LogError(ex, "OtOpcUaNodeManager: HistoryRead failed for node {0}", handle.NodeId);
 #pragma warning restore CS0618
@@ -2115,7 +2337,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
     /// <param name="startTimeUtc">The request window's (inclusive) lower bound, used for a fresh read.</param>
     /// <param name="endUtc">The (inclusive) upper bound of the read window; unchanged across pages.</param>
     /// <param name="numValuesPerNode">The client's per-page cap; <c>0</c> means "all values, no paging".</param>
-    private void ServeRawPaged(
+    private async Task ServeRawPagedAsync(
         NodeHandle handle,
         ISession? session,
         IList<HistoryReadValueId> nodesToRead,
@@ -2123,7 +2345,8 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
         IList<ServiceResult> errors,
         DateTime startTimeUtc,
         DateTime endUtc,
-        uint numValuesPerNode)
+        uint numValuesPerNode,
+        CancellationToken ct)
     {
         var inboundCp = nodesToRead[handle.Index].ContinuationPoint;
 
@@ -2165,10 +2388,10 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
                 startUtc = startTimeUtc;
             }
 
-            // HistoryRead is NOT under the node-manager Lock — block-bridging the async source is safe.
+            // HistoryRead is NOT under the node-manager Lock — awaiting the async source is safe.
-            var sourceResult = HistorianDataSource
+            var sourceResult = await HistorianDataSource
-                .ReadRawAsync(tagname, startUtc, endUtc, numValuesPerNode, CancellationToken.None)
+                .ReadRawAsync(tagname, startUtc, endUtc, numValuesPerNode, ct)
-                .GetAwaiter().GetResult();
+                .ConfigureAwait(false);
 
             var backendFull = HistoryPaging.IsFullPage(sourceResult.Samples.Count, numValuesPerNode);
 
@@ -2198,9 +2421,9 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
                 // Compute in uint so +1 can never wrap: Math.Max(1,…) also guards against a zero/negative
                 // config value slipping through (Validate() warns but does not block startup).
                 var overfetchCap = (uint)Math.Max(1, MaxTieClusterOverfetch) + 1u;
-                var cluster = HistorianDataSource
+                var cluster = (await HistorianDataSource
-                    .ReadRawAsync(tagname, startUtc, startUtc, overfetchCap, CancellationToken.None)
+                    .ReadRawAsync(tagname, startUtc, startUtc, overfetchCap, ct)
-                    .GetAwaiter().GetResult().Samples;
+                    .ConfigureAwait(false)).Samples;
 
                 // Absurd burst: more ties than we're willing to buffer in memory. Preserve today's loud-fail
                 // for that node rather than over-fetch an unbounded cluster; the operator's remedy is a
@@ -2272,6 +2495,12 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
             };
             errors[handle.Index] = ServiceResult.Good;
         }
+        catch (OperationCanceledException)
+        {
+            // The per-request deadline elapsed while paging this node — BadTimeout for THIS node only.
+            errors[handle.Index] = StatusCodes.BadTimeout;
+            results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadTimeout };
+        }
         catch (Exception ex)
         {
             // One node's backend failure must not throw out of the batch — Bad for THIS node only.

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/ZB.MOM.WW.OtOpcUa.OpcUaServer.csproj

@@ -23,6 +23,9 @@
 
     <ItemGroup>
         <InternalsVisibleTo Include="ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests"/>
+        <!-- R2-08 (03/S2): the outage-sim integration test drives the internal static
+             OpcUaApplicationHost.HandleImpersonation against the real LDAP library. -->
+        <InternalsVisibleTo Include="ZB.MOM.WW.OtOpcUa.Host.IntegrationTests"/>
     </ItemGroup>
 
 </Project>

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Historian/ServerHistorianOptions.cs

@@ -68,6 +68,32 @@ public sealed class ServerHistorianOptions
     /// </summary>
     public int MaxTieClusterOverfetch { get; init; } = 65536;
 
+    /// <summary>
+    ///     Maximum number of historized nodes served CONCURRENTLY within one HistoryRead batch (arch-review
+    ///     03/S3). The SDK's <c>HistoryRead*</c> override surface is synchronous, so each arm block-bridges
+    ///     once at its boundary; this bound turns the per-node fan-out inside that boundary from sequential
+    ///     (worst case <c>N × CallTimeout</c>) into <c>⌈N / P⌉ × CallTimeout</c>. It also caps the gateway-load
+    ///     multiplication a single client can command. Default 4.
+    /// </summary>
+    public int HistoryReadBatchParallelism { get; init; } = 4;
+
+    /// <summary>
+    ///     Process-wide limit on the number of HistoryRead batches served CONCURRENTLY (arch-review 03/S3).
+    ///     A flood of history clients degrades to <c>BadTooManyOperations</c> rather than exhausting the SDK
+    ///     request-thread pool. Together with <see cref="HistoryReadBatchParallelism"/> this caps total
+    ///     in-flight gateway reads at <c>MaxConcurrentHistoryReadBatches × HistoryReadBatchParallelism</c>
+    ///     (default 16 × 4 = 64). Default 16.
+    /// </summary>
+    public int MaxConcurrentHistoryReadBatches { get; init; } = 16;
+
+    /// <summary>
+    ///     Per-request wall-clock deadline for a HistoryRead batch (arch-review 03/S3). A single request can
+    ///     no longer hold an SDK request thread longer than this regardless of node count; a node still
+    ///     in flight at the deadline surfaces <c>BadTimeout</c>. Non-positive = unbounded (a
+    ///     <see cref="Validate"/> warning). Default 60 seconds.
+    /// </summary>
+    public TimeSpan HistoryReadDeadline { get; init; } = TimeSpan.FromSeconds(60);
+
     /// <summary>Returns operator-facing misconfiguration warnings for an <c>Enabled</c> historian
     /// (empty when disabled or correctly configured). Pure — the registration logs each entry.
     /// <para>
@@ -100,6 +126,14 @@ public sealed class ServerHistorianOptions
         // so a zero/negative value is harmless (and noise-free) when the historian is disabled.
         if (MaxTieClusterOverfetch <= 0)
             warnings.Add($"ServerHistorian:MaxTieClusterOverfetch is {MaxTieClusterOverfetch} — must be > 0; HistoryRead-Raw cannot page within an oversized tie cluster and will surface BadHistoryOperationUnsupported for those reads.");
+        // S3 HistoryRead concurrency knobs — a non-positive value disables the corresponding bound, so warn
+        // (these gate the per-node fan-out / batch limiter that only run when a real data source is wired).
+        if (HistoryReadBatchParallelism <= 0)
+            warnings.Add($"ServerHistorian:HistoryReadBatchParallelism is {HistoryReadBatchParallelism} — must be > 0; HistoryRead falls back to serving each node sequentially.");
+        if (MaxConcurrentHistoryReadBatches <= 0)
+            warnings.Add($"ServerHistorian:MaxConcurrentHistoryReadBatches is {MaxConcurrentHistoryReadBatches} — must be > 0; the process-wide HistoryRead batch limiter is disabled.");
+        if (HistoryReadDeadline <= TimeSpan.Zero)
+            warnings.Add($"ServerHistorian:HistoryReadDeadline is {HistoryReadDeadline} — must be > 0; HistoryRead requests run unbounded (no per-request deadline).");
         return warnings;
     }
 }

src/Server/ZB.MOM.WW.OtOpcUa.Security/Ldap/LdapAuthResult.cs

@@ -7,4 +7,15 @@ public sealed record LdapAuthResult(
     string? Username,
     IReadOnlyList<string> Groups,
     IReadOnlyList<string> Roles,
-    string? Error);
+    string? Error)
+{
+    /// <summary>
+    ///     <c>true</c> when this failure is a SYSTEM-side directory fault (the directory is unreachable or
+    ///     the service account is misconfigured) rather than a user-credential deny. Feeds the OPC UA
+    ///     data-plane authenticator's outage circuit (03/S2): a run of consecutive system-side failures
+    ///     opens the circuit and fails fast, whereas user-side denies never trip it. Always <c>false</c>
+    ///     on success and on user-side denies; set only on the delegated real path for the library's
+    ///     directory-unreachable bucket.
+    /// </summary>
+    public bool IsSystemFailure { get; init; }
+}

src/Server/ZB.MOM.WW.OtOpcUa.Security/Ldap/LdapOptions.cs

@@ -83,6 +83,49 @@ public sealed class LdapOptions
     /// </summary>
     public Dictionary<string, string> GroupToRole { get; set; } = new(StringComparer.OrdinalIgnoreCase);
 
+    /// <summary>
+    ///     Bounds (in milliseconds) the shared-library directory client's socket connect AND each
+    ///     per-operation search inside a single bind flow. Projected into the library's
+    ///     <see cref="LibLdapOptions.ConnectionTimeoutMs"/> via <see cref="ToLibraryOptions"/>. The
+    ///     default (10000) matches the library default, so behaviour is unchanged unless configured.
+    ///     This is the FIRST line of defence against a directory outage; the whole-flow
+    ///     <see cref="AuthTimeoutMs"/> is the backstop (03/S2).
+    /// </summary>
+    public int ConnectionTimeoutMs { get; set; } = 10_000;
+
+    /// <summary>
+    ///     Hard whole-flow wall-clock bound (in milliseconds) the OPC UA data-plane authenticator
+    ///     (<c>LdapOpcUaUserAuthenticator</c>) applies across the ENTIRE authenticate flow — bind plus
+    ///     role-map — so the OPC UA SDK's synchronous impersonation callback (raised under a
+    ///     <c>SessionManager</c>-wide event lock) can never be wedged by a slow / hung directory.
+    ///     Deliberately &gt; <see cref="ConnectionTimeoutMs"/> (default 15000) so a healthy-but-slow
+    ///     full flow is not cut off while an outage is bounded by the library timeout first (03/S2).
+    /// </summary>
+    public int AuthTimeoutMs { get; set; } = 15_000;
+
+    /// <summary>
+    ///     Maximum number of in-flight OPC UA authentication flows the data-plane authenticator admits
+    ///     at once. A caller that cannot acquire a slot within its budget is denied "backend busy"
+    ///     (local backpressure, not counted against the outage circuit). Bounds accumulation of
+    ///     orphaned / timed-out bind tasks during an outage (03/S2). Default 8.
+    /// </summary>
+    public int MaxConcurrentAuthentications { get; set; } = 8;
+
+    /// <summary>
+    ///     Number of CONSECUTIVE system-side failures (boundary timeout, directory-unreachable result,
+    ///     or unexpected throw) after which the data-plane authenticator's outage circuit opens and
+    ///     denies instantly for <see cref="OutageCooldownSeconds"/> without touching LDAP. Any success
+    ///     or user-side (credential) deny resets the counter. <c>0</c> disables the circuit (03/S2).
+    ///     Default 3.
+    /// </summary>
+    public int OutageFailureThreshold { get; set; } = 3;
+
+    /// <summary>
+    ///     How long (in seconds) the outage circuit stays open before a half-open probe is allowed
+    ///     through. Only meaningful when <see cref="OutageFailureThreshold"/> &gt; 0. Default 15 (03/S2).
+    /// </summary>
+    public int OutageCooldownSeconds { get; set; } = 15;
+
     /// <summary>
     ///     Projects the wire fields onto the shared <c>ZB.MOM.WW.Auth.Ldap</c>
     ///     <see cref="LibLdapOptions"/> the directory client consumes. App-only concerns
@@ -104,5 +147,6 @@ public sealed class LdapOptions
         UserNameAttribute = UserNameAttribute,
         DisplayNameAttribute = DisplayNameAttribute,
         GroupAttribute = GroupAttribute,
+        ConnectionTimeoutMs = ConnectionTimeoutMs,
     };
 }

src/Server/ZB.MOM.WW.OtOpcUa.Security/Ldap/OtOpcUaLdapAuthService.cs

@@ -113,7 +113,13 @@ public sealed class OtOpcUaLdapAuthService : ILdapAuthService
         if (result.Succeeded)
             return new(true, result.DisplayName, result.Username, result.Groups, [], null);
 
-        return new(false, null, username, [], [], FailureToError(result.Failure));
+        // ServiceAccountBindFailed is the library's directory-unreachable / service-account-misconfigured
+        // bucket (its enum has no dedicated DirectoryUnavailable) — flag it as system-side so the OPC UA
+        // authenticator's outage circuit can distinguish an outage from a user-credential deny (03/S2).
+        return new(false, null, username, [], [], FailureToError(result.Failure))
+        {
+            IsSystemFailure = result.Failure is LdapAuthFailure.ServiceAccountBindFailed,
+        };
     }
 
     /// <summary>Folds a structured library failure code into the app's user-facing error text.</summary>

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/LdapAuthResilienceTests.cs

@@ -0,0 +1,389 @@
+using System.Diagnostics;
+using System.Text;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using Opc.Ua;
+using Opc.Ua.Server;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.Auth.Abstractions.Roles;
+using ZB.MOM.WW.OtOpcUa.Host.OpcUa;
+using ZB.MOM.WW.OtOpcUa.OpcUaServer;
+using ZB.MOM.WW.OtOpcUa.Security.Ldap;
+using LdapTransport = ZB.MOM.WW.Auth.Abstractions.Ldap.LdapTransport;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.IntegrationTests;
+
+/// <summary>
+/// Resilience guard for <see cref="LdapOpcUaUserAuthenticator"/> (arch-review 03/S2). The OPC UA SDK
+/// raises the impersonation callback synchronously under a <c>SessionManager</c>-wide event lock, so a
+/// slow / hung LDAP bind inside it does not merely stall one SDK thread — it serializes EVERY session
+/// activation server-wide. These tests prove the authenticator enforces a hard wall-clock bound,
+/// bounded in-flight concurrency, and a directory-outage circuit at its own boundary (the SDK callback
+/// being irreducibly synchronous), all fail-closed. No real LDAP server is used — a delaying fake
+/// reproduces the outage shape offline.
+/// </summary>
+public sealed class LdapAuthResilienceTests
+{
+    /// <summary>
+    /// RED repro (03/S2): a backend that takes 5 s must NOT hold the caller for 5 s — the authenticator's
+    /// wall-clock bound denies (fail-closed) well before then. On current code (no bound) this fails: the
+    /// call returns Allow after the full 5 s.
+    /// </summary>
+    [Fact]
+    public async Task Authenticate_slow_backend_denies_within_AuthTimeout()
+    {
+        var ldap = new DelayingFakeLdap(
+            TimeSpan.FromSeconds(5),
+            new LdapAuthResult(true, "Alice", "alice", new[] { "configeditor" }, Array.Empty<string>(), null));
+        var mapper = new FakeMapper(g => g.ToArray());
+        var sut = Build(ldap, mapper, new LdapOptions { AuthTimeoutMs = 200 });
+
+        var sw = Stopwatch.StartNew();
+        var result = await sut
+            .AuthenticateUserNameAsync("alice", "secret", CancellationToken.None)
+            .WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+        sw.Stop();
+
+        result.Success.ShouldBeFalse();
+        result.Error.ShouldNotBeNull();
+        result.Error!.ShouldContain("timed out");
+        sw.Elapsed.ShouldBeLessThan(TimeSpan.FromSeconds(1.5));
+    }
+
+    /// <summary>A fast, healthy backend is unaffected by the resilience wrapping — success + roles pass
+    /// through untouched (03/S2).</summary>
+    [Fact]
+    public async Task Authenticate_fast_success_unaffected()
+    {
+        var ldap = new DelayingFakeLdap(
+            TimeSpan.Zero,
+            new LdapAuthResult(true, "Alice", "alice", new[] { "configeditor" }, Array.Empty<string>(), null));
+        var mapper = new FakeMapper(g => g.Select(x => x == "configeditor" ? "Designer" : x).ToArray());
+        var sut = Build(ldap, mapper, new LdapOptions());
+
+        var result = await sut
+            .AuthenticateUserNameAsync("alice", "secret", CancellationToken.None)
+            .WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+
+        result.Success.ShouldBeTrue();
+        result.DisplayName.ShouldBe("Alice");
+        result.Roles.ShouldBe(new[] { "Designer" });
+    }
+
+    /// <summary>With the concurrency cap saturated (a parked in-flight bind holding the only slot), a
+    /// second call is denied "backend busy" within its own budget — it never waits behind the parked
+    /// bind indefinitely (03/S2).</summary>
+    [Fact]
+    public async Task Authenticate_saturated_concurrency_denies_busy()
+    {
+        var ldap = new GatedFakeLdap(
+            new LdapAuthResult(true, "Alice", "alice", new[] { "configeditor" }, Array.Empty<string>(), null));
+        var mapper = new FakeMapper(g => g.ToArray());
+        // MaxConcurrentAuthentications = 1 → the second call cannot get a slot; small AuthTimeoutMs so the
+        // "busy" denial is quick and the first call's boundary also trips fast (its core stays parked,
+        // holding the slot).
+        var sut = Build(ldap, mapper, new LdapOptions { MaxConcurrentAuthentications = 1, AuthTimeoutMs = 300 });
+
+        // First call: acquires the only slot; its core parks in the gated fake.
+        var first = sut.AuthenticateUserNameAsync("alice", "secret", CancellationToken.None);
+
+        // Wait until the fake has actually been entered (slot acquired, core parked).
+        var spin = Stopwatch.StartNew();
+        while (ldap.Entered == 0 && spin.Elapsed < TimeSpan.FromSeconds(5))
+            await Task.Delay(10, TestContext.Current.CancellationToken);
+        ldap.Entered.ShouldBe(1);
+
+        // Second concurrent call: the slot is held by the parked core, so acquisition times out → busy.
+        var second = await sut
+            .AuthenticateUserNameAsync("bob", "secret", CancellationToken.None)
+            .WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+
+        second.Success.ShouldBeFalse();
+        second.Error.ShouldNotBeNull();
+        second.Error!.ShouldContain("busy");
+        ldap.Entered.ShouldBe(1, "the second call must be denied without ever reaching the backend");
+
+        // Release the parked bind so the first call's core completes cleanly (no orphaned resources).
+        ldap.Release();
+        await first.WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+    }
+
+    /// <summary>After <c>OutageFailureThreshold</c> consecutive system-side failures the circuit opens and
+    /// the next call is fast-denied "backend unavailable" WITHOUT touching LDAP (03/S2).</summary>
+    [Fact]
+    public async Task Consecutive_system_failures_open_circuit_and_fast_deny()
+    {
+        var ldap = new DelayingFakeLdap(TimeSpan.Zero, SystemFailure());
+        var mapper = new FakeMapper(g => g.ToArray());
+        var sut = Build(ldap, mapper,
+            new LdapOptions { OutageFailureThreshold = 3, OutageCooldownSeconds = 60, AuthTimeoutMs = 5000 });
+
+        for (var i = 0; i < 3; i++)
+        {
+            var r = await sut.AuthenticateUserNameAsync("u", "p", CancellationToken.None)
+                .WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+            r.Success.ShouldBeFalse();
+        }
+
+        ldap.Calls.ShouldBe(3);
+
+        var sw = Stopwatch.StartNew();
+        var blocked = await sut.AuthenticateUserNameAsync("u", "p", CancellationToken.None)
+            .WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+        sw.Stop();
+
+        blocked.Success.ShouldBeFalse();
+        blocked.Error.ShouldNotBeNull();
+        blocked.Error!.ShouldContain("unavailable");
+        ldap.Calls.ShouldBe(3, "an open circuit must fast-deny without reaching the backend");
+        sw.Elapsed.ShouldBeLessThan(TimeSpan.FromSeconds(1));
+    }
+
+    /// <summary>A user-side (credential) deny between system failures resets the streak, so the circuit
+    /// never opens — user denies are not directory evidence (03/S2).</summary>
+    [Fact]
+    public async Task User_side_deny_resets_the_system_failure_streak()
+    {
+        var ldap = new ScriptedFakeLdap(
+            SystemFailure(), SystemFailure(), UserDeny(), SystemFailure(), SystemFailure());
+        var mapper = new FakeMapper(g => g.ToArray());
+        var sut = Build(ldap, mapper,
+            new LdapOptions { OutageFailureThreshold = 3, OutageCooldownSeconds = 60, AuthTimeoutMs = 5000 });
+
+        // 5 scripted calls: sys, sys, user(reset), sys, sys → streak ends at 2 (< 3), circuit stays closed.
+        for (var i = 0; i < 5; i++)
+        {
+            await sut.AuthenticateUserNameAsync("u", "p", CancellationToken.None)
+                .WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+        }
+
+        // A 6th call must still REACH the backend (circuit never opened).
+        await sut.AuthenticateUserNameAsync("u", "p", CancellationToken.None)
+            .WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+        ldap.Calls.ShouldBe(6);
+    }
+
+    /// <summary>After the cooldown elapses the circuit half-opens: the next call probes the backend and a
+    /// success closes the circuit (03/S2).</summary>
+    [Fact]
+    public async Task Half_open_probe_after_cooldown_closes_on_success()
+    {
+        var ldap = new ScriptedFakeLdap(
+            SystemFailure(), SystemFailure(), SystemFailure(),
+            new LdapAuthResult(true, "Alice", "alice", new[] { "configeditor" }, Array.Empty<string>(), null));
+        var mapper = new FakeMapper(g => g.ToArray());
+        var sut = Build(ldap, mapper,
+            new LdapOptions { OutageFailureThreshold = 3, OutageCooldownSeconds = 1, AuthTimeoutMs = 5000 });
+
+        for (var i = 0; i < 3; i++)
+            await sut.AuthenticateUserNameAsync("u", "p", CancellationToken.None)
+                .WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+        ldap.Calls.ShouldBe(3);
+
+        // Circuit open: this call is fast-denied without touching the backend.
+        var blocked = await sut.AuthenticateUserNameAsync("u", "p", CancellationToken.None)
+            .WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+        blocked.Error!.ShouldContain("unavailable");
+        ldap.Calls.ShouldBe(3);
+
+        // Wait past the cooldown, then a probe reaches the backend and succeeds → circuit closes.
+        await Task.Delay(TimeSpan.FromMilliseconds(1200), TestContext.Current.CancellationToken);
+
+        var probe = await sut.AuthenticateUserNameAsync("alice", "secret", CancellationToken.None)
+            .WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+        probe.Success.ShouldBeTrue();
+        ldap.Calls.ShouldBe(4, "the half-open probe reaches the backend");
+    }
+
+    /// <summary><c>OutageFailureThreshold = 0</c> disables the circuit — it never opens, every call reaches
+    /// the backend (03/S2).</summary>
+    [Fact]
+    public async Task Circuit_disabled_never_opens()
+    {
+        var ldap = new DelayingFakeLdap(TimeSpan.Zero, SystemFailure());
+        var mapper = new FakeMapper(g => g.ToArray());
+        var sut = Build(ldap, mapper,
+            new LdapOptions { OutageFailureThreshold = 0, OutageCooldownSeconds = 60, AuthTimeoutMs = 5000 });
+
+        for (var i = 0; i < 6; i++)
+            await sut.AuthenticateUserNameAsync("u", "p", CancellationToken.None)
+                .WaitAsync(TimeSpan.FromSeconds(10), CancellationToken.None);
+
+        ldap.Calls.ShouldBe(6, "a disabled circuit never fast-denies");
+    }
+
+    /// <summary>
+    /// Outage-sim (offline-safe): the REAL <see cref="OtOpcUaLdapAuthService"/> + real shared library
+    /// pointed at an UNROUTABLE host (a TCP blackhole — the same outage shape as "stop the GLAuth
+    /// container"), driven through the REAL internal <see cref="OpcUaApplicationHost.HandleImpersonation"/>
+    /// block-bridge. The activation must fail bounded (well under the SDK's default ~10-20 s stall) with
+    /// <see cref="StatusCodes.BadIdentityTokenRejected"/> and no granted identity (03/S2).
+    /// </summary>
+    [Fact]
+    public async Task Activation_with_unreachable_directory_fails_within_bound()
+    {
+        var options = new LdapOptions
+        {
+            Enabled = true,
+            DevStubMode = false,
+            Server = "10.255.255.1", // RFC-blackhole: SYN goes nowhere
+            Port = 3893,
+            Transport = LdapTransport.None,
+            AllowInsecure = true,
+            SearchBase = "dc=zb,dc=local",
+            ConnectionTimeoutMs = 500,
+            AuthTimeoutMs = 1000,
+        };
+
+        var ldap = new OtOpcUaLdapAuthService(Options.Create(options), NullLogger<OtOpcUaLdapAuthService>.Instance);
+        var mapper = new FakeMapper(g => g.ToArray());
+        var authenticator = new LdapOpcUaUserAuthenticator(
+            ldap, ScopeFactoryWith(mapper), Options.Create(options),
+            NullLogger<LdapOpcUaUserAuthenticator>.Instance);
+
+        var token = new UserNameIdentityToken { UserName = "alice", DecryptedPassword = Encoding.UTF8.GetBytes("secret") };
+        var policy = new UserTokenPolicy(UserTokenType.UserName) { PolicyId = "username_basic256sha256" };
+        var args = new ImpersonateEventArgs(token, policy, new EndpointDescription());
+
+        var sw = Stopwatch.StartNew();
+        await Task.Run(() => OpcUaApplicationHost.HandleImpersonation(
+                authenticator, args, NullLogger<object>.Instance), TestContext.Current.CancellationToken)
+            .WaitAsync(TimeSpan.FromSeconds(10), TestContext.Current.CancellationToken);
+        sw.Stop();
+
+        args.Identity.ShouldBeNull();
+        args.IdentityValidationError.ShouldNotBeNull();
+        args.IdentityValidationError.Code.ShouldBe(Opc.Ua.StatusCodes.BadIdentityTokenRejected);
+        sw.Elapsed.ShouldBeLessThan(TimeSpan.FromSeconds(3));
+    }
+
+    private static LdapAuthResult SystemFailure() =>
+        new(false, null, "u", Array.Empty<string>(), Array.Empty<string>(), "Unexpected authentication error")
+        { IsSystemFailure = true };
+
+    private static LdapAuthResult UserDeny() =>
+        new(false, null, "u", Array.Empty<string>(), Array.Empty<string>(), "Invalid username or password")
+        { IsSystemFailure = false };
+
+    private static LdapOpcUaUserAuthenticator Build(
+        ILdapAuthService ldap, IGroupRoleMapper<string> mapper, LdapOptions options) =>
+        new(ldap, ScopeFactoryWith(mapper), Options.Create(options),
+            NullLogger<LdapOpcUaUserAuthenticator>.Instance);
+
+    /// <summary>Builds an IServiceScopeFactory whose scopes resolve the supplied mapper.</summary>
+    private static IServiceScopeFactory ScopeFactoryWith(IGroupRoleMapper<string> mapper)
+    {
+        var services = new ServiceCollection();
+        services.AddScoped(_ => mapper);
+        return services.BuildServiceProvider().GetRequiredService<IServiceScopeFactory>();
+    }
+
+    /// <summary>
+    /// A fake <see cref="ILdapAuthService"/> that awaits a configurable delay (observing the token) before
+    /// returning a fixed result — the offline stand-in for a slow / hung directory. Counts invocations so
+    /// tests can assert the outage circuit denies WITHOUT reaching the backend.
+    /// </summary>
+    private sealed class DelayingFakeLdap : ILdapAuthService
+    {
+        private readonly TimeSpan _delay;
+        private readonly LdapAuthResult _then;
+        private int _calls;
+
+        /// <summary>Initializes the fake with a per-call delay and the result to return afterwards.</summary>
+        /// <param name="delay">How long each authenticate call blocks before returning.</param>
+        /// <param name="then">The result returned once the delay elapses.</param>
+        public DelayingFakeLdap(TimeSpan delay, LdapAuthResult then)
+        {
+            _delay = delay;
+            _then = then;
+        }
+
+        /// <summary>The number of times the backend was actually invoked.</summary>
+        public int Calls => Volatile.Read(ref _calls);
+
+        /// <summary>Awaits the configured delay, then returns the canned result.</summary>
+        /// <param name="username">The username (ignored).</param>
+        /// <param name="password">The password (ignored).</param>
+        /// <param name="ct">Cancellation token, observed by the delay.</param>
+        public async Task<LdapAuthResult> AuthenticateAsync(string username, string password, CancellationToken ct = default)
+        {
+            Interlocked.Increment(ref _calls);
+            await Task.Delay(_delay, ct).ConfigureAwait(false);
+            return _then;
+        }
+    }
+
+    /// <summary>
+    /// A fake <see cref="ILdapAuthService"/> that parks ASYNCHRONOUSLY (awaits a
+    /// <see cref="TaskCompletionSource"/>) until the test calls <see cref="Release"/> — used to hold an
+    /// in-flight bind so the concurrency cap is deterministically saturated WITHOUT blocking a thread.
+    /// Records how many calls have entered (past the slot acquire).
+    /// </summary>
+    private sealed class GatedFakeLdap : ILdapAuthService
+    {
+        private readonly TaskCompletionSource _park = new(TaskCreationOptions.RunContinuationsAsynchronously);
+        private readonly LdapAuthResult _then;
+        private int _entered;
+
+        /// <summary>Initializes the fake with the result to return once released.</summary>
+        /// <param name="then">The result returned once <see cref="Release"/> is called.</param>
+        public GatedFakeLdap(LdapAuthResult then) => _then = then;
+
+        /// <summary>Number of authenticate calls that reached the backend body.</summary>
+        public int Entered => Volatile.Read(ref _entered);
+
+        /// <summary>Releases the parked bind so its awaiting call can complete.</summary>
+        public void Release() => _park.TrySetResult();
+
+        /// <summary>Marks entry, awaits the parking gate, then returns the canned result.</summary>
+        /// <param name="username">The username (ignored).</param>
+        /// <param name="password">The password (ignored).</param>
+        /// <param name="ct">Cancellation token, observed while parked.</param>
+        public async Task<LdapAuthResult> AuthenticateAsync(string username, string password, CancellationToken ct = default)
+        {
+            Interlocked.Increment(ref _entered);
+            await _park.Task.WaitAsync(ct).ConfigureAwait(false);
+            return _then;
+        }
+    }
+
+    /// <summary>
+    /// A fake <see cref="ILdapAuthService"/> that returns a scripted sequence of results (one per call;
+    /// the last entry repeats once exhausted). Counts calls so the outage-circuit tests can assert the
+    /// backend was — or was NOT — reached on a given call.
+    /// </summary>
+    private sealed class ScriptedFakeLdap : ILdapAuthService
+    {
+        private readonly IReadOnlyList<LdapAuthResult> _script;
+        private int _calls;
+
+        /// <summary>Initializes the fake with the scripted results, returned in order.</summary>
+        /// <param name="script">The results to return, one per call; the last repeats once exhausted.</param>
+        public ScriptedFakeLdap(params LdapAuthResult[] script) => _script = script;
+
+        /// <summary>The number of times the backend was invoked.</summary>
+        public int Calls => Volatile.Read(ref _calls);
+
+        /// <summary>Returns the next scripted result (clamped to the last entry).</summary>
+        /// <param name="username">The username (ignored).</param>
+        /// <param name="password">The password (ignored).</param>
+        /// <param name="ct">The cancellation token (ignored).</param>
+        public Task<LdapAuthResult> AuthenticateAsync(string username, string password, CancellationToken ct = default)
+        {
+            var idx = Interlocked.Increment(ref _calls) - 1;
+            return Task.FromResult(_script[Math.Min(idx, _script.Count - 1)]);
+        }
+    }
+
+    /// <summary>Test group→role mapper driven by a delegate over the supplied groups.</summary>
+    private sealed class FakeMapper(Func<IReadOnlyList<string>, IReadOnlyList<string>> map) : IGroupRoleMapper<string>
+    {
+        /// <summary>Maps groups to roles via the configured delegate; Scope is always null.</summary>
+        /// <param name="groups">The LDAP groups to map.</param>
+        /// <param name="ct">The cancellation token.</param>
+        public Task<GroupRoleMapping<string>> MapAsync(IReadOnlyList<string> groups, CancellationToken ct)
+            => Task.FromResult(new GroupRoleMapping<string>(map(groups), Scope: null));
+    }
+}

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/LdapOpcUaUserAuthenticatorTests.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
 using Shouldly;
 using Xunit;
 using ZB.MOM.WW.Auth.Abstractions.Roles;
@@ -26,7 +27,7 @@ public sealed class LdapOpcUaUserAuthenticatorTests
         // group "configeditor" -> "Designer" (canonical, Task 1.7).
         var ldap = new FakeLdap(new LdapAuthResult(true, "Alice", "alice", new[] { "configeditor" }, Array.Empty<string>(), null));
         var mapper = new FakeMapper(g => g.Select(x => x == "configeditor" ? "Designer" : x).ToArray());
-        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
+        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), Options.Create(new LdapOptions()), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
 
         var result = await sut.AuthenticateUserNameAsync("alice", "secret", CancellationToken.None);
 
@@ -44,7 +45,7 @@ public sealed class LdapOpcUaUserAuthenticatorTests
         // nothing, so the union is exactly {Administrator}.
         var ldap = new FakeLdap(new LdapAuthResult(true, "dev", "dev", new[] { "dev" }, new[] { "Administrator" }, null));
         var mapper = new FakeMapper(_ => Array.Empty<string>());
-        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
+        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), Options.Create(new LdapOptions()), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
 
         var result = await sut.AuthenticateUserNameAsync("dev", "anything", CancellationToken.None);
 
@@ -59,7 +60,7 @@ public sealed class LdapOpcUaUserAuthenticatorTests
     {
         var ldap = new FakeLdap(new LdapAuthResult(true, "dev", "dev", new[] { "dev" }, new[] { "Administrator" }, null));
         var mapper = new FakeMapper(_ => throw new InvalidOperationException("DB down"));
-        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
+        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), Options.Create(new LdapOptions()), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
 
         var result = await sut.AuthenticateUserNameAsync("dev", "anything", CancellationToken.None);
 
@@ -73,7 +74,7 @@ public sealed class LdapOpcUaUserAuthenticatorTests
     {
         var ldap = new FakeLdap(new LdapAuthResult(false, null, "mallory", Array.Empty<string>(), Array.Empty<string>(), "Invalid username or password"));
         var mapper = new FakeMapper(g => g.ToArray());
-        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
+        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), Options.Create(new LdapOptions()), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
 
         var result = await sut.AuthenticateUserNameAsync("mallory", "wrong", CancellationToken.None);
 
@@ -87,7 +88,7 @@ public sealed class LdapOpcUaUserAuthenticatorTests
     {
         var ldap = new FakeLdap(_ => throw new InvalidOperationException("LDAP unreachable"));
         var mapper = new FakeMapper(g => g.ToArray());
-        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
+        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), Options.Create(new LdapOptions()), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
 
         var result = await sut.AuthenticateUserNameAsync("anyone", "x", CancellationToken.None);
 
@@ -102,7 +103,7 @@ public sealed class LdapOpcUaUserAuthenticatorTests
     {
         var ldap = new FakeLdap(new LdapAuthResult(true, null, "alice", new[] { "ReadOnly" }, Array.Empty<string>(), null));
         var mapper = new FakeMapper(g => g.ToArray());
-        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
+        var sut = new LdapOpcUaUserAuthenticator(ldap, ScopeFactoryWith(mapper), Options.Create(new LdapOptions()), NullLogger<LdapOpcUaUserAuthenticator>.Instance);
 
         var result = await sut.AuthenticateUserNameAsync("alice", "x", CancellationToken.None);
 

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/LdapOptionsBindingTests.cs

@@ -61,6 +61,56 @@ public sealed class LdapOptionsBindingTests
 
         options.DevStubMode.ShouldBeFalse();
     }
+
+    /// <summary>
+    /// The five S2 resilience keys bind from the real <c>Security:Ldap</c> section (03/S2).
+    /// </summary>
+    [Fact]
+    public void Binding_reads_resilience_keys()
+    {
+        var configuration = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?>
+            {
+                ["Security:Ldap:ConnectionTimeoutMs"] = "7000",
+                ["Security:Ldap:AuthTimeoutMs"] = "12000",
+                ["Security:Ldap:MaxConcurrentAuthentications"] = "5",
+                ["Security:Ldap:OutageFailureThreshold"] = "4",
+                ["Security:Ldap:OutageCooldownSeconds"] = "20",
+            })
+            .Build();
+
+        var options = configuration.GetSection(LdapOptions.SectionName).Get<LdapOptions>();
+
+        options.ShouldNotBeNull();
+        options.ConnectionTimeoutMs.ShouldBe(7000);
+        options.AuthTimeoutMs.ShouldBe(12000);
+        options.MaxConcurrentAuthentications.ShouldBe(5);
+        options.OutageFailureThreshold.ShouldBe(4);
+        options.OutageCooldownSeconds.ShouldBe(20);
+    }
+
+    /// <summary>The resilience keys carry the documented defaults when absent from config.</summary>
+    [Fact]
+    public void Resilience_key_defaults_hold_when_absent()
+    {
+        var options = new LdapOptions();
+
+        options.ConnectionTimeoutMs.ShouldBe(10000);
+        options.AuthTimeoutMs.ShouldBe(15000);
+        options.MaxConcurrentAuthentications.ShouldBe(8);
+        options.OutageFailureThreshold.ShouldBe(3);
+        options.OutageCooldownSeconds.ShouldBe(15);
+    }
+
+    /// <summary><c>ConnectionTimeoutMs</c> is projected onto the shared-library options so the
+    /// socket-connect + per-search timeout is bounded inside the directory client.</summary>
+    [Fact]
+    public void ToLibraryOptions_projects_ConnectionTimeoutMs()
+    {
+        var options = new LdapOptions { ConnectionTimeoutMs = 4321 };
+
+        options.ToLibraryOptions().ConnectionTimeoutMs.ShouldBe(4321);
+    }
 }
 
 /// <summary>

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/LdapOptionsValidatorTests.cs

@@ -203,6 +203,95 @@ public sealed class LdapOptionsValidatorTests
         result.Failures.ShouldContain("Ldap:SearchBase is required when LDAP login is enabled.");
     }
 
+    /// <summary>Enabled with a non-positive <c>ConnectionTimeoutMs</c> fails validation (03/S2).</summary>
+    [Fact]
+    public void Enabled_with_nonpositive_ConnectionTimeoutMs_fails()
+    {
+        var options = new LdapOptions
+        {
+            Enabled = true,
+            Server = "ldap",
+            SearchBase = "dc=x",
+            Port = 389,
+            Transport = LdapTransport.Ldaps,
+            ConnectionTimeoutMs = 0,
+        };
+
+        Sut.Validate(null, options).Failed.ShouldBeTrue();
+    }
+
+    /// <summary>Enabled with a non-positive <c>AuthTimeoutMs</c> fails validation (03/S2).</summary>
+    [Fact]
+    public void Enabled_with_nonpositive_AuthTimeoutMs_fails()
+    {
+        var options = new LdapOptions
+        {
+            Enabled = true,
+            Server = "ldap",
+            SearchBase = "dc=x",
+            Port = 389,
+            Transport = LdapTransport.Ldaps,
+            AuthTimeoutMs = 0,
+        };
+
+        Sut.Validate(null, options).Failed.ShouldBeTrue();
+    }
+
+    /// <summary>Enabled with a non-positive <c>MaxConcurrentAuthentications</c> fails validation (03/S2).</summary>
+    [Fact]
+    public void Enabled_with_nonpositive_MaxConcurrentAuthentications_fails()
+    {
+        var options = new LdapOptions
+        {
+            Enabled = true,
+            Server = "ldap",
+            SearchBase = "dc=x",
+            Port = 389,
+            Transport = LdapTransport.Ldaps,
+            MaxConcurrentAuthentications = 0,
+        };
+
+        Sut.Validate(null, options).Failed.ShouldBeTrue();
+    }
+
+    /// <summary>A positive <c>OutageFailureThreshold</c> with a zero <c>OutageCooldownSeconds</c> fails —
+    /// an open circuit with no cooldown would probe on every call (03/S2).</summary>
+    [Fact]
+    public void OutageFailureThreshold_positive_with_zero_cooldown_fails()
+    {
+        var options = new LdapOptions
+        {
+            Enabled = true,
+            Server = "ldap",
+            SearchBase = "dc=x",
+            Port = 389,
+            Transport = LdapTransport.Ldaps,
+            OutageFailureThreshold = 3,
+            OutageCooldownSeconds = 0,
+        };
+
+        Sut.Validate(null, options).Failed.ShouldBeTrue();
+    }
+
+    /// <summary>A zero <c>OutageFailureThreshold</c> (circuit disabled) passes regardless of the
+    /// cooldown value (03/S2).</summary>
+    [Fact]
+    public void OutageFailureThreshold_zero_disables_circuit_and_passes()
+    {
+        var options = new LdapOptions
+        {
+            Enabled = true,
+            Server = "ldap",
+            SearchBase = "dc=x",
+            Port = 389,
+            Transport = LdapTransport.Ldaps,
+            OutageFailureThreshold = 0,
+            OutageCooldownSeconds = 0,
+        };
+
+        Sut.Validate(null, options).Succeeded.ShouldBeTrue();
+    }
+
     /// <summary>Enabled with port 0 reports the port-range failure using the shared primitive wording.</summary>
     [Fact]
     public void Enabled_with_zero_port_fails()

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/NodeManagerHistoryReadConcurrencyTests.cs

@@ -0,0 +1,466 @@
+using System.Diagnostics;
+using Microsoft.Extensions.Logging.Abstractions;
+using Opc.Ua;
+using Opc.Ua.Server;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using HistorianRead = ZB.MOM.WW.OtOpcUa.Core.Abstractions.HistoryReadResult;
+using SdkHistoryReadResult = Opc.Ua.HistoryReadResult;
+
+namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests;
+
+/// <summary>
+///     Arch-review 03/S3 — the node-manager's HistoryRead arms must serve a multi-node batch with BOUNDED
+///     per-node parallelism, a per-request deadline, and a process-wide concurrent-batch limiter, rather
+///     than block-bridging the gateway per node sequentially on an SDK request thread. Boots the same real
+///     <see cref="OtOpcUaSdkServer"/> harness as <see cref="NodeManagerHistoryReadTests"/> and drives the
+///     public <c>HistoryRead</c> with a delaying fake <see cref="IHistorianDataSource"/> that records the
+///     maximum observed concurrent reads (deterministic — no raw-timing dependence for the primary
+///     assertions).
+/// </summary>
+public sealed class NodeManagerHistoryReadConcurrencyTests : IDisposable
+{
+    private static CancellationToken Ct => TestContext.Current.CancellationToken;
+
+    private readonly string _pkiRoot = Path.Combine(
+        Path.GetTempPath(),
+        $"otopcua-historyread-conc-{Guid.NewGuid():N}");
+
+    /// <summary>
+    ///     RED repro (03/S3): a single Raw HistoryRead naming 8 historized nodes against a 150 ms-per-read
+    ///     backend must serve at least two nodes concurrently. On the current sequential <c>foreach</c> the
+    ///     max observed concurrency is pinned at 1, so this fails.
+    /// </summary>
+    [Fact]
+    public async Task Raw_batch_is_served_with_bounded_parallelism()
+    {
+        var (host, server) = await BootAsync();
+        var nm = server.NodeManager!;
+        var fake = new ConcurrencyTrackingHistorianDataSource(TimeSpan.FromMilliseconds(150));
+        nm.HistorianDataSource = fake;
+
+        var nodeIds = MaterializeHistorizedNodes(nm, count: 8);
+
+        var details = new ReadRawModifiedDetails
+        {
+            StartTime = DateTime.UtcNow.AddHours(-1),
+            EndTime = DateTime.UtcNow,
+            NumValuesPerNode = 10,
+            IsReadModified = false,
+        };
+
+        var sw = Stopwatch.StartNew();
+        await Task.Run(() => InvokeHistoryRead(server, nm, details, nodeIds), Ct)
+            .WaitAsync(TimeSpan.FromSeconds(15), Ct);
+        sw.Stop();
+
+        // Primary (deterministic): the batch was served with >1 node in flight at once.
+        fake.MaxObservedConcurrency.ShouldBeGreaterThanOrEqualTo(2);
+
+        await host.DisposeAsync();
+    }
+
+    /// <summary>
+    ///     Parallelism is bounded ABOVE by <c>HistoryReadBatchParallelism</c>: with the property set to 3 and
+    ///     12 nodes in flight, no more than 3 reads are ever concurrent (03/S3).
+    /// </summary>
+    [Fact]
+    public async Task Parallelism_is_bounded_above()
+    {
+        var (host, server) = await BootAsync();
+        var nm = server.NodeManager!;
+        nm.HistoryReadBatchParallelism = 3;
+        var fake = new ConcurrencyTrackingHistorianDataSource(TimeSpan.FromMilliseconds(100));
+        nm.HistorianDataSource = fake;
+
+        var nodeIds = MaterializeHistorizedNodes(nm, count: 12);
+        var details = new ReadRawModifiedDetails
+        {
+            StartTime = DateTime.UtcNow.AddHours(-1),
+            EndTime = DateTime.UtcNow,
+            NumValuesPerNode = 10,
+            IsReadModified = false,
+        };
+
+        await Task.Run(() => InvokeHistoryRead(server, nm, details, nodeIds), Ct)
+            .WaitAsync(TimeSpan.FromSeconds(15), Ct);
+
+        fake.MaxObservedConcurrency.ShouldBeGreaterThanOrEqualTo(2);
+        fake.MaxObservedConcurrency.ShouldBeLessThanOrEqualTo(3);
+
+        await host.DisposeAsync();
+    }
+
+    /// <summary>
+    ///     One node's backend throw does not poison a parallel batch: that node surfaces
+    ///     <c>BadHistoryOperationUnsupported</c> while the other seven return Good (per-node isolation holds
+    ///     under parallelism — 03/S3).
+    /// </summary>
+    [Fact]
+    public async Task One_node_failure_does_not_poison_a_parallel_batch()
+    {
+        var (host, server) = await BootAsync();
+        var nm = server.NodeManager!;
+
+        var src = DateTime.UtcNow.AddSeconds(-5);
+        var srv = DateTime.UtcNow;
+        // Node index 3's tagname throws; all others return a single good sample.
+        var fake = new PerTagnameFake(tagname => tagname == "WW.Tag3"
+            ? throw new InvalidOperationException("backend boom for Tag3")
+            : new HistorianRead(new[] { new DataValueSnapshot(1.0f, StatusCodes.Good, src, srv) }, null));
+        nm.HistorianDataSource = fake;
+
+        var nodeIds = MaterializeHistorizedNodes(nm, count: 8);
+        var details = new ReadRawModifiedDetails
+        {
+            StartTime = DateTime.UtcNow.AddHours(-1),
+            EndTime = DateTime.UtcNow,
+            NumValuesPerNode = 10,
+            IsReadModified = false,
+        };
+
+        var (results, errors) = await Task.Run(() => InvokeHistoryRead(server, nm, details, nodeIds), Ct)
+            .WaitAsync(TimeSpan.FromSeconds(15), Ct);
+
+        for (var i = 0; i < 8; i++)
+        {
+            if (i == 3)
+            {
+                StatusCode.IsBad(errors[i].StatusCode).ShouldBeTrue();
+                errors[i].StatusCode.Code.ShouldBe(StatusCodes.BadHistoryOperationUnsupported);
+            }
+            else
+            {
+                errors[i].StatusCode.Code.ShouldBe(StatusCodes.Good);
+                results[i].StatusCode.Code.ShouldBe(StatusCodes.Good);
+            }
+        }
+
+        await host.DisposeAsync();
+    }
+
+    /// <summary>
+    ///     A per-request deadline bounds a hung backend: reads that only complete on token cancellation
+    ///     surface <c>BadTimeout</c> for every node and the call returns within the deadline (plus slack) —
+    ///     it does not hang an SDK request thread indefinitely (03/S3).
+    /// </summary>
+    [Fact]
+    public async Task Deadline_bounds_a_hung_backend()
+    {
+        var (host, server) = await BootAsync();
+        var nm = server.NodeManager!;
+        nm.HistoryReadDeadline = TimeSpan.FromMilliseconds(500);
+        nm.HistorianDataSource = new HangingHistorianDataSource();
+
+        var nodeIds = MaterializeHistorizedNodes(nm, count: 4);
+        var details = new ReadRawModifiedDetails
+        {
+            StartTime = DateTime.UtcNow.AddHours(-1),
+            EndTime = DateTime.UtcNow,
+            NumValuesPerNode = 10,
+            IsReadModified = false,
+        };
+
+        var (_, errors) = await Task.Run(() => InvokeHistoryRead(server, nm, details, nodeIds), Ct)
+            .WaitAsync(TimeSpan.FromSeconds(10), Ct);
+
+        for (var i = 0; i < 4; i++)
+            errors[i].StatusCode.Code.ShouldBe(StatusCodes.BadTimeout);
+
+        await host.DisposeAsync();
+    }
+
+    /// <summary>
+    ///     A saturated process-wide batch limiter degrades a second concurrent HistoryRead to
+    ///     <c>BadTooManyOperations</c> (rather than exhausting the SDK request-thread pool); once the parked
+    ///     batch completes the slot frees and a later read succeeds (03/S3).
+    /// </summary>
+    [Fact]
+    public async Task Saturated_limiter_rejects_with_BadTooManyOperations()
+    {
+        var (host, server) = await BootAsync();
+        var nm = server.NodeManager!;
+        nm.MaxConcurrentHistoryReadBatches = 1;
+        nm.HistoryReadDeadline = TimeSpan.FromMilliseconds(200);
+        var fake = new GatedRawFake();
+        nm.HistorianDataSource = fake;
+
+        var ids = MaterializeHistorizedNodes(nm, count: 2);
+        var details = new ReadRawModifiedDetails
+        {
+            StartTime = DateTime.UtcNow.AddHours(-1),
+            EndTime = DateTime.UtcNow,
+            NumValuesPerNode = 10,
+            IsReadModified = false,
+        };
+
+        // First batch parks in the gated fake, holding the single limiter permit.
+        var first = Task.Run(() => InvokeHistoryRead(server, nm, details, ids[0]), Ct);
+
+        var spin = Stopwatch.StartNew();
+        while (fake.Entered == 0 && spin.Elapsed < TimeSpan.FromSeconds(5))
+            await Task.Delay(10, Ct);
+        fake.Entered.ShouldBe(1);
+
+        // Second concurrent batch: the limiter is saturated → all handles BadTooManyOperations.
+        var (_, errors2) = await Task.Run(() => InvokeHistoryRead(server, nm, details, ids[1]), Ct)
+            .WaitAsync(TimeSpan.FromSeconds(5), Ct);
+        errors2[0].StatusCode.Code.ShouldBe(StatusCodes.BadTooManyOperations);
+        fake.Entered.ShouldBe(1, "the rejected batch must never reach the backend");
+
+        // Release the parked batch; the slot frees and a later read succeeds.
+        fake.Release();
+        await first.WaitAsync(TimeSpan.FromSeconds(10), Ct);
+
+        var (_, errors3) = await Task.Run(() => InvokeHistoryRead(server, nm, details, ids[1]), Ct)
+            .WaitAsync(TimeSpan.FromSeconds(10), Ct);
+        errors3[0].StatusCode.Code.ShouldBe(StatusCodes.Good);
+
+        await host.DisposeAsync();
+    }
+
+    /// <summary>Materialises <paramref name="count"/> historized Float variables and returns their NodeIds.</summary>
+    private static NodeId[] MaterializeHistorizedNodes(OtOpcUaNodeManager nm, int count)
+    {
+        var ids = new NodeId[count];
+        for (var i = 0; i < count; i++)
+        {
+            var key = $"eq/tag{i}";
+            nm.EnsureVariable(key, parentFolderNodeId: null, displayName: $"Tag{i}", dataType: "Float",
+                writable: false, historianTagname: $"WW.Tag{i}");
+            ids[i] = nm.TryGetVariable(key)!.NodeId;
+        }
+
+        return ids;
+    }
+
+    /// <summary>Issues one HistoryRead batch for all supplied node ids in order.</summary>
+    private static (IList<SdkHistoryReadResult> Results, IList<ServiceResult> Errors) InvokeHistoryRead(
+        OtOpcUaSdkServer server, OtOpcUaNodeManager nm, HistoryReadDetails details, params NodeId[] nodeIds)
+    {
+        var context = new OperationContext(
+            new RequestHeader(), secureChannelContext: null, RequestType.HistoryRead, identity: null);
+
+        var nodesToRead = nodeIds.Select(id => new HistoryReadValueId { NodeId = id }).ToList();
+        var results = Enumerable.Repeat<SdkHistoryReadResult>(null!, nodeIds.Length).ToList();
+        var errors = Enumerable.Repeat<ServiceResult>(null!, nodeIds.Length).ToList();
+
+        nm.HistoryRead(
+            context,
+            details,
+            TimestampsToReturn.Both,
+            releaseContinuationPoints: false,
+            nodesToRead,
+            results,
+            errors);
+
+        return (results, errors);
+    }
+
+    /// <summary>
+    ///     A delaying fake historian source that tracks the maximum number of reads in flight at once
+    ///     (via interlocked live-counter) so a test can prove per-node parallelism deterministically.
+    /// </summary>
+    private sealed class ConcurrencyTrackingHistorianDataSource(TimeSpan delay) : IHistorianDataSource
+    {
+        private int _current;
+        private int _max;
+
+        /// <summary>The highest number of concurrently in-flight reads observed.</summary>
+        public int MaxObservedConcurrency => Volatile.Read(ref _max);
+
+        private async Task<HistorianRead> TrackAsync(CancellationToken ct)
+        {
+            var now = Interlocked.Increment(ref _current);
+            UpdateMax(now);
+            try
+            {
+                await Task.Delay(delay, ct).ConfigureAwait(false);
+            }
+            finally
+            {
+                Interlocked.Decrement(ref _current);
+            }
+
+            return new HistorianRead(
+                new[] { new DataValueSnapshot(1.0f, StatusCodes.Good, DateTime.UtcNow, DateTime.UtcNow) }, null);
+        }
+
+        private void UpdateMax(int observed)
+        {
+            int cur;
+            while ((cur = Volatile.Read(ref _max)) < observed &&
+                   Interlocked.CompareExchange(ref _max, observed, cur) != cur)
+            {
+            }
+        }
+
+        public Task<HistorianRead> ReadRawAsync(
+            string fullReference, DateTime startUtc, DateTime endUtc, uint maxValuesPerNode,
+            CancellationToken cancellationToken) => TrackAsync(cancellationToken);
+
+        public Task<HistorianRead> ReadProcessedAsync(
+            string fullReference, DateTime startUtc, DateTime endUtc, TimeSpan interval,
+            HistoryAggregateType aggregate, CancellationToken cancellationToken) => TrackAsync(cancellationToken);
+
+        public Task<HistorianRead> ReadAtTimeAsync(
+            string fullReference, IReadOnlyList<DateTime> timestampsUtc, CancellationToken cancellationToken) =>
+            TrackAsync(cancellationToken);
+
+        public Task<HistoricalEventsResult> ReadEventsAsync(
+            string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents,
+            CancellationToken cancellationToken) =>
+            Task.FromResult(new HistoricalEventsResult(Array.Empty<HistoricalEvent>(), null));
+
+        public HistorianHealthSnapshot GetHealthSnapshot() => NullHistorianDataSource.Instance.GetHealthSnapshot();
+
+        public void Dispose()
+        {
+        }
+    }
+
+    /// <summary>A fake whose reads complete ONLY on token cancellation — used to prove the per-request
+    /// deadline surfaces <c>BadTimeout</c> rather than hanging.</summary>
+    private sealed class HangingHistorianDataSource : IHistorianDataSource
+    {
+        private static async Task<HistorianRead> HangAsync(CancellationToken ct)
+        {
+            await Task.Delay(Timeout.Infinite, ct).ConfigureAwait(false);
+            return new HistorianRead(Array.Empty<DataValueSnapshot>(), null);
+        }
+
+        public Task<HistorianRead> ReadRawAsync(
+            string fullReference, DateTime startUtc, DateTime endUtc, uint maxValuesPerNode,
+            CancellationToken cancellationToken) => HangAsync(cancellationToken);
+
+        public Task<HistorianRead> ReadProcessedAsync(
+            string fullReference, DateTime startUtc, DateTime endUtc, TimeSpan interval,
+            HistoryAggregateType aggregate, CancellationToken cancellationToken) => HangAsync(cancellationToken);
+
+        public Task<HistorianRead> ReadAtTimeAsync(
+            string fullReference, IReadOnlyList<DateTime> timestampsUtc, CancellationToken cancellationToken) =>
+            HangAsync(cancellationToken);
+
+        public async Task<HistoricalEventsResult> ReadEventsAsync(
+            string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents,
+            CancellationToken cancellationToken)
+        {
+            await Task.Delay(Timeout.Infinite, cancellationToken).ConfigureAwait(false);
+            return new HistoricalEventsResult(Array.Empty<HistoricalEvent>(), null);
+        }
+
+        public HistorianHealthSnapshot GetHealthSnapshot() => NullHistorianDataSource.Instance.GetHealthSnapshot();
+
+        public void Dispose() { }
+    }
+
+    /// <summary>A fake whose raw read parks (awaits a <see cref="TaskCompletionSource"/>) until the test
+    /// releases it — used to hold the process-wide batch limiter permit while a second batch is issued.</summary>
+    private sealed class GatedRawFake : IHistorianDataSource
+    {
+        private readonly TaskCompletionSource _gate = new(TaskCreationOptions.RunContinuationsAsynchronously);
+        private int _entered;
+
+        /// <summary>Number of raw reads that reached the backend body.</summary>
+        public int Entered => Volatile.Read(ref _entered);
+
+        /// <summary>Releases the parked read so its awaiting batch can complete.</summary>
+        public void Release() => _gate.TrySetResult();
+
+        public async Task<HistorianRead> ReadRawAsync(
+            string fullReference, DateTime startUtc, DateTime endUtc, uint maxValuesPerNode,
+            CancellationToken cancellationToken)
+        {
+            Interlocked.Increment(ref _entered);
+            // Deliberately ignore the per-request deadline token: the first batch must keep HOLDING the
+            // single limiter permit until the test releases it, so the second batch deterministically hits
+            // a saturated limiter (rather than the first batch's own deadline freeing the slot early).
+            await _gate.Task.ConfigureAwait(false);
+            return new HistorianRead(
+                new[] { new DataValueSnapshot(1.0f, StatusCodes.Good, DateTime.UtcNow, DateTime.UtcNow) }, null);
+        }
+
+        public Task<HistorianRead> ReadProcessedAsync(
+            string fullReference, DateTime startUtc, DateTime endUtc, TimeSpan interval,
+            HistoryAggregateType aggregate, CancellationToken cancellationToken) =>
+            NullHistorianDataSource.Instance.ReadProcessedAsync(fullReference, startUtc, endUtc, interval, aggregate, cancellationToken);
+
+        public Task<HistorianRead> ReadAtTimeAsync(
+            string fullReference, IReadOnlyList<DateTime> timestampsUtc, CancellationToken cancellationToken) =>
+            NullHistorianDataSource.Instance.ReadAtTimeAsync(fullReference, timestampsUtc, cancellationToken);
+
+        public Task<HistoricalEventsResult> ReadEventsAsync(
+            string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents,
+            CancellationToken cancellationToken) =>
+            NullHistorianDataSource.Instance.ReadEventsAsync(sourceName, startUtc, endUtc, maxEvents, cancellationToken);
+
+        public HistorianHealthSnapshot GetHealthSnapshot() => NullHistorianDataSource.Instance.GetHealthSnapshot();
+
+        public void Dispose() { }
+    }
+
+    /// <summary>A per-tagname fake that delegates each raw read to a caller-supplied func (which may throw
+    /// to simulate a per-node backend failure). Processed/AtTime/Events delegate to the null source.</summary>
+    private sealed class PerTagnameFake(Func<string, HistorianRead> rawHandler) : IHistorianDataSource
+    {
+        public Task<HistorianRead> ReadRawAsync(
+            string fullReference, DateTime startUtc, DateTime endUtc, uint maxValuesPerNode,
+            CancellationToken cancellationToken) => Task.FromResult(rawHandler(fullReference));
+
+        public Task<HistorianRead> ReadProcessedAsync(
+            string fullReference, DateTime startUtc, DateTime endUtc, TimeSpan interval,
+            HistoryAggregateType aggregate, CancellationToken cancellationToken) =>
+            NullHistorianDataSource.Instance.ReadProcessedAsync(fullReference, startUtc, endUtc, interval, aggregate, cancellationToken);
+
+        public Task<HistorianRead> ReadAtTimeAsync(
+            string fullReference, IReadOnlyList<DateTime> timestampsUtc, CancellationToken cancellationToken) =>
+            NullHistorianDataSource.Instance.ReadAtTimeAsync(fullReference, timestampsUtc, cancellationToken);
+
+        public Task<HistoricalEventsResult> ReadEventsAsync(
+            string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents,
+            CancellationToken cancellationToken) =>
+            NullHistorianDataSource.Instance.ReadEventsAsync(sourceName, startUtc, endUtc, maxEvents, cancellationToken);
+
+        public HistorianHealthSnapshot GetHealthSnapshot() => NullHistorianDataSource.Instance.GetHealthSnapshot();
+
+        public void Dispose() { }
+    }
+
+    private async Task<(OpcUaApplicationHost Host, OtOpcUaSdkServer Server)> BootAsync()
+    {
+        var host = new OpcUaApplicationHost(
+            new OpcUaApplicationHostOptions
+            {
+                ApplicationName = "OtOpcUa.HistoryReadConcTest",
+                ApplicationUri = $"urn:OtOpcUa.HistoryReadConcTest:{Guid.NewGuid():N}",
+                OpcUaPort = AllocateFreePort(),
+                PublicHostname = "localhost",
+                PkiStoreRoot = _pkiRoot,
+            },
+            NullLogger<OpcUaApplicationHost>.Instance);
+
+        var server = new OtOpcUaSdkServer();
+        await host.StartAsync(server, Ct);
+        return (host, server);
+    }
+
+    private static int AllocateFreePort()
+    {
+        using var listener = new System.Net.Sockets.TcpListener(System.Net.IPAddress.Loopback, 0);
+        listener.Start();
+        var port = ((System.Net.IPEndPoint)listener.LocalEndpoint).Port;
+        listener.Stop();
+        return port;
+    }
+
+    /// <summary>Cleans up the PKI root directory.</summary>
+    public void Dispose()
+    {
+        if (Directory.Exists(_pkiRoot))
+        {
+            try { Directory.Delete(_pkiRoot, recursive: true); }
+            catch { /* best-effort cleanup */ }
+        }
+    }
+}

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Historian/ServerHistorianOptionsTests.cs

@@ -50,4 +50,55 @@ public sealed class ServerHistorianOptionsTests
             .Validate();
         Assert.Contains(w, m => m.Contains("MaxTieClusterOverfetch"));
     }
+
+    // ── S3 HistoryRead concurrency knobs (arch-review 03/S3) ────────────────────────────────────
+
+    [Fact]
+    public void HistoryRead_knob_defaults()
+    {
+        var o = new ServerHistorianOptions();
+        Assert.Equal(4, o.HistoryReadBatchParallelism);
+        Assert.Equal(16, o.MaxConcurrentHistoryReadBatches);
+        Assert.Equal(TimeSpan.FromSeconds(60), o.HistoryReadDeadline);
+    }
+
+    [Fact]
+    public void Enabled_with_nonpositive_HistoryReadBatchParallelism_warns()
+    {
+        var w = new ServerHistorianOptions
+        { Enabled = true, Endpoint = "https://h:5222", ApiKey = "histgw_x_y", HistoryReadBatchParallelism = 0 }
+            .Validate();
+        Assert.Contains(w, m => m.Contains("HistoryReadBatchParallelism"));
+    }
+
+    [Fact]
+    public void Enabled_with_nonpositive_MaxConcurrentHistoryReadBatches_warns()
+    {
+        var w = new ServerHistorianOptions
+        { Enabled = true, Endpoint = "https://h:5222", ApiKey = "histgw_x_y", MaxConcurrentHistoryReadBatches = 0 }
+            .Validate();
+        Assert.Contains(w, m => m.Contains("MaxConcurrentHistoryReadBatches"));
+    }
+
+    [Fact]
+    public void Enabled_with_nonpositive_HistoryReadDeadline_warns()
+    {
+        var w = new ServerHistorianOptions
+        { Enabled = true, Endpoint = "https://h:5222", ApiKey = "histgw_x_y", HistoryReadDeadline = TimeSpan.Zero }
+            .Validate();
+        Assert.Contains(w, m => m.Contains("HistoryReadDeadline"));
+    }
+
+    [Fact]
+    public void Disabled_with_nonpositive_HistoryRead_knobs_is_silent()
+    {
+        var w = new ServerHistorianOptions
+        {
+            Enabled = false,
+            HistoryReadBatchParallelism = 0,
+            MaxConcurrentHistoryReadBatches = 0,
+            HistoryReadDeadline = TimeSpan.Zero,
+        }.Validate();
+        Assert.Empty(w);
+    }
 }

tests/Server/ZB.MOM.WW.OtOpcUa.Security.Tests/OtOpcUaLdapAuthServiceTests.cs

@@ -121,6 +121,56 @@ public sealed class OtOpcUaLdapAuthServiceTests
         inner.Called.ShouldBeFalse();
     }
 
+    /// <summary>A system-side library failure (<c>ServiceAccountBindFailed</c> — the directory-unreachable
+    /// bucket) adapts to <c>IsSystemFailure == true</c> so the OPC UA authenticator's outage circuit can
+    /// distinguish it from a user-credential deny (03/S2).</summary>
+    [Fact]
+    public async Task ServiceAccountBindFailed_adapts_to_IsSystemFailure_true()
+    {
+        var inner = new RecordingLibService(LibLdapAuthResult.Fail(LdapAuthFailure.ServiceAccountBindFailed));
+        var sut = Build(
+            new LdapOptions { Enabled = true, DevStubMode = false, Transport = LdapTransport.Ldaps },
+            inner);
+
+        var result = await sut.AuthenticateAsync("alice", "secret", CancellationToken.None);
+
+        result.Success.ShouldBeFalse();
+        result.IsSystemFailure.ShouldBeTrue();
+    }
+
+    /// <summary>User-side library failures and success adapt to <c>IsSystemFailure == false</c> — they must
+    /// NOT trip the outage circuit (03/S2).</summary>
+    [Theory]
+    [InlineData(LdapAuthFailure.BadCredentials)]
+    [InlineData(LdapAuthFailure.UserNotFound)]
+    public async Task User_side_failures_adapt_to_IsSystemFailure_false(LdapAuthFailure failure)
+    {
+        var inner = new RecordingLibService(LibLdapAuthResult.Fail(failure));
+        var sut = Build(
+            new LdapOptions { Enabled = true, DevStubMode = false, Transport = LdapTransport.Ldaps },
+            inner);
+
+        var result = await sut.AuthenticateAsync("alice", "wrong", CancellationToken.None);
+
+        result.Success.ShouldBeFalse();
+        result.IsSystemFailure.ShouldBeFalse();
+    }
+
+    /// <summary>A successful bind is never a system failure.</summary>
+    [Fact]
+    public async Task Success_is_not_a_system_failure()
+    {
+        var inner = new RecordingLibService(LibLdapAuthResult.Success("alice", "Alice", new[] { "ReadOnly" }));
+        var sut = Build(
+            new LdapOptions { Enabled = true, DevStubMode = false, Transport = LdapTransport.Ldaps },
+            inner);
+
+        var result = await sut.AuthenticateAsync("alice", "secret", CancellationToken.None);
+
+        result.Success.ShouldBeTrue();
+        result.IsSystemFailure.ShouldBeFalse();
+    }
+
     /// <summary>Records whether the library service was invoked and returns a canned result.</summary>
     private sealed class RecordingLibService(LibLdapAuthResult result) : LibILdapAuthService
     {