movies-diary/docs/adr/0015-transactional-outbox.md

Events go through a transactional outbox and are delivered idempotently

Domain events used to be published after the write that caused them, from the use case, on a channel the server owned. A crash between the write and the publish lost the event (no poster, no federation delivery, no search index entry), and with NATS the server needed a live broker connection just to log a review. Delivery was also at-least-once without any guard, so a redelivered event ran its handlers twice.

Decision

The outbox is event_queue. The same SQLite table the DB-backed bus already used is now the outbox for every backend. Migration 0101 adds event_queue.event_id (UUID, unique when set) and claimed_at, and creates processed_events (event_id, handler, status, started_at, owner token). Legacy rows have a NULL event_id and are still delivered; they just get no dedup.

Writes are atomic with their events. Repository and command-port methods that change state take the events they cause and write them in the same transaction as the data. shared::enqueue_events is the single path that inserts outbox rows. Covered: review log/edit/delete (including orphan movie deletion and ReviewDeleted/MovieDeleted), watchlist add/remove, movie discovery and enrichment requests, wrap-up, integration writes, local follow/unfollow, remote review save. Use cases never open transactions; the adapter does.

The server never talks to NATS. With EVENT_BUS_BACKEND=nats the server still only inserts outbox rows. The worker runs a relay that claims pending rows and publishes them to JetStream with Nats-Msg-Id set to the event id, then marks the row done. With db the worker consumes the rows directly. max_deliver on the JetStream consumer equals EVENT_QUEUE_MAX_ATTEMPTS, so both backends dead-letter after the same number of attempts.

Three dedup layers.

1. Broker. JetStream drops a repeated Nats-Msg-Id inside duplicate_window, set explicitly to 1 hour (longer than the relay's retry horizon) rather than the 2-minute server default.
2. Relay lease. A row claimed by a relay or DB consumer that then died is reclaimed once claimed_at is older than 600 s, so a crash never strands an event in processing.
3. Processed-event ledger. Before a handler runs, (event_id, handler) is claimed in processed_events (INSERT in_progress, or take over a claim older than the 1800 s lease). Success marks it done; failure releases it; both are conditioned on an owner token so a stale worker cannot clobber a newer claim. A delivery that sees a fresh claim held elsewhere is deferred (AckHandle::defer: rescheduled to the lease expiry without consuming an attempt; JetStream nak with delay, which does count toward max_deliver). A done row skips the handler. Ledger rows are kept 30 days. A ledger read error skips the handler and nacks.

Guarantees

• No two deliveries run the same handler for the same event concurrently.
• At-least-once after a crash: a lost claim expires and the event is retried.
• Handlers are idempotent, so the remaining duplicate window (crash after the effect, before the ledger mark) is harmless. The O5 audit made each handler safe to re-run: image cleanup (NotFound is Ok), image conversion (probes the exact key; missing original means done), follow backfill and inbound review import (upserts, deterministic Create ids), person/movie enrichment (upserts; the movie profile write is last because it is the "done" marker), wrap-up (a Generating record is resumed; completion is atomic so WrapUpCompleted is emitted once), search handlers (keyed upserts and removes), poster sync (skips when the poster is already stored under the movie id, so no second PosterSynced/ImageStored).

Known gaps

• Remote and block follow events are best-effort. CompositeSocialAdapter routes remote follows and blocks through k-ap, which owns that transaction. Its events are published after the fact through an injected publisher, so a crash between the k-ap write and the publish loses them. Fixing it needs transaction hooks in k-ap.
• k-ap 0.5.0 mints random ids for some activities. Create, Announce and Like ids are deterministic (broadcast.rs:22-35, :283-288; backfill backfill.rs:208-212). Delete (broadcast.rs:157), Add/Undo (:210), Update (:321), Move (:377) and the actor update (:339-344) use Uuid::new_v4() via url_scheme.rs:52-53 with no override; only broadcast_raw_to_followers (:395) takes a caller id. A duplicate of those after a crash is a fresh-id activity. They are semantically idempotent for remote servers (deleting twice, updating to the same state), but it is a new activity, not a replay. The ledger makes this rare; closing it means building those activities ourselves or a k-ap change.
• Core NATS has no safety net. NATS_MODE=core is fire-and-forget: no broker dedup, no redelivery. Rows are marked done after publish; a subscriber that is down misses the event. The ledger still prevents double handling of a repeated id.
• ImageStored stays best-effort. It is raised after an object-storage write, which no database transaction can cover.

Consequences

• Existing deployments keep their JetStream stream. get_or_create_stream applies config only on creation, so the consumer now compares an existing stream and consumer with what this build needs and updates duplicate_window (1 h) and the consumer's max_deliver/ack_wait in place, with a warning in the log. Both are mutable on a live stream/consumer, so no recreate is needed; immutable consumer fields (name, filter, deliver policy) are never touched.
• Starting server and worker at the same moment against a database that still needs migration 0101 can make one fail with duplicate column name; start the server first, or restart the loser.
• Handler authors must keep new handlers idempotent; the ledger narrows duplicates but cannot remove the crash-after-effect case.