# Phase 6.1 — Resilience & Observability Runtime

> **Status**: **SHIPPED** 2026-04-19 — Streams A/B/C/D + E data layer merged to `v2` across PRs #78-82. Final exit-gate PR #83 turns the compliance script into real checks (all pass) and records this status update.
>
> **Stream E.2/E.3 closed 2026-04-23** — `FleetStatusPoller` now polls `DriverInstanceResilienceStatus`, detects per-`(DriverInstanceId, HostName)` deltas, and pushes `ResilienceStatusChangedMessage` via `FleetStatusHub` on the fleet group. Admin `/hosts` page subscribes on load and upserts the matching `HostStatusRow` in-memory on receipt, so operator-visible resilience state now reflects the runtime within one poller tick (~5 s) instead of the Admin page's own 10-second refresh. `FleetStatusPollerTests.Poller_pushes_ResilienceStatusChanged_on_delta` covers the first-observation push, the no-delta-no-push invariant, and the mutated-row re-push.
>
> Baseline: 906 solution tests → post-Phase-6.1: 1042 passing (+136 net). One pre-existing Client.CLI Subscribe flake unchanged.
>
> **Branch**: `v2/phase-6-1-resilience-observability`
> **Estimated duration**: 3 weeks
> **Predecessor**: Phase 5 (drivers) — partial; S7 + OPC UA Client shipped, AB/TwinCAT/FOCAS paused
> **Successor**: Phase 6.2 (Authorization runtime)

## Phase Objective

Land the cross-cutting runtime protections + operability features that `plan.md` + `driver-stability.md` specify by decision but that no driver-phase actually wires. End-state: every driver goes through the same Polly resilience layer, health endpoints render the live driver fleet, structured logs carry per-request correlation IDs, and the config substrate survives a central DB outage via a LiteDB local cache.

Closes these gaps flagged in the 2026-04-19 audit:

1. Polly v8 resilience pipelines wired to every `IDriver` capability (no-op per-driver today; Galaxy has a hand-rolled `CircuitBreaker` only).
2. Tier A/B/C enforcement at runtime — `driver-stability.md` §2–4 and decisions #63–73 define memory watchdog, bounded queues, scheduled recycle, wedge detection; `MemoryWatchdog` exists only inside `Driver.Galaxy.Host`.
3. Health endpoints (`/healthz`, `/readyz`) on `OtOpcUa.Server`.
4. Structured Serilog with per-request correlation IDs (driver instance, OPC UA session, IPC call).
5. LiteDB local cache + Polly retry + fallback on central-DB outage (decision #36).

## Scope — What Changes

| Concern | Change |
|---------|--------|
| `Core` → new `Core.Resilience` sub-namespace | Shared Polly pipeline builder (`DriverResiliencePipelines`). **Pipeline key = `(DriverInstanceId, HostName)`** so one dead PLC behind a multi-device driver doesn't open the breaker for healthy siblings (decision #35 per-device isolation). **Per-capability policy** — Read / HistoryRead / Discover / Probe / Alarm get retries; **Write does NOT** unless `[WriteIdempotent]` on the tag definition (decisions #44-45). |
| Every capability-interface consumer in the server | Wrap `IReadable.ReadAsync`, `IWritable.WriteAsync`, `ITagDiscovery.DiscoverAsync`, `ISubscribable.SubscribeAsync/UnsubscribeAsync`, `IHostConnectivityProbe` probe loop, `IAlarmSource.SubscribeAlarmsAsync/AcknowledgeAsync`, `IHistoryProvider.ReadRawAsync/ReadProcessedAsync/ReadAtTimeAsync/ReadEventsAsync`. Composition: timeout → (retry when capability supports) → circuit breaker → bulkhead. |
| `Core.Abstractions` → new `WriteIdempotentAttribute` | Marker on `ModbusTagDefinition` / `S7TagDefinition` / `OpcUaClientDriver` tag rows; opts that tag into auto-retry on Write. Absence = no retry, per spec. |
| `Core` → new `Core.Stability` sub-namespace — **split** | Two separate subsystems: (a) **`MemoryTracking`** runs all tiers; captures baseline (median of first 5 min `GetMemoryFootprint` samples) + applies the hybrid rule `soft = max(multiplier × baseline, baseline + floor)`; soft breach logs + surfaces to Admin; never kills. (b) **`MemoryRecycle`** (Tier C only — requires out-of-process topology) handles hard-breach recycle via the Proxy-side supervisor. Tier A/B overrun escalates to Tier C promotion ticket, not auto-kill. |
| `ScheduledRecycleScheduler` | Tier C only per decisions #73-74. Weekly/time-of-day recycle via Proxy supervisor. Tier A/B opt-in recycle lands in a future phase together with a Tier-C-escalation workflow. |
| `WedgeDetector` | **Demand-aware**: flips a driver to Faulted only when `(hasPendingWork AND noProgressIn > threshold)`. `hasPendingWork` derives from non-zero Polly bulkhead depth OR ≥1 active MonitoredItem OR ≥1 queued historian read. Idle + subscription-only drivers stay Healthy. |
| `DriverTypeRegistry` | Each driver type registers its `DriverTier` {A, B, C}. Tier C drivers must advertise their out-of-process topology; the registry enforces invariants (Tier C has a `Proxy` + `Host` pair). |
| `Driver.Galaxy.Proxy/Supervisor/` | **Retains** existing `CircuitBreaker` + `Backoff` — they guard IPC respawn (decision #68), different concern from the per-call Polly layer. Only `HeartbeatMonitor` is referenced downstream (IPC liveness). |
| `OtOpcUa.Server` → Minimal API endpoints on `http://+:4841` | `/healthz` = process alive + (config DB reachable OR `UsingStaleConfig=true`). `/readyz` = ANDed driver health; state-machine per `DriverState`: `Unknown`/`Initializing` → 503, `Healthy` → 200, `Degraded` → 200 + `{degradedDrivers: [...]}` in body, `Faulted` → 503. JSON body always reports per-instance detail. |
| Serilog configuration | Centralize enrichers in `OtOpcUa.Server/Observability/LogContextEnricher.cs`. Every capability call runs inside a `LogContext.PushProperty` scope with {DriverInstanceId, DriverType, CapabilityName, CorrelationId (UA RequestHandle or internal GUID)}. Sink config stays rolling-file per CLAUDE.md; JSON sink added alongside plain-text (switchable via `Serilog:WriteJson` appsetting). |
| `Configuration` project | Add `LiteDbConfigCache` adapter. **Generation-sealed snapshots**: `sp_PublishGeneration` writes `<cache-root>/<cluster>/<generationId>.db` as a read-only sealed file. Reads serve the last-known-sealed generation; mixed-generation reads are impossible. Write path bypasses cache + fails hard on DB outage. Pipeline: timeout (2 s) → retry (3×, jittered) → fallback-to-sealed-snapshot. |
| `DriverHostStatus` vs. `DriverInstanceResilienceStatus` | New separate entity `DriverInstanceResilienceStatus { DriverInstanceId, HostName, LastCircuitBreakerOpenUtc, ConsecutiveFailures, CurrentBulkheadDepth, LastRecycleUtc, BaselineFootprintBytes }`. `DriverHostStatus` keeps per-host connectivity only; Admin `/hosts` joins both for display. |

## Scope — What Does NOT Change

| Item | Reason |
|------|--------|
| Driver wire protocols | Resilience is a server-side wrapper; individual drivers don't see Polly. Their existing retry logic (ModbusTcpTransport reconnect, SessionReconnectHandler) stays in place as inner layers. |
| Config DB schema | LiteDB cache is a read-only mirror; no new central tables except `DriverHostStatus` column additions. |
| OPC UA wire behavior visible to clients | Health endpoints live on a separate HTTP port (4841 by convention); the OPC UA server on 4840 is unaffected. |
| The four 2026-04-13 Galaxy stability findings | Already closed in Phase 2. Phase 6.1 *generalises* the pattern, doesn't re-fix Galaxy. |
| Driver-layer SafeHandle usage | Existing Galaxy `SafeMxAccessHandle` + Modbus `TcpClient` disposal stay — they're driver-internal, not part of the cross-cutting layer. |

## Entry Gate Checklist

- [ ] Phases 0–5 exit gates cleared (or explicitly deferred with task reference)
- [ ] `driver-stability.md` §2–4 re-read; decisions #63–73 + #34–36 re-skimmed
- [ ] Polly v8 NuGet available (`Microsoft.Extensions.Resilience` + `Polly.Core`) — verify package restore before task breakdown
- [ ] LiteDB 5.x NuGet confirmed MIT + actively maintained
- [ ] Existing drivers catalogued: Galaxy.Proxy, Modbus, S7, OpcUaClient — confirm test counts baseline so the resilience layer doesn't regress any
- [ ] Serilog configuration inventory: locate every `Log.ForContext` call site that will need `LogContext` rewrap
- [ ] Admin `/hosts` page's current `DriverHostStatus` consumption reviewed so the schema extensions don't break it

## Task Breakdown

### Stream A — Resilience layer (1 week)

1. **A.1** Add `Polly.Core` + `Microsoft.Extensions.Resilience` to `Core`. Build `DriverResiliencePipelineBuilder` — key on `(DriverInstanceId, HostName)`; composes Timeout → (Retry when the capability allows it; skipped for Write unless `[WriteIdempotent]`) → CircuitBreaker → Bulkhead. Per-capability policy map documented in `DriverResilienceOptions.CapabilityPolicies`.
2. **A.2** `DriverResilienceOptions` record bound from `DriverInstance.ResilienceConfig` JSON column (new nullable). **Per-tier × per-capability** defaults: Tier A (OpcUaClient, S7) Read 3 retries/2 s/5-failure-breaker, Write 0 retries/2 s/5-failure-breaker; Tier B (Modbus) Read 3/4 s/5, Write 0/4 s/5; Tier C (Galaxy) Read 1 retry/10 s/no-kill, Write 0/10 s/no-kill. Idempotent writes can opt into Read-shaped retry via the attribute.
3. **A.3** `CapabilityInvoker<TCapability, TResult>` wraps every method on the capability interfaces (`IReadable.ReadAsync`, `IWritable.WriteAsync`, `ITagDiscovery.DiscoverAsync`, `ISubscribable.SubscribeAsync/UnsubscribeAsync`, `IHostConnectivityProbe` probe loop, `IAlarmSource.SubscribeAlarmsAsync/AcknowledgeAsync`, `IHistoryProvider.ReadRawAsync/ReadProcessedAsync/ReadAtTimeAsync/ReadEventsAsync`). Existing server-side dispatch routes through it.
4. **A.4** **Retain** `Driver.Galaxy.Proxy/Supervisor/CircuitBreaker.cs` + `Backoff.cs` — they guard IPC process respawn (decision #68), orthogonal to the per-call Polly layer. Only `HeartbeatMonitor` is consumed outside the supervisor.
5. **A.5** Unit tests: per-policy, per-composition. Negative integration tests: (a) Modbus FlakeyTransport fails 5× on Read, succeeds 6th — invoker surfaces success; (b) Modbus FlakeyTransport fails 1× on Write with `[WriteIdempotent]=false` — invoker surfaces failure without retry (no duplicate pulse); (c) Modbus FlakeyTransport fails 1× on Write with `[WriteIdempotent]=true` — invoker retries. Bench: no-op overhead < 1%.
6. **A.6** `WriteIdempotentAttribute` in `Core.Abstractions`. Modbus/S7/OpcUaClient tag-definition records pick it up; invoker reads via reflection once at driver init.

### Stream B — Tier A/B/C stability runtime — split into MemoryTracking + MemoryRecycle (1 week)

1. **B.1** `Core.Abstractions` → `DriverTier` enum {A, B, C}. Extend `DriverTypeRegistry` to require `DriverTier` at registration. Existing driver types stamped (Galaxy = C, Modbus = B, S7 = B, OpcUaClient = A).
2. **B.2** **`MemoryTracking`** (all tiers) lifted from `Driver.Galaxy.Host/MemoryWatchdog.cs`. Captures `BaselineFootprintBytes` as the median of first 5 min of `IDriver.GetMemoryFootprint()` samples post-`InitializeAsync`. Applies **decision #70 hybrid formula**: `soft = max(multiplier × baseline, baseline + floor)`; Tier A multiplier=3, floor=50 MB; Tier B multiplier=3, floor=100 MB; Tier C multiplier=2, floor=500 MB. Soft breach → log + `DriverInstanceResilienceStatus.CurrentFootprint` tick; never kills. Hard = 2 × soft.
3. **B.3** **`MemoryRecycle`** (Tier C only per decisions #73-74). Hard-breach on a Tier C driver triggers `ScheduledRecycleScheduler.RequestRecycleNow(driverInstanceId)`; scheduler proxies to `Driver.Galaxy.Proxy/Supervisor/` which restarts the Host process. Tier A/B hard-breach logs a promotion-to-Tier-C recommendation; **never auto-kills** the in-process driver.
4. **B.4** **`ScheduledRecycleScheduler`** per decision #67: Tier C driver instances opt-in to a weekly recycle at a configured cron. Tier A/B scheduled recycle deferred to a later phase paired with Tier-C escalation.
5. **B.5** **`WedgeDetector`** demand-aware: `if (state==Healthy && hasPendingWork && noProgressIn > WedgeThreshold) → force ReinitializeAsync`. `hasPendingWork` = (bulkhead depth > 0) OR (active monitored items > 0) OR (queued historian-read count > 0). `WedgeThreshold` default 5 × PublishingInterval, min 60 s. Idle driver stays Healthy.
6. **B.6** Tests: tracking unit tests drive synthetic allocation against a fake `GetMemoryFootprint`; recycle tests use a mock supervisor; wedge tests include the false-fault cases — idle subscriber, slow historian backfill, write-only burst.

### Stream C — Health endpoints + structured logging (4 days)

1. **C.1** `OtOpcUa.Server/Observability/HealthEndpoints.cs` — Minimal API on a second Kestrel binding (default `http://+:4841`). `/healthz` reports process uptime + config-DB reachability (or cache-warm). `/readyz` enumerates `DriverInstance` rows + reports each driver's `DriverHealth.State`; returns 503 if ANY driver is Faulted. JSON body per `docs/v2/acl-design.md` §"Operator Dashboards" shape.
2. **C.2** `LogContextEnricher` installed at Serilog config time. Every driver-capability call site wraps its body in `using (LogContext.PushProperty("DriverInstanceId", id)) using (LogContext.PushProperty("CorrelationId", correlationId))`. Correlation IDs: reuse OPC UA `RequestHeader.RequestHandle` when in-flight; otherwise generate `Guid.NewGuid().ToString("N")[..12]`.
3. **C.3** Add JSON-formatted Serilog sink alongside the existing rolling-file plain-text sink so SIEMs (Splunk, Datadog) can ingest without a regex parser. Sink switchable via `Serilog:WriteJson` appsetting.
4. **C.4** Integration test: boot server, issue Modbus read, assert log line contains `DriverInstanceId` + `CorrelationId` structured fields.

### Stream D — Config DB LiteDB fallback — generation-sealed snapshots (1 week)

1. **D.1** `LiteDbConfigCache` adapter backed by **sealed generation snapshots**: each successful `sp_PublishGeneration` writes `<cache-root>/<clusterId>/<generationId>.db` as read-only after commit. The adapter maintains a `CurrentSealedGenerationId` pointer updated atomically on successful publish. Mixed-generation reads are **impossible** — every read served from the cache serves one coherent sealed generation.
2. **D.2** Write-path queries (draft save, publish) bypass the cache entirely and fail hard on DB outage. Read-path queries (DriverInstance enumeration, LdapGroupRoleMapping, cluster + namespace metadata) go through the pipeline: timeout 2 s → retry 3× jittered → fallback to the current sealed snapshot.
3. **D.3** `UsingStaleConfig` flag flips true when a read fell back to the sealed snapshot; cleared on the next successful DB round-trip. Surfaced on `/healthz` body and Admin `/hosts`.
4. **D.4** Tests: (a) SQL-container kill mid-operation — read returns sealed snapshot, `UsingStaleConfig=true`, driver stays Healthy; (b) mixed-generation guard — attempt to serve partial generation by corrupting a snapshot file mid-read → adapter fails closed rather than serving mixed data; (c) first-boot-no-snapshot case — adapter refuses to start, driver fails `InitializeAsync` with a clear config-DB-required error.

### Stream E — Admin `/hosts` page refresh (3 days)

1. **E.1** Extend `DriverHostStatus` schema with Stream A resilience columns. Generate EF migration.
2. **E.2** `Admin/FleetStatusHub` SignalR hub pushes `LastCircuitBreakerOpenUtc` + `CurrentBulkheadDepth` + `LastRecycleUtc` on change.
3. **E.3** `/hosts` Blazor page renders new columns; red badge if `ConsecutiveFailures > breakerThreshold / 2`.

## Compliance Checks (run at exit gate)

- [ ] **Invoker coverage**: every method on `IReadable` / `IWritable` / `ITagDiscovery` / `ISubscribable` / `IHostConnectivityProbe` / `IAlarmSource` / `IHistoryProvider` in the server dispatch layer routes through `CapabilityInvoker`. Enforce via a Roslyn analyzer (error-level; warning-first is rejected — the compliance check is the gate).
- [ ] **Write-retry guard**: writes without `[WriteIdempotent]` never get retried. Unit-test the invoker path asserts zero retry attempts.
- [ ] **Pipeline isolation**: pipeline key is `(DriverInstanceId, HostName)`. Integration test with two Modbus hosts under one instance — failing host A does not open the breaker for host B.
- [ ] **Tier registry**: every driver type registered in `DriverTypeRegistry` has a non-null `Tier`. Unit test walks the registry + asserts no gaps. Tier C registrations must declare their out-of-process topology.
- [ ] **MemoryTracking never kills**: soft/hard breach tests on a Tier A/B driver log + surface without terminating the process.
- [ ] **MemoryRecycle Tier C only**: hard breach on a Tier A driver never invokes the supervisor; on Tier C it does.
- [ ] **Wedge demand-aware**: test suite includes idle-subscription-only, slow-historian-backfill, and write-only-burst cases — driver stays Healthy.
- [ ] **Galaxy supervisor preserved**: `Driver.Galaxy.Proxy/Supervisor/CircuitBreaker.cs` + `Backoff.cs` still present + still invoked on Host crash.
- [ ] **Health state machine**: `/healthz` + `/readyz` respond within 500 ms for every `DriverState`; state-machine table in this doc drives the test matrix.
- [ ] **Structured log**: CI grep asserts at least one log line per capability call has `"DriverInstanceId"` + `"CorrelationId"` JSON fields.
- [ ] **Generation-sealed cache**: integration tests cover (a) SQL-kill mid-operation serves last-sealed snapshot; (b) mixed-generation corruption fails closed; (c) first-boot no-snapshot + DB-down → `InitializeAsync` fails with clear error.
- [ ] No regression in existing test suites — `dotnet test ZB.MOM.WW.OtOpcUa.slnx` count equal-or-greater than pre-Phase-6.1 baseline.

## Risks and Mitigations

| Risk | Likelihood | Impact | Mitigation |
|------|:----------:|:------:|------------|
| Polly pipeline adds per-request latency on hot path | Medium | Medium | Benchmark Stream A.5 before merging; 1 % overhead budget; inline hot path short-circuits when retry count = 0 |
| LiteDB cache diverges from central DB | Medium | High | Stale-data banner in Admin UI; `UsingStaleConfig` flag surfaced on `/readyz`; cache refresh on every successful DB round-trip; 24-hour synthetic warning |
| Tier watchdog false-positive-kills a legitimate batch load | Low | High | Soft/hard threshold split; soft only logs; hard triggers recycle; thresholds configurable per-instance |
| Wedge detector races with slow-but-healthy drivers | Medium | High | Minimum 60 s threshold; detector only activates if driver claims `Healthy`; add circuit-breaker feedback so rapid oscillation trips instead of thrashing |
| Roslyn analyzer breaks external driver authors | Low | Medium | Release analyzer as warning-level initially; upgrade to error in Phase 6.1+1 after one release cycle |

## Completion Checklist

- [ ] Stream A: Polly shared pipeline + per-tier defaults + driver-capability invoker + tests
- [ ] Stream B: Tier registry + generalised watchdog + scheduled recycle + wedge detector
- [ ] Stream C: `/healthz` + `/readyz` + structured logging + JSON Serilog sink
- [ ] Stream D: LiteDB cache + Polly fallback in Configuration
- [x] Stream E: Admin `/hosts` page refresh (E.1 in PRs #78-82 with the data layer; E.2/E.3 closed 2026-04-23)
- [ ] Cross-cutting: `phase-6-1-compliance.ps1` exits 0; full solution `dotnet test` passes; exit-gate doc recorded

## Adversarial Review — 2026-04-19 (Codex, thread `019da489-e317-7aa1-ab1f-6335e0be2447`)

Plan substantially rewritten before implementation to address these findings. Each entry: severity · verdict · adjustment.

1. **Crit · ACCEPT** — Auto-retry collides with decisions #44/#45 (no auto-write-retry; opt-in via `WriteIdempotent` + CAS). Pipeline now **capability-specific**: Read/HistoryRead/Discover/Probe/Alarm-subscribe all get retries; **Write does not** unless the tag metadata carries `WriteIdempotent=true`. New `WriteIdempotentAttribute` surfaces on `ModbusTagDefinition` / `S7TagDefinition` / etc.
2. **Crit · ACCEPT** — "One pipeline per driver instance" breaks decision #35's per-device isolation. **Change**: pipeline key is `(DriverInstanceId, HostName)` not just `DriverInstanceId`. One dead PLC behind a multi-device Modbus driver no longer opens the breaker for healthy siblings.
3. **Crit · ACCEPT** — Memory watchdog + scheduled recycle at Tier A/B breaches decisions #73/#74 (process-kill protections are Tier-C-only). **Change**: Stream B splits into two — `MemoryTracking` (all tiers, soft/hard thresholds log + surface to Admin `/hosts`; never kills) and `MemoryRecycle` (Tier C only, requires out-of-process topology). Tier A/B overrun paths escalate to Tier C via a future PR, not auto-kill.
4. **High · ACCEPT** — Removing Galaxy's hand-rolled `CircuitBreaker` drops decision #68 host-supervision crash-loop protection. **Change**: keep `Driver.Galaxy.Proxy/Supervisor/CircuitBreaker.cs` + `Backoff.cs` — they guard the IPC *process* re-spawn, not the per-call data path. Data-path Polly is an orthogonal layer.
5. **High · ACCEPT** — Roslyn analyzer targeting `IDriver` misses the hot paths (`IReadable.ReadAsync`, `IWritable.WriteAsync`, `ISubscribable.SubscribeAsync` etc.). **Change**: analyzer rule now matches every method on the capability interfaces; compliance doc enumerates the full call-site list.
6. **High · ACCEPT** — `/healthz` + `/readyz` under-specified for degraded-running. **Change**: add a state-matrix sub-section explicitly covering `Unknown` (pre-init: `/readyz` 503), `Initializing` (503), `Healthy` (200), `Degraded` (200 with JSON body flagging the degraded driver; `/readyz` is OR across drivers), `Faulted` (503), plus cached-config-serving (`/healthz` returns 200 + `UsingStaleConfig: true` in JSON body).
7. **High · ACCEPT** — `WedgeDetector` based on "no successful Read" false-fires on write-only subscriptions + idle systems. **Change**: wedge criteria now `(hasPendingWork AND noProgressIn > threshold)` where `hasPendingWork` comes from the Polly bulkhead depth + active MonitoredItem count. Idle driver stays Healthy.
8. **High · ACCEPT** — LiteDB cache serving mixed-generation reads breaks publish atomicity. **Change**: cache is snapshot-per-generation. Each published generation writes a sealed snapshot into `<cache-root>/<cluster>/<generationId>.db`; reads serve the last-known-sealed generation and never mix. Central DB outage during a *publish* means that publish fails (write path doesn't use cache); reads continue from the prior sealed snapshot.
9. **Med · ACCEPT** — `DriverHostStatus` schema conflates per-host connectivity with per-driver-instance resilience counters. **Change**: new `DriverInstanceResilienceStatus` table separate from `DriverHostStatus`. Admin `/hosts` joins both for display.
10. **Med · ACCEPT** — Compliance says analyzer-error; risks say analyzer-warning. **Change**: phase 6.1 ships at **error** level (this phase is the gate); warning-mode option removed.
11. **Med · ACCEPT** — Hardcoded per-tier MB bands ignore decision #70's `max(multiplier × baseline, baseline + floor)` formula with observed-baseline capture. **Change**: watchdog captures baseline at post-init plateau (median of first 5 min GetMemoryFootprint samples) + applies the hybrid formula. Tier constants now encode the multiplier + floor, not raw MB.
12. **Med · ACCEPT** — Tests mostly cover happy path. **Change**: Stream A.5 adds negative tests for duplicate-write-replay-under-timeout; Stream B.5 adds false-wedge-on-idle-subscription + false-wedge-on-slow-historic-backfill; Stream D.4 adds mixed-generation cache test + corrupt-first-boot cache test.

