docs(grpc): transport matrix + correct the obsolete v12 version-gate note

Document the WCF-vs-gRPC surface and fix a stale claim.

- README: add a "Transport matrix (WCF vs gRPC)" section with a per-operation
  table. Mark the config ops the gRPC server exposes-but-untooled with a distinct
  legend state (recovered RPC + bytes buffer named) vs genuinely unavailable, so
  "not tooled" is not conflated with "not possible".
- README: document the gRPC live-test env vars (HISTORIAN_GRPC_HOST/_PORT/_TLS/
  _DNSID/_TIMEOUT/_WRITE_SANDBOX_TAG) and refresh the Status section (test count
  + the live-verified gRPC surface).
- The "gRPC requires VerifyServerInterfaceVersion=false against a v12 server" note
  was obsolete: the gate already accepts History 11 AND 12 (AcceptedVersions), and
  the live gRPC suite runs with the default verification on. Corrected in the
  README, CLAUDE.md, and the HistorianServerVersionGate docstring.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B6mcaT2PjRFKcogzp9UkfC

CLAUDE.md

@@ -72,7 +72,7 @@ Three layered subsystems, intentionally decoupled so protocol parsing can be uni
 - **`Wcf/`** — managed WCF/MDAS layer. The Historian uses Net.TCP on port `32568` with a custom `application/x-mdas` content type wrapping a binary SOAP 1.2 / WS-Addressing 1.0 envelope. `MdasMessageEncoder` + `MdasMessageEncodingBindingElement` implement that wrapper. `HistorianWcfBindingFactory` produces three flavors: plain MDAS, MDAS+Windows transport (used for `/Hist-Integrated`), and MDAS+certificate (used for `/HistCert`). Service paths live in `HistorianWcfServiceNames`. WCF data contracts (`Wcf/Contracts/`) are reproduced from server-side static analysis and are versioned per native interface (e.g., `IRetrievalServiceContract2..4`).
 - **`Protocol/`** — binary frame layer (`HistorianFrameReader`/`Writer`, `HistorianBinaryPrimitives`, `HistorianMessageType`). `Historian2020ProtocolDialect` is the version-anchored bridge between `HistorianClient` and the frame layer; methods without sufficient evidence throw `ProtocolEvidenceMissingException` rather than guessing wire bytes.
 - **`Transport/`** — pluggable `IHistorianTransport` (default: TCP). Tests inject a fake transport.
-- **`Grpc/`** — 2023 R2 gRPC transport (`HistorianTransport.RemoteGrpc`). The recovered protobuf contract lives in `Grpc/Protos/*.proto` and is compiled to client stubs at build time by `Grpc.Tools`. `HistorianGrpcChannelFactory` builds a gRPC-Web/HTTP-1.1 channel (default port `32565`, optional TLS, gzip) matching the stock 2023 R2 client. `HistorianGrpcReadOrchestrator` mirrors `HistorianWcfReadOrchestrator` but over gRPC: it reuses the exact native serializers/parsers — the same Open2 buffer, SSPI/NTLM tokens, and `DataQueryRequest`/result buffers travel inside protobuf `bytes` fields. The 2020→gRPC op map: `Hist.ValCl`→`StorageService.ValidateClientCredential` (the SSPI/Negotiate token loop), `Hist.Open2`→`HistoryService.OpenConnection`, `Retr.StartQuery2`→`RetrievalService.StartQuery`, `Retr.GetNextQueryResultBuffer2`→`RetrievalService.GetNextQueryResultBuffer`. The transport-agnostic handshake (Open2 request builder + SSPI token loop + response decode) is shared via `Wcf/HistorianNativeHandshake`. **Live-verified 2026-06-21 against a real 2023 R2 server** (interface versions History=12, Retrieval=4, Storage=4): the full read chain returns rows. NOTE: `HistoryService.ExchangeKey` is a SEPARATE key-exchange/cert-path op, NOT the Negotiate loop — an earlier revision wrongly routed the token loop there and it was rejected at round 0 regardless of credentials; the loop belongs on `StorageService.ValidateClientCredential` (which kept the 2020 inBuff/outBuff token framing). The byte payloads are the proven 2020 protocol and transfer unchanged; only the History interface integer differs (12 vs 11) and is buffer-compatible, so `VerifyServerInterfaceVersion=false` is currently required against a v12 server (the gate still pins History=11). Gated live test: set `HISTORIAN_GRPC_HOST` (+ `HISTORIAN_TEST_TAG`, optional `HISTORIAN_GRPC_PORT`/`HISTORIAN_GRPC_TLS`/`HISTORIAN_GRPC_DNSID`); reach the live 2023 R2 box via [[reference_2023r2_live_server_access]].
+- **`Grpc/`** — 2023 R2 gRPC transport (`HistorianTransport.RemoteGrpc`). The recovered protobuf contract lives in `Grpc/Protos/*.proto` and is compiled to client stubs at build time by `Grpc.Tools`. `HistorianGrpcChannelFactory` builds a gRPC-Web/HTTP-1.1 channel (default port `32565`, optional TLS, gzip) matching the stock 2023 R2 client. `HistorianGrpcReadOrchestrator` mirrors `HistorianWcfReadOrchestrator` but over gRPC: it reuses the exact native serializers/parsers — the same Open2 buffer, SSPI/NTLM tokens, and `DataQueryRequest`/result buffers travel inside protobuf `bytes` fields. The 2020→gRPC op map: `Hist.ValCl`→`StorageService.ValidateClientCredential` (the SSPI/Negotiate token loop), `Hist.Open2`→`HistoryService.OpenConnection`, `Retr.StartQuery2`→`RetrievalService.StartQuery`, `Retr.GetNextQueryResultBuffer2`→`RetrievalService.GetNextQueryResultBuffer`. The transport-agnostic handshake (Open2 request builder + SSPI token loop + response decode) is shared via `Wcf/HistorianNativeHandshake`. **Live-verified 2026-06-21 against a real 2023 R2 server** (interface versions History=12, Retrieval=4, Storage=4): the full read chain returns rows. NOTE: `HistoryService.ExchangeKey` is a SEPARATE key-exchange/cert-path op, NOT the Negotiate loop — an earlier revision wrongly routed the token loop there and it was rejected at round 0 regardless of credentials; the loop belongs on `StorageService.ValidateClientCredential` (which kept the 2020 inBuff/outBuff token framing). The byte payloads are the proven 2020 protocol and transfer unchanged; only the History interface integer differs (12 vs 11) and is buffer-compatible. The version gate now accepts BOTH 11 and 12 for History (`HistorianServerVersionGate.AcceptedVersions`), so a v12 server passes with the default `VerifyServerInterfaceVersion=true` — no opt-out needed (the earlier requirement to set it false is obsolete). Gated live test: set `HISTORIAN_GRPC_HOST` (+ `HISTORIAN_TEST_TAG`, optional `HISTORIAN_GRPC_PORT`/`HISTORIAN_GRPC_TLS`/`HISTORIAN_GRPC_DNSID`); reach the live 2023 R2 box via [[reference_2023r2_live_server_access]].
 - **`Models/`** — public DTOs and enums (`HistorianSample`, `RetrievalMode`, etc.). `HistorianDataValue` represents the discriminated value type.
 
 `InternalsVisibleTo` exposes internals to the test assembly and the reverse-engineering tool.

README.md

@@ -56,6 +56,126 @@ auth) and `RemoteTcpCertificate` (server-cert TLS) are now live-verified for
 `ProbeAsync`; `RemoteTcpIntegrated` is additionally live-verified for the full
 read / browse / metadata / event / status surface.
 
+## Transport matrix (WCF vs gRPC)
+
+The SDK speaks two wire protocols. **WCF** is the 2020 binary MDAS protocol over
+Net.TCP `32568` (transports `LocalPipe`, `RemoteTcpIntegrated`,
+`RemoteTcpCertificate`); **gRPC** is the 2023 R2 HCAL transport over HTTP/2
+`32565` (transport `RemoteGrpc`). The core read chain reuses the *same* native
+Open2 buffers and SSPI tokens on both — on gRPC they simply ride inside protobuf
+`bytes` fields — so reads are at parity. The surfaces diverge at the edges.
+
+Legend: ✅ tooled + live-verified · ⚠️ tooled, partial/synthesized ·
+🔌 **the gRPC server exposes the RPC (recovered in `Grpc/Protos/*.proto`) but the
+SDK doesn't drive it yet** — untooled/uncaptured, *not* a protocol gap ·
+❌ unavailable on that transport.
+
+| Operation | WCF | gRPC | Notes |
+|---|:---:|:---:|---|
+| `ProbeAsync` | ✅ | ✅ | |
+| `ReadRawAsync` | ✅ | ✅ | |
+| `ReadAggregateAsync` | ✅ | ✅ | all 16 retrieval modes |
+| `ReadAtTimeAsync` | ✅ | ✅ | |
+| `BrowseTagNamesAsync` | ✅ | ✅ | |
+| `GetTagMetadataAsync` | ✅ | ✅ | |
+| `GetSystemParameterAsync` | ✅ | ✅ | |
+| `AddHistoricalValuesAsync` | ❌ | ✅ | historical/backfill writes ride `HistoryService.AddStreamValues`; non-gRPC throws `ProtocolEvidenceMissingException` |
+| `GetServerTimeZoneAsync` | ❌ | ✅ | 2020 `GetSystemTimeZoneName` is a client-side stub (empty); WCF throws |
+| `GetStoreForwardStatusAsync` | ⚠️ | ✅ | gRPC contacts the server (measured idle-state, reports `ErrorOccurred`); WCF returns synthesized all-false. Active-SF magnitude is D2-gated on both |
+| `ReadEventsAsync` | ✅ | 🔌 | gRPC `RetrievalService.StartEventQuery` / `GetNextEventQueryResultBuffer` / `EndEventQuery` recovered (`bytes btRequest` + handle); not tooled over gRPC |
+| `SendEventAsync` | ✅ | 🔌 | rides `AddStreamValues` family; no distinct event-send RPC, framing uncaptured over gRPC |
+| `EnsureTagAsync` / `DeleteTagAsync` / `RenameTagsAsync` | ✅ | 🔌 | gRPC `HistoryService.EnsureTags` / `DeleteTags` / `StartJob`(+`GetJobStatus`) recovered (`bytes btTagInfos`/`btTagnames`/`btInput` + handle) |
+| `GetTagExtendedPropertiesAsync` / `AddTagExtendedPropertiesAsync` | ✅ | 🔌 | gRPC `RetrievalService.GetTagExtendedPropertiesFromName` + `HistoryService.AddTagExtendedProperties`; gRPC also exposes `DeleteTagExtendedProperties` (WCF delete was server-blocked) |
+| `ExecuteSqlCommandAsync` | ✅ | 🔌 | gRPC `RetrievalService.ExecuteSqlCommand` (`StrCommand` + `uiOption`, mirrors WCF `ExeC`/`GetR`) |
+| `GetRuntimeParameterAsync` | ✅ | 🔌 | gRPC `StatusService.GetRuntimeParameter` (`bytes btRequest` + handle) |
+| `GetConnectionStatusAsync` | ✅ | ❌ | synthesized from an authenticated probe — no dedicated RPC on either transport (gRPC `PingServer`/`GetHistorianConsoleStatus` could synthesize it) |
+| `ReadBlocksAsync` | ❌ | ❌ | `StartBlockRetrievalQuery` never captured on either transport — throws `ProtocolEvidenceMissingException` |
+
+In short: **WCF is the broad, mature surface** (every config write, events, SQL,
+and all reads), while **gRPC is the narrower *tooled* surface** — but the 2023 R2
+gRPC *contract* is actually a **superset** of WCF. Every 🔌 row above has a
+recovered RPC carrying the **same opaque `bytes` buffers the existing WCF
+serializers already emit**, keyed by the same `strHandle`/`uiHandle` session
+handle the gRPC read path already obtains. So these are **capture-and-wire** items
+(route the existing serializer into a gRPC orchestrator + golden-capture the
+framing), **not** protocol-discovery items. We have only *buffer-verified* two
+gRPC families live — the read chain and `AddStreamValues` — so per the
+"capture first, never guess wire bytes" rule the 🔌 rows stay untooled until each
+is captured. The natural production pattern today remains WCF for config/reads and
+`RemoteGrpc` reserved for `AddHistoricalValuesAsync`.
+
+> A 2023 R2 server reports History interface version 12 (vs. 11 on 2020). The
+> connect-time version gate accepts both — they are byte-compatible — so gRPC
+> works against a v12 server with the default `VerifyServerInterfaceVersion=true`;
+> no opt-out is required.
+
+## Resilience subsystems (M4)
+
+Two **pure client-side** subsystems layered on top of `HistorianClient`. They use
+only the public SDK surface — no extra reverse-engineering, no server-side
+protocol — and are fully unit-tested without a live server.
+
+### Store-and-forward (`AVEVA.Historian.Client.StoreForward`)
+
+A durable local outbox that buffers writes when the Historian is unreachable and
+replays them on reconnect. `HistorianStoreForwardWriter` wraps a `HistorianClient`
+(or any `IHistorianWriteSink`) and persists each enqueued write to an
+`IHistorianOutboxStore` — `FileHistorianOutboxStore` (crash-durable, atomic
+JSON-per-entry, FIFO by filename sequence, corrupt-file quarantine) or
+`InMemoryHistorianOutboxStore`. A background drain loop retries with FIFO
+head-of-line ordering, optional `MaxDeliveryAttempts` dead-lettering, and a
+`DropOldest`/`Reject` overflow policy.
+
+```csharp
+using AVEVA.Historian.Client.StoreForward;
+
+await using HistorianStoreForwardWriter writer = new(
+    client,
+    new FileHistorianOutboxStore(@"C:\ProgramData\histsdk\outbox"));
+await writer.StartAsync();                       // background drain on reconnect
+
+await writer.EnqueueHistoricalValuesAsync("MyTag",
+    [new HistorianHistoricalValue(DateTime.UtcNow, 42.0)]);   // returns immediately, durable
+
+HistorianStoreForwardStatusSnapshot status = await writer.GetStatusAsync();
+// status.PendingCount / .Storing / .ErrorOccurred — mirrors the server SF semantics
+```
+
+This is a pragmatic outbox, **not** the bit-faithful native SF cache
+(`Forward*Snapshot` decode), which stays deferred.
+
+### Multi-historian redundancy (`AVEVA.Historian.Client.Redundancy`)
+
+`HistorianRedundantClient` fronts N members as one logical client. Reads fail over
+to the next healthy member in priority order (streaming reads fail over only
+*before the first row* — mid-stream failures propagate to avoid duplicated/skipped
+rows); writes fan out (`AllMembers`/`PreferredOnly`) under an `All`/`Any`
+acknowledgement policy and return a per-member `HistorianRedundantWriteResult`. A
+failure-threshold demotion + background watchdog restores recovered members.
+
+```csharp
+using AVEVA.Historian.Client.Redundancy;
+
+await using HistorianRedundantClient cluster = new(
+[
+    new HistorianClientMember("primary",   primaryClient),
+    new HistorianClientMember("secondary", secondaryClient),
+]);
+await cluster.StartAsync();                       // health watchdog
+
+await foreach (HistorianSample s in cluster.ReadRawAsync("MyTag", startUtc, endUtc, 100))
+{
+    // served by the first healthy member; transparently fails over on connect
+}
+
+HistorianRedundantWriteResult w = await cluster.AddHistoricalValuesAsync("MyTag",
+    [new HistorianHistoricalValue(DateTime.UtcNow, 42.0)]);   // fanned out across members
+```
+
+Backing a member's writes with a `HistorianStoreForwardWriter` gives the pragmatic
+equivalent of native ReSyncTags: a down member buffers locally and replays on
+recovery.
+
 ## Build & test
 
 ```powershell
@@ -80,6 +200,28 @@ $env:HISTORIAN_USER, $env:HISTORIAN_PASSWORD
 $env:HISTORIAN_TAG_FILTER = 'Sys*'             # or any LIKE-pattern
 ```
 
+The 2023 R2 `RemoteGrpc` transport has its own gated live suite
+(`HistorianGrpcIntegrationTests`) covering the full tooled gRPC surface — probe,
+raw / aggregate (incl. multiple retrieval modes) / at-time reads, browse,
+metadata, system-parameter, server time-zone, and measured store-forward status.
+It skips cleanly unless `HISTORIAN_GRPC_HOST` is set:
+
+```powershell
+$env:HISTORIAN_GRPC_HOST  = 'my-2023r2-host'  # gates the gRPC suite
+$env:HISTORIAN_TEST_TAG   = 'SysTimeSec'
+$env:HISTORIAN_USER, $env:HISTORIAN_PASSWORD  # required for the authenticated ops
+# Optional:
+$env:HISTORIAN_GRPC_PORT     = '32565'        # default 32565
+$env:HISTORIAN_GRPC_TLS      = 'true'         # gRPC over TLS
+$env:HISTORIAN_GRPC_DNSID    = 'my-2023r2-host'  # cert DNS name when connecting by IP
+$env:HISTORIAN_GRPC_TIMEOUT  = '120'          # per-call deadline (s); raise for slow links
+$env:HISTORIAN_WRITE_SANDBOX_TAG = 'MyFloatTag'  # gates the AddHistoricalValues write test
+```
+
+The aggregate tests self-calibrate their query window from a real raw sample, so
+they pass against an idle 2023 R2 server (no recent collection) as well as a
+live-collecting one.
+
 ## Repository layout
 
 ```
@@ -165,10 +307,17 @@ property dictionary → Retr.EndEventQuery → Hist.Close2
 
 ## Status
 
-165 unit + live integration tests pass (`dotnet test --logger "console;verbosity=minimal"`).
+313 unit + live integration tests pass (`dotnet test --logger "console;verbosity=minimal"`).
 Full SDK surface — reads, browse, metadata, status, plus the two write ops
 (`EnsureTagAsync` / `DeleteTagAsync`) — verified end-to-end against both a
 local Historian (`LocalPipe`) and a remote Historian (`RemoteTcpIntegrated`
 over Net.TCP with Windows transport auth). `RemoteTcpCertificate` ProbeAsync
 is live-verified; deeper coverage over the cert transport plus the
 explicit-credentials path await additional verification.
+
+The 2023 R2 `RemoteGrpc` transport's full tooled surface is live-verified against
+a real 2023 R2 server: probe, raw / aggregate (TimeWeightedAverage +
+Minimum/MaximumWithTime + BestFit) / at-time reads, browse, metadata,
+system-parameter, server time-zone, and measured store-forward status — plus the
+gRPC-only `AddHistoricalValuesAsync` backfill write. The remaining gRPC config ops
+are exposed by the server but untooled (see the transport matrix above).

src/AVEVA.Historian.Client/HistorianServerVersionGate.cs

@@ -32,10 +32,11 @@ internal enum HistorianServiceInterface
 /// The Status (<c>Stat</c>) service's <c>GetInterfaceVersion</c> returns 0 (not a real
 /// version), so the Status interface is validated for reachability only, never value.
 ///
-/// A 2023 R2 gRPC server may report different integers even though it carries the same
-/// proven 2020 native buffers; until those integers are captured, point such a server at
-/// this gate with <see cref="HistorianClientOptions.VerifyServerInterfaceVersion"/> set to
-/// <see langword="false"/>.
+/// A 2023 R2 gRPC server reports History interface version 12 even though it carries the
+/// same proven 2020 native buffers. That value is captured and accepted (see
+/// <see cref="AcceptedVersions"/>), so a v12 server passes with the default
+/// <see cref="HistorianClientOptions.VerifyServerInterfaceVersion"/>=<see langword="true"/>;
+/// the opt-out is only a safety valve for some future, not-yet-captured interface integer.
 /// </summary>
 internal static class HistorianServerVersionGate
 {