Feature/projection message outbox 84#100
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…ssages Closes #84. Marten projections can publish side-effect messages on commit (transactionally with the projection write) via Wolverine.Marten's MartenToWolverineMessageBatch. Polecat's projection daemon had no equivalent — the IProjectionBatch.PublishMessageAsync hook was a Task.CompletedTask stub. This adds the Polecat-side plumbing only — a future Wolverine.Polecat will implement IMessageOutbox to bridge into Wolverine's outgoing- message machinery. The user explicitly scoped this PR Polecat-only. ## Added - `Polecat.Events.Aggregation.IMessageBatch` — extends `JasperFx.Events.IMessageSink` with `BeforeCommitAsync(token)` and `AfterCommitAsync(token)` hooks. The batch buffers messages in PublishAsync; the implementer chooses which hook to flush in based on the desired delivery guarantee. - `Polecat.Events.Aggregation.IMessageOutbox` — factory that vends a fresh `IMessageBatch` per projection daemon batch. The signature takes the public `IDocumentSession` (rather than internal `DocumentSessionBase`) so downstream integrations don't need InternalsVisibleTo; `IDocumentSession` already exposes `ITransactionParticipantRegistrar` for enrollment. - `Polecat.Events.Aggregation.NulloMessageOutbox` — singleton no-op default (drops messages, fires no hooks). Apps that don't integrate a message bus pay zero overhead. - `EventStoreOptions.MessageOutbox` property defaulting to NulloMessageOutbox. This is the registration point for downstream integrations. ## Wired through PolecatProjectionBatch - New `_messageBatch` field + `_messageBatchGate` semaphore for thread-safe lazy initialization. Stays null when no projection in this batch publishes. - `PublishMessageAsync(object, string)` — was a Task.CompletedTask stub; now lazily creates the batch via `IMessageOutbox.CreateBatch` and forwards the message via cached generic `IMessageSink.PublishAsync<T>` MethodInfo. The reflection lookup is done once at static init; per-publish cost is one MakeGenericMethod + one Invoke (off the per-event hot path — only fires when a projection explicitly emits a side-effect). - New `PublishMessageAsync(object, MessageMetadata)` overload mirroring the same pattern; satisfies the JFx.Events.IProjectionBatch contract added in the canonical interface (default impl forwards tenant-only; we wire the metadata-aware path so downstream integrations can stamp correlation / causation / headers). - ExecuteAsync passes the snapshot batch into the resilience lambda's state tuple. `BeforeCommitAsync` fires inside the SQL transaction (right before `tx.CommitAsync`) so an at-least-once outbox can persist its envelopes in the same transaction. `AfterCommitAsync` fires outside the resilience pipeline (after success) so it does not re-fire on a transient SQL retry. ## Tests `src/Polecat.Tests/Daemon/projection_message_outbox_tests.cs` — 4 cases: - No publish in the batch → `IMessageOutbox.CreateBatch` is never called and no hooks fire (the common no-op path) - Publish creates the batch exactly once and forwards every published message with the right type and tenant id - BeforeCommit + AfterCommit hooks both fire after `ExecuteAsync` completes, in the right order (publish → before → after) - The metadata overload correctly extracts tenant id (default IMessageSink impl) and forwards through the same path All 4 pass against SQL Server 2025 docker on net9.0. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Adds docs/events/projections/side-effects.md adapted from the Marten equivalent: how to override RaiseSideEffects, what AppendEvent and PublishMessage do, and how to register a custom IMessageOutbox to actually deliver the published messages. Notes that a Wolverine.Polecat integration is on the roadmap. Polecat does not (yet) support side effects on Inline projections, so the corresponding Marten section is omitted. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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