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feat(transport): observability for the import-session cap (register row 53)

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Nothing counted open import sessions and nothing logged a rejection at
MaxConcurrentImportSessions (default 8), so the register-row-43 session leak
could only be inferred from rotating test failures.

- Gauge scadabridge.central.transport.import_sessions.open on the existing
  ZB.MOM.WW.ScadaBridge meter — already in SiteServiceRegistration.ObservedMeters
  (the ZbTelemetryOptions.Meters allowlist), so no telemetry config change is
  needed and the series is not silently dropped. Provider-backed, mirroring the
  StoreAndForward queue-depth gauge: registered by BundleSessionEvictionService
  .StartAsync (Transport's only lifecycle seam) and identity-checked-cleared on
  StopAsync so a late stop cannot clobber a newer registrant.
- IBundleSessionStore.OpenSessionCount counts only UNEXPIRED sessions. The raw
  dictionary count would report the node as pinned at the cap for up to a
  one-minute sweep interval after a burst had drained — the exact false positive
  the instrument exists to rule out.
- BundleSessionStore.Open logs Warning with open count, cap and TTL when it
  rejects. Fires only on a real cap hit: a store full of expired sessions is
  reclaimed first and logs nothing.
- Docs: docs/components/Transport.md + docs/requirements/Component-Transport.md.

Tests: Transport 173 passed (+6 new — Warning content, two negative controls,
unexpired-only counting, gauge register/clear, no-clobber-on-late-stop).
This commit is contained in:
Joseph Doherty
2026-08-15 11:42:31 -04:00
parent b449fe201a
commit 372ea4df1b
7 changed files with 407 additions and 2 deletions
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docs/components/Transport.md
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@@ -49,6 +49,11 @@ After `LoadAsync` validates and decrypts a bundle, the plaintext content is stor
Without this, an abandoned wizard or a plain CLI preview held one of the eight `MaxConcurrentImportSessions` slots — and the decrypted bundle content — for the full TTL; eight of them locked out every import on the active node (deferred register row 43, observed live).
**Session-cap observability (register row 53).** That leak was only ever inferable from rotating test failures, because nothing counted open sessions and nothing logged a rejection. Two signals now close the gap:
- **Gauge `scadabridge.central.transport.import_sessions.open`** — open (unexpired) sessions on this node, on the `ZB.MOM.WW.ScadaBridge` meter, which is already in `SiteServiceRegistration.ObservedMeters` (the `ZbTelemetryOptions.Meters` allowlist; an unlisted meter exports nothing, silently). The provider is registered by `BundleSessionEvictionService.StartAsync` and identity-checked-cleared on `StopAsync`, mirroring the StoreAndForward queue-depth gauge. `IBundleSessionStore.OpenSessionCount` deliberately excludes expired-but-unswept entries — counting the raw dictionary would report the node as pinned at the cap for up to a sweep interval after a burst had actually drained, which is the exact false positive this instrument exists to rule out. Transport is registered only in the host's Central branch, so the gauge reports 0 on a site node.
- **Warning log on rejection** — `BundleSessionStore.Open` logs the open count, the cap, and the TTL when it rejects. It fires only on a real cap hit: a store full of *expired* sessions is reclaimed first and logs nothing. A count that stays pinned at the cap across repeated warnings is a leak; an occasional warning is genuine concurrency, and the fix there is to raise `ScadaBridge:Transport:MaxConcurrentImportSessions` (each slot pins up to `MaxBundleSizeMb` of decrypted plaintext, so the cap is a memory budget).
### Conflict resolution and `BundleImportId` correlation
`PreviewAsync` compares each bundle artifact against the target environment using `ArtifactDiff`, classifying items as `Identical`, `Modified`, `New`, or `Blocker`. The operator assigns a `ResolutionAction` (`Add`, `Overwrite`, `Skip`, `Rename`) per item. `ApplyAsync` honours those resolutions in a single EF transaction and threads a new `BundleImportId` GUID through every per-entity audit row via the scoped `IAuditCorrelationContext`. This makes every configuration row written by a bundle import queryable as a group from the audit log.