# DCL connection arbitration — which config source owns a live connection **Date:** 2026-08-17 **Scope:** Site Runtime (`DeploymentManagerActor`), deferred-work-register rows **89** and **90** **Status:** implemented **Predecessor:** `docs/plans/2026-08-15-instance-sweep-self-heal-design.md` §7.4, which designed a store-level connection sweep, **rejected** it, and recorded rows 89/90 as the residual — explicitly blocked on "the arbitration decision above (which source wins), not another sweep pass". --- ## 1. Verified topology — and two premise corrections Both register rows are worded in terms of "the peer node's DCL". That framing is wrong, and the correction changes what the fix has to do, so it comes first. ### 1.1 What actually runs where | Actor | Placement | Verified at | |---|---|---| | `DeploymentManagerActor` | **Cluster singleton**, role-scoped to `site-{SiteId}` (`SingletonRegistrar.Start(…, "deployment-manager", …, role: siteRole)`) | `Host/Actors/AkkaHostedService.cs` | | `DataConnectionManagerActor` (`/user/dcl-manager`) | **Every site node**, unconditional `ActorOf` — *not* a singleton | `Host/Actors/AkkaHostedService.cs` | | `CertStoreActor` | Every site node (documented; re-confirmed) | same file | `CreateConnectionCommand` and `RemoveConnectionCommand` have exactly **one** sender in the whole repo — `DeploymentManagerActor` — and it sends them to the `_dclManager` ref it was constructed with, which is *its own node's* local `dcl-manager` (the Props are built per node, so whichever node materialises the singleton uses that node's manager). ### 1.2 Premise correction 1 — the standby's DCL is EMPTY, not stale Because the singleton is the only sender, **the standby node's `dcl-manager` holds no connection actors at all**. It is not "serving the old endpoint"; it is serving nothing, and nothing subscribes through it. So there is no per-node divergence to reconcile with a cross-node message. The real failure mode is one layer over: > `_createdConnections` — the per-name hash cache that records what the DM last pushed — is > **in-memory and per singleton incarnation**. When the singleton restarts (failover, or an > ordinary node restart), the new incarnation starts with an empty cache, loads > `deployed_configurations`, and rebuilds every connection **from the `Connections` snapshot frozen > into each instance's flattened config at that instance's deploy time**. Nothing has ever read > `data_connection_definitions` — that table had **no production reader** — so a newer, > connection-only artifact deploy is silently discarded and the site goes back to the old endpoint. This also means **row 89 is not replication-specific**. Replication only puts the definition row on both nodes, which is if anything helpful. The identical loss happens on an unreplicated single-node restart. The row-89 fix is therefore a *create-time arbitration* fix, not a *message-delivery* fix. Row 90 is the same shape: the delete is durable in the store, but the flattened configs still name the connection, so the next instance start re-creates it. ### 1.3 Premise correction 2 — a changed config never reached a LIVE connection either `EnsureDclConnection`'s contract, as documented in the code before this change, was that a changed endpoint/credential *"re-issues a `CreateConnectionCommand` so the DCL adopts the new configuration instead of keeping a stale connection"*. **That is false.** `DataConnectionManagerActor.HandleCreateConnection` is create-only: ```csharp if (_connectionActors.ContainsKey(command.ConnectionName)) { _log.Warning("Connection {0} already exists", command.ConnectionName); return; } ``` So the system-wide artifact deploy's "make it live immediately" push has never worked on the deploy-handling node, and the DM's hash cache recorded it as applied regardless. This is a **separate, pre-existing defect** discovered while validating rows 89/90; see §6. --- ## 2. The arbitration rule > **The definitions store wins wherever it has an opinion. The flattened config is authoritative > only for names the store has never owned.** 1. **A name with a row in `data_connection_definitions`** — that row's config configures the live DCL connection actor. The identically-named entry in an instance's flattened `Connections` map is a deploy-time snapshot and never overrides it. 2. **A name with no definition row** — the flattened config is the source, exactly as before. This is not a fallback; it is the *correct* answer for a deploy-only site, where the definitions table is legitimately empty because nothing has ever sent that site a system-wide artifact deploy while every connection that matters lives inline in the instance configs. §7.4 of the predecessor memo identified this as the reason a blanket store-level sweep is wrong, and it remains the reason rule 2 exists. 3. **Removal** semantics on the deploy-handling node are unchanged: the actor is stopped even if deployed instances still reference it, and their attributes go bad quality until redeploy. What is *added* is a tombstone (§4). ### 2.1 The overlay is an arbiter, not a creation source `_connectionDefinitions` decides **which configuration** a connection the runtime already wants is built from. It never creates a connection on its own: a definition row naming a connection no deployed instance references produces no actor, because none is wanted. This is what makes it safe to consult on every sweep tick, and it is why re-adding a centrally deleted connection brings the actor back at the *next instance start* rather than instantly (pinned by `ReAddedDefinition_ClearsTheTombstone_SoTheNextInstanceStartRecreatesIt`). --- ## 3. The ping-pong, and why keying the hash on the winner makes it unrepresentable The predecessor memo rejected a store-level sweep partly because of this: > `_createdConnections` holds one hash per name. The flattened-config hash and the > artifact-definition hash for the same connection can legitimately differ (the supported > version-skew case). A sweep re-hashing from the definitions table would flip the slot every tick > against the deploy path flipping it back, **recreating the connection actor and dropping its live > subscriptions on a loop.** That analysis is correct, and it is a property of *where the hash is computed*, not of the sweep. The fix is one line of ordering discipline: > **Every ensure path resolves the winning configuration BEFORE hashing or pushing.** `EnsureDclConnections(configJson)` no longer hands the flattened `ConnectionConfig` straight to `EnsureDclConnection`. It first asks the overlay whether the store owns that name; if it does, the *definition's* protocol/config/backup/failover fields are what get hashed and pushed. The artifact-deploy path pushes those same fields. Therefore, for any given name, **only one configuration can ever be hashed**, no matter which path calls — the two sources cannot occupy the slot alternately because there is only ever one candidate. The flip is not guarded against; it is unrepresentable. The guard test `DisagreeingSources_DoNotPingPongTheConnection_AcrossManySweeps` pins exactly this with the two sources deliberately disagreeing: one create, zero removes, across many ticks. It passes before *and* after the fix — before, because nothing read the definitions at all — which is the point: it is a guard against the **rejected naive design**, not a fail-before test. ### 3.1 A second flip source, closed in the same way `HandleApplyArtifactDataConnectionsToDcl` now writes the artifact's rows into the overlay in the same pass it pushes them. Without that there is a window between an artifact deploy and the next sweep read in which an instance start would resolve the name to its flattened snapshot while the artifact path had already pushed the definition — the same flip, arriving through a different door. --- ## 4. Removal and the tombstone `_centrallyRemovedConnections` is a `HashSet` of names this singleton incarnation has seen removed centrally. It is populated by both removal paths, which share one helper (`RemoveConnectionCentrally`) so they cannot drift: - the artifact reconcile's `RemovedConnectionNames` (the existing path); and - **the sweep's disappearance detection** (new — §5). `EnsureDclConnections` skips a tombstoned name. Without the tombstone the removal does not survive one tick: the removal evicts the hash slot, and the next instance start whose flattened config still names the connection re-creates it. That is not hypothetical — the sweep starts instances — and it is a pre-existing bug this change also closes (`ArtifactReconcileRemoval_IsNotResurrectedByALaterInstanceStart` fails on the old code). **Tombstones are deliberately in-memory only.** Persisting them was rejected: a durable "this name is gone" marker is a second divergent authority over the same names, and the flattened configs already resurrect a removed connection across a restart today. That pre-existing semantic is accepted and unchanged — see the residual in §6. A tombstone is cleared the moment a definition row for the name reappears (a definition row *is* the store re-asserting ownership), so a centrally re-added connection is creatable again. --- ## 5. The mechanism: the definitions table as an overlay on the existing reads No new message, no new actor, no new cross-node path. The two durable tables the DM arbitrates over are read in **one off-thread task**, `ReadStoreSnapshotAsync`, used by both existing reads: ``` PreStart → LoadDeployedConfigs ─┐ ├─ ReadStoreSnapshotAsync (off-thread, one task) InstanceSweepTick ──────────────┘ deployed_configurations data_connection_definitions │ ▼ piped back StartupConfigsLoaded / InstanceSweepLoaded (actor thread) │ ApplyConnectionDefinitionOverlay │ ┌───────────────────┴───────────────────┐ row present row disappeared adopt + push if live tombstone + stop actor ``` Nothing blocks the actor thread on SQLite; the overlay mutation, the hash cache and the tombstone set are all actor-thread-confined, matching the existing `StartupConfigsLoaded` / `ApplyArtifactDataConnectionsToDcl` patterns. **Ordering is load-bearing in both handlers.** The overlay is applied *before* the boot batch creates any Instance Actor and *before* the sweep's repair loop, so a connection is created from the winning config on its **first** push rather than created stale and corrected a tick later. On failover that is the difference between a clean takeover and a needless recreate on every one. ### 5.1 The two reads fail differently, on purpose A failed **config** read propagates and re-arms the whole load (the existing S5 path, unchanged). A failed **definitions** read is swallowed into a `null` result, and `null` means *keep the overlay exactly as it is* — deliberately distinct from an empty list, which is a real answer (a deploy-only site, or a site after central deletes every data connection). Clearing the overlay on a transient read failure would hand arbitration back to the flattened snapshots for one pass and take it away on the next successful read: the §3 ping-pong, reintroduced through the error path. ### 5.2 Disappearance detection, and precisely how far it reaches A name that **was** in the overlay and now has no row is a removal this incarnation can *prove*: the store owned it a moment ago and does not now. It is tombstoned and its connection actor stopped. A name that was **never** in the overlay is indistinguishable from one the store has never owned — a fresh incarnation cannot tell "central deleted this" from "this site keeps its connections inline". It is left to the flattened config. That asymmetry is deliberate; the alternative (infer deletion from "the definitions table is non-empty, therefore it is the complete set") would stop live connections on an inference, and is recorded as a candidate follow-up in §6. ### 5.3 Cost One extra `SELECT` over `data_connection_definitions` per sweep tick (default 60 s) and once at boot, on the same task as the existing deployed-configs read. Steady state is otherwise silent: the hash guard means an unchanged connection produces no message and no log. --- ## 6. Residuals — what this does NOT close These are precise, and each is a candidate register row. 1. **A live connection actor is still not reconfigured in place** (premise correction 1.3). The DCL manager's `HandleCreateConnection` is create-only, so a changed configuration for a name whose actor already exists takes effect only when the connection is next created from scratch — a node restart, a singleton failover, or the connection being removed and re-added. This change makes that **honest** rather than silent: `EnsureDclConnection` now logs a Warning naming exactly that, once per changed configuration (the hash guard suppresses repeats, so a healthy site stays silent). It is not *fixed* here because the obvious fix is destructive: stopping and re-creating the connection actor drops every Instance Actor's tag subscription, and `InstanceActor` cancels its per-connection subscribe-retry timer on the first success, so **nothing re-subscribes** — trading "stale endpoint, data flowing" for "correct endpoint, no data until redeploy". The non-destructive fix is a `ReconfigureConnectionCommand` handled *inside* `DataConnectionActor`, reusing the adapter-swap machinery the primary→backup failover path already has (`CountFailureAndMaybeFailover`: dispose adapter, `_factory.Create` the new one, bump `_adapterGeneration`, re-enter the connect cycle, which re-issues everything from `_subscriptionsByInstance`). That is a Data Connection Layer change and is out of rows 89/90's scope. 2. **A removal is still not durable across a restart.** Tombstones are in-memory, so a connection deleted centrally and then resurrected by an instance's flattened config on the next node restart behaves as it always has. Closing this needs a durable marker or — better — flattened configs that stop carrying connection bodies at all. 3. **A delete that arrives while this node has never adopted the row is undetectable** (§5.2). The narrow case that remains open: the singleton moves to a node whose file already has the row deleted, and some instance's flattened config still names the connection. That node re-creates it. A stronger rule — "a non-empty definitions table is the complete set, so absence means deleted" — would close it, at the cost of stopping live connections on an inference about what central shipped; deliberately not taken. 4. **Dual-active (a real partition under `auto-down`)** is the only routine way a definition change reaches a singleton that did not process the deploy itself. It is handled correctly by §5, but it is worth naming that this is not the common path — the common path is failover/restart (§1.2). --- ## 7. Tests `tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/DclConnectionArbitrationTests.cs` — 8 tests, asserting on the `CreateConnectionCommand` / `RemoveConnectionCommand` messages the DM sends to a recording stand-in for the DCL manager, and on the **endpoint inside** those commands (which is what distinguishes the two config sources). Every test gates on `DeploymentManagerStartupProbe` for the same reason the row-78/85 suites do: a row written before the boot load reads storage is consumed by the boot load, and an ungated test would silently prove the boot load works instead of the path it names. Fail-before evidence (behavioral revert: `ApplyConnectionDefinitionOverlay` made inert and the tombstone check disabled — i.e. the exact pre-fix arbitration) — **5 fail, 3 pass**: | Test | Pre-fix | |---|---| | `DefinitionRow_WinsOverAnOlderFlattenedSnapshot_WhenTheSingletonBoots` | **FAIL** — the row-89 symptom in its failover shape (create carries the old endpoint) | | `DefinitionArrivingAfterBoot_IsAdoptedByTheNextSweepTick` | **FAIL** — the CDC-delivered change is never read | | `DefinitionRowDisappearing_StopsTheConnection_AndItIsNotResurrected` | **FAIL** — the row-90 symptom (orphan actor stays live) | | `ArtifactReconcileRemoval_IsNotResurrectedByALaterInstanceStart` | **FAIL** — removal undone by the next instance start | | `ReAddedDefinition_ClearsTheTombstone_SoTheNextInstanceStartRecreatesIt` | **FAIL** | | `DisagreeingSources_DoNotPingPongTheConnection_AcrossManySweeps` | pass (guard against the rejected naive design) | | `NoDefinitionRow_LeavesTheFlattenedConfigAuthoritative_AtBoot` | pass (deploy-only regression guard) | | `NoDefinitionRow_LeavesTheFlattenedConfigAuthoritative_OnTheSweepPath` | pass (deploy-only regression guard) | The three passers are the point of passing both before and after: rule 2 must be byte-for-byte unchanged for a deploy-only site, and the ping-pong must never appear. --- ## 8. Configuration **None added.** The overlay refresh rides `ScadaBridge:SiteRuntime:InstanceSweepIntervalSeconds` (default 60; `0` disables the sweep and therefore the post-boot half of this convergence — the boot-time arbitration still applies, because it hangs off the startup load, not the sweep timer).