Joseph Doherty
30d0697c28
`IHostConnectivityProbe` was a dead surface: eleven drivers implement it, `GetHostStatuses()` had ZERO production call sites, and `OnHostStatusChanged` had no subscriber outside the Galaxy driver's own aggregator. Per-host connectivity was computed by every driver and read by nobody. The issue offered "build the publisher or delete it". The publisher as its entity doc described it — driver nodes upserting `DriverHostStatus` rows — is not buildable: per-cluster mesh Phase 4 gates `AddOtOpcUaConfigDb` on the `admin` role, so a driver-only node has no ConfigDb connection to write rows with. So the capability is kept and the transport changed. `DriverHealthChanged.HostStatuses` now carries the probe result to `/hosts` as a Hosts column. That channel already reached the page, already survives the mesh split via the Phase 5 gRPC telemetry stream, and already replays a last-value snapshot on re-subscribe — so per-host state re-primes after a reconnect without a durable store. Both halves of the interface finally do what they are for: the event triggers a prompt publish, the pull is the source of truth. The point of the column is the case the driver-level state chip structurally cannot express: a multi-device driver stays aggregate-Healthy while ONE of its devices is unreachable. Two traps, both pinned by tests that were falsified against the prod code: - The host digest MUST be in the publish fingerprint. On a single-host-down transition every other fingerprint component is unchanged, so the dedup would swallow exactly the publish carrying the news — the trap that already bit the rediscovery signal. Removing it turns the guard test red, verified. - null (no probe) must stay distinct from empty (probe with no hosts). proto3 cannot tell an absent repeated field from an empty one, hence the explicit `has_host_statuses` flag; collapsing them would render every probe-less driver as one whose devices are all fine. Dropped: the DriverHostStatus entity, enum, DbSet, model config and table (migration DropDriverHostStatusTable — empty on every deployment, so the scaffolder's data-loss warning is moot, and Down() recreates it exactly). Found en route, NOT fixed here: `DriverInstanceResilienceStatus` is the identical defect — no writer, no reader, only a DbSet declaration, while the live data rides the `driver-resilience-status` telemetry channel. Its doc-comment now states that rather than describing the sampler and AdminUI join that were never built. Filed as #524 rather than widening this schema change beyond what was asked. Claude-Session: https://claude.ai/code/session_015p7wGqy3YpZNCpDzTpGMKo
256 lines
17 KiB
Markdown
256 lines
17 KiB
Markdown
# Live Telemetry Transport (v2)
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## Overview
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The AdminUI's live observability panels (`/alerts`, `/script-log`, `/hosts` driver table, driver
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resilience status) are fed by four node→central event channels. Historically all four rode
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DistributedPubSub (DPS) on the shared Akka mesh — the same gossip ring that carries redundancy
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state and the command-plane topics. Per-cluster mesh **Phase 5** adds a second transport for these
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four channels only: one gRPC server-streaming contract, selected per node/central pair by
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`Telemetry:Mode` / `TelemetryDial:Mode` (`Dps` default | `Grpc`).
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This is the same motivation as [Phase 2's `MeshTransport`](Configuration.md#meshtransport-centralnode-command-transport)
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and [Phase 3's `ConfigSource`/`ConfigServe`](Configuration.md#configsource--configserve-config-fetch-and-cache):
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DPS only works when central and the node share a gossip ring, and [Phase 6](plans/2026-07-21-per-cluster-mesh-design.md)
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splits the fleet into one Akka mesh per application `Cluster`. Once that split lands, a driver node
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in a site's mesh is no longer a cluster member of central's mesh, so DPS can no longer reach it —
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telemetry needs its own out-of-band transport, same as commands (Phase 2) and config bytes (Phase 3).
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## Direction: the node hosts, central dials
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**Load-bearing and easy to get backwards:** each **driver** node hosts the telemetry gRPC server
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(a dedicated Kestrel h2c listener); **central (admin role) is the client** and dials in. This
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mirrors the inversion ScadaBridge already uses for the same problem (its `SiteStreamService`).
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```
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driver node central (admin)
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┌─────────────────────┐ ┌──────────────────────────┐
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│ 4 publish seams │ │ TelemetryDialSupervisor │
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│ -> node-local hub │ Subscribe │ - discovers ClusterNode │
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│ -> DPS (unchanged) │◄─────────────│ rows (Host+GrpcPort) │
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│ │ (gRPC │ - one reconnecting │
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│ TelemetryStreamGrpc- │ stream) │ dialer per node │
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│ Service (h2c server) │─────────────►│ - feeds the SAME sinks │
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│ Telemetry:GrpcListen- │ TelemetryEvent │ the DPS bridges feed │
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│ Port │ (oneof, 4 kinds)│ today │
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└─────────────────────┘ └──────────────────────────┘
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```
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A fused `admin,driver` node both hosts (as driver) and dials (as admin) — it dials itself plus its
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redundant pair peer, the same way central dials site nodes.
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Central discovers driver-node telemetry endpoints from the `ClusterNode` table (`Host` +
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`GrpcPort`, the latter added in Phase 1 for exactly this purpose) — the same DB-sourced discovery
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Phase 1's ack set and Phase 2's `ClusterClient` contact set already use. No shared gossip
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membership is required to find or dial a node.
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## The dark switch
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`Telemetry:Mode` (node/serve side) and `TelemetryDial:Mode` (central/dial side) each default to
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`Dps` and can be independently set to `Grpc`. Both code paths are compiled into every binary —
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flipping either flag is an appsettings/env change plus a restart, **not a redeploy or rebuild** —
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the same discipline as `MeshTransport:Mode` and `ConfigSource:Mode`.
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What actually changes per mode, precisely:
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- **The node ALWAYS does both things, regardless of `Telemetry:Mode`.** Every one of the four
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publish seams emits into the node-local `ITelemetryLocalHub` **and** publishes to DPS,
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unconditionally. The node additionally **always** hosts the gRPC server whenever
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`Telemetry:GrpcListenPort > 0` — hosting is not mode-gated at all. `Telemetry:Mode` only affects
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central's own bookkeeping about which side it expects to be consulted (it is otherwise inert on
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the node).
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- **Only central's ingest source switches**, driven by `TelemetryDial:Mode`:
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- `Dps` (default) — today's four DPS SignalR bridges (alert, script-log, driver-status,
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resilience) subscribe and feed the AdminUI sinks. Unchanged behavior.
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- `Grpc` — those four DPS bridges are **not spawned**. Instead the `TelemetryDialSupervisor`
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actor dials every driver node's gRPC stream and feeds the **identical** in-process sinks
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(`IInProcessBroadcaster<AlarmTransitionEvent>`, `IInProcessBroadcaster<ScriptLogEntry>`,
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`IDriverStatusSnapshotStore`, `IDriverResilienceStatusStore`).
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- **The AdminUI panels themselves are untouched.** `/alerts`, `/script-log`, and the `/hosts`
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driver table all read from the same sinks in both modes — only the sinks' upstream feed swaps.
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The `fleet-status` bridge (out of Phase 5 scope — see below) stays on DPS in both modes.
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Because hosting is unconditional and both ingest paths are always compiled in, a fleet can be
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flipped node-by-node and central-by-central with no coordination window where telemetry is lost —
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the node is always serving, so central can switch to `Grpc` whenever it likes.
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## The four migrated channels
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| DPS topic (today) | Domain record | Central sink fed |
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|---|---|---|
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| `alerts` | `AlarmTransitionEvent` | `IInProcessBroadcaster<AlarmTransitionEvent>` (+ `AlertHub`); `/alerts` page; live/disconnected pill |
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| `script-logs` | `ScriptLogEntry` | `IInProcessBroadcaster<ScriptLogEntry>` (+ `ScriptLogHub`); `/script-log` page; live pill |
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| `driver-health` | `DriverHealthChanged` | `IDriverStatusSnapshotStore`; `/hosts` driver table |
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| `driver-resilience-status` | `DriverResilienceStatusChanged` | `IDriverResilienceStatusStore` |
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These four are carried as `oneof` event kinds on one `TelemetryStreamService.Subscribe` RPC — a
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single stream per (central, driver-node) pair carries all four, rather than one stream per topic.
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Proto field evolution is additive-only (never renumber/reuse a tag), locked by a contract test that
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reflects over the `oneof` cases.
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### Per-host connectivity rides `driver-health` (Gitea #521)
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`DriverHealthChanged.HostStatuses` carries each driver's `IHostConnectivityProbe.GetHostStatuses()`
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result, rendered as the **Hosts** column on `/hosts`. It exists to show the one thing the driver-level
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state chip structurally cannot: a **multi-device driver stays aggregate-`Healthy` while one of its
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devices is unreachable**. A FOCAS or AbLegacy instance owning several PLCs previously hid that
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entirely.
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Three things to know before touching it:
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- **This deliberately does NOT go through SQL.** A `DriverHostStatus` table existed, with an entity
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doc describing a publisher hosted service that upserted rows from each driver node. That publisher
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was never written, and **per-cluster mesh Phase 4 made it unbuildable as described** — `Program.cs`
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gates `AddOtOpcUaConfigDb` on the `admin` role, so a driver-only node has no ConfigDb connection to
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write rows with. The table was dropped (migration `DropDriverHostStatusTable`); it was empty on
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every deployment. This channel needs no DB and already survives the mesh split.
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- **null ≠ empty, on the wire too.** Null means "the driver has no probe"; an empty list means "it has
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one that currently knows no hosts". proto3 cannot distinguish an absent repeated field from an empty
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one, so `DriverHealth.has_host_statuses` carries that bit explicitly. Collapsing the two would render
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every probe-less driver as one whose devices are all fine.
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- **The host digest is part of the publish fingerprint, and must stay there.** `PublishHealthSnapshot`
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dedups on that fingerprint, and on a single-host-down transition *every other component is
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unchanged* — so without it the dedup swallows precisely the publish carrying the news. Same trap
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that bit the rediscovery signal; pinned by
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`DriverInstanceActorHostStatusTests.A_single_host_going_down_is_not_swallowed_by_the_unchanged_health_dedup`.
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The digest is a flattened, host-name-ordered **string** for the converse reason: a tuple holding an
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`IReadOnlyList` compares by reference and would re-publish on every 30 s heartbeat forever.
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⚠️ **`DriverInstanceResilienceStatus` is the same defect, still open.** That table also has no writer
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and no reader — the only reference in the repo is its DbSet declaration — while the live data reaches
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the AdminUI over the `driver-resilience-status` channel above. Keep-or-delete is Gitea **#524**.
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## The three deferred channels (NOT migrated in Phase 5 — do not read this as "seven done")
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The program sketch originally named seven observability topics for Phase 5. Three were scoped out,
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each for a distinct, settled reason:
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- **`redundancy-state`** — bidirectional, built directly from `Cluster.State`, and **pair-local**:
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it drives ServiceLevel and the Primary data-plane gate, consumed in-process by
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`OpcUaPublishActor`, `ScriptedAlarmHostActor`, `DriverHostActor`, and `HistorianAdapterActor`.
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It stays on DPS in **both** `MeshTransport` modes today, and it is genuinely mesh-bound by
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design — under Phase 6 each pair keeps sharing its own small mesh, so DPS keeps working for it
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in-mesh. Central's *display* of each pair's redundancy state is a Phase 6 cross-mesh concern
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(a future observability channel, once central no longer shares gossip with any site pair), not a
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Phase 5 telemetry-panel migration.
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- **`fleet-status`** — **central-internal**, not a node→central stream at all. The admin singleton
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`FleetStatusBroadcaster` builds it from the admin node's own cluster membership/reachability/
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leader events, and `Fleet.razor` **polls the Config DB** and ignores the feed entirely. There is
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nothing here for a per-node gRPC stream to carry. Revisit in Phase 6, once central genuinely
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loses gossip visibility of site nodes and needs another way to know a pair's membership.
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- **`deployment-acks`** — already migrated, but onto a different transport: it rides the Phase 2
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`ClusterClient` transport (`MeshTransport:Mode=ClusterClient`) as a command-plane reply, not an
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observability broadcast. It was never a Phase 5 candidate.
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See [`docs/Redundancy.md`](Redundancy.md#command-transport-centralnode) for how `redundancy-state`
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and the command transports fit together.
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## Authentication — fail-closed from day one
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A shared node bearer key gates the stream: `Telemetry:ApiKey` (node/serve side) must equal
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`TelemetryDial:ApiKey` (central/dial side). `TelemetryStreamAuthInterceptor` enforces it — path-scoped
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to `/telemetry.v1.TelemetryStreamService/`, comparing the `Authorization: Bearer` token with
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`CryptographicOperations.FixedTimeEquals`, and rejecting with `PermissionDenied`. An **unset** key
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rejects every call rather than allowing an open stream — the same fail-closed posture as
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`ConfigServeAuthInterceptor` and `LocalDbSyncAuthInterceptor`.
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This **supersedes** [the design doc's §6.3](plans/2026-07-21-per-cluster-mesh-design.md), which had
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provisionally decided to "match ScadaBridge's unauthenticated posture for now" for inter-cluster
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transports. ScadaBridge itself has since closed that gap with this identical
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interceptor pattern, so Phase 5 ships authenticated from the start rather than deferring auth to a
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later hardening pass. See the design doc's superseded note for the full history.
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## Upgrading an existing deployment: populate `ClusterNode.GrpcPort` first (#493)
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`GrpcPort` is a **nullable column added by Phase 5**, and nothing backfills it onto rows that
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already existed. On an upgraded deployment every `ClusterNode` row therefore carries `NULL`,
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central finds **no dial targets**, and flipping `TelemetryDial:Mode` to `Grpc` connects to
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nothing. Fresh installs are unaffected. `AkkaPort` did not have this problem only because it is
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`NOT NULL` with a default of 4053.
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Nothing fails loudly, which is what makes this worth stating up front — the code degrades
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gracefully, once per node, throttled:
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```
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ClusterNode <id> has no GrpcPort; it exposes no telemetry stream and is skipped in this dial refresh
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(further skips of this node are silent)
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```
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Other symptoms: no `ESTABLISHED` sockets on the telemetry port (a `LISTEN` but nothing else in
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`docker exec <node> cat /proc/net/tcp6`), and the `/alerts` / `/script-log` pill never turning
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live in `Grpc` mode.
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**Before flipping any node to `Grpc`**, set each driver node's `GrpcPort` to that node's own
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`Telemetry:GrpcListenPort` — via the AdminUI node editor, or directly:
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```sql
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UPDATE dbo.ClusterNode SET GrpcPort = <that node's Telemetry:GrpcListenPort> WHERE NodeId = '<node>';
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```
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There is deliberately **no EF data migration** backfilling this. The correct value is that node's
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own listener port, which is per-deployment configuration the database cannot derive; a migration
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could only invent one, and a *wrong* dial target is worse than the `NULL` the dialer already skips
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explicitly. The docker-dev seed does backfill (`GrpcPort IS NULL AND CreatedBy = 'docker-dev-seed'`
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→ 4056) because there the port is fixed by `docker-compose.yml` and therefore actually knowable.
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## Reconnect and reliability
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Central's `TelemetryDialSupervisor` (an admin-role actor) runs **one reconnecting dialer per
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driver node**:
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- **Discovery** — enabled, non-maintenance `ClusterNode` rows are read on start and refreshed every
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`TelemetryDial:ContactRefreshSeconds` (default 60), plus on an admin-side topology change. A row
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with a null `GrpcPort` is skipped (that node exposes no telemetry endpoint — logged, not an
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error). Dial targets are added/removed as the row set changes.
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- **Reconnect backoff** — immediate first retry, then a fixed ~5 second backoff, indefinitely. The
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dialer never gives up permanently: this is an observability channel, not the data plane, so a
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persistently-unreachable node just means a persistently-stale panel, not a fleet fault.
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- **Generation stamping** — each dialer carries a monotonically-increasing generation counter, so
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a late error or late event from a superseded stream (e.g. one that raced a reconnect) is
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recognized and ignored rather than corrupting the current stream's state.
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- **Snapshot replay on reconnect** — the node-local hub keeps a last-value cache for
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`driver-health` and `driver-resilience-status` (there is nothing meaningful to replay for
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`alerts`/`script-logs`, which are append logs). On every new `Subscribe` — including a
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reconnect — the hub drains the cached snapshots to the new stream before live deltas, so
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`IDriverStatusSnapshotStore` / `IDriverResilienceStatusStore` re-prime immediately rather than
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sitting stale until the next natural event. `alerts`/`script-logs` simply tolerate the gap — a
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reconnect loses whatever transitions happened while disconnected, same as a DPS resubscribe would.
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- **Connection indicator** — the `/alerts` and `/script-log` live/disconnected pill is driven by
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**aggregate** stream health across all dialers: connected when **at least one** node stream is
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up, disconnected when all are down. This matches today's DPS `SubscribeAck`/`PostStop` pill
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semantics — the pill has never meant "every node is live," only "the panel has a source." The
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two store-backed panels (`driver-health`, `driver-resilience-status`) carry no fleet-wide
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connection flag today, on gRPC or on DPS; per-row staleness is unchanged.
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## Configuration reference
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See [`docs/Configuration.md` § `Telemetry` / `TelemetryDial`](Configuration.md#telemetry--telemetrydial-live-telemetry-transport)
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for the full keys table. In brief:
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- **`Telemetry`** (node/serve side): `Mode` (`Dps`|`Grpc`, default `Dps`), `GrpcListenPort` (`0` =
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disabled — the driver node's dedicated telemetry h2c port), `ApiKey` (the shared node key; supply
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via `${secret:}`/env, never commit).
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- **`TelemetryDial`** (central side): `Mode` (`Dps`|`Grpc`), `ApiKey` (must equal the nodes'
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`Telemetry:ApiKey`), `ContactRefreshSeconds` (default `60`), `CallTimeoutSeconds` (default `30`).
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A driver-role node with `Telemetry:Mode=Grpc` must set `GrpcListenPort > 0` (nothing to serve
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otherwise), and `Grpc` mode on either side requires a non-empty `ApiKey` — both are enforced by
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`ValidateOnStart` validators, fail-fast at boot.
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## Relationship to the rest of the mesh program
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- **Same dark-switch discipline as Phases 2 and 3.** `MeshTransport:Mode`, `ConfigSource:Mode`, and
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`Telemetry:Mode`/`TelemetryDial:Mode` are all independently-flippable, restart-only config
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changes, and in every case the "new" side is always wired so the flip needs no coordinated
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redeploy.
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- **Why the inversion matters for Phase 6.** Every other cross-boundary transport in this program
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(`MeshTransport`, `ConfigSource`/`ConfigServe`) already has central as the addressable, discoverable
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side and the node as the caller or the callee-by-address. Telemetry is the one channel that is
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naturally node-sourced and fleet-wide-fanned, so putting central in the client role — dialing out
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to each node by its `ClusterNode`-recorded address — is what lets telemetry survive the mesh split
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cleanly: central never needs cluster membership with a site's mesh to keep watching it.
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- **Status:** code-complete on `feat/mesh-phase5`; the live gate (flip the docker-dev rig to `Grpc`,
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confirm all panels stay green with the DPS telemetry bridges unspawned, kill-and-reconnect a site
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node) has not yet run. See `docs/plans/2026-07-22-per-cluster-mesh-program.md` § Phase 5 and
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`docs/plans/2026-07-23-mesh-phase5-grpc-telemetry-stream.md` for the implementation plan.
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