# Work Summary — 2026-08-05 20:17

## Task
Insight #2 from the `domains/amazon` design review: fire-and-forget listener notifications meant a dropped/failed SSE push was silently permanent, with the underlying event already marked `processed`. Fixed per a design worked out collaboratively with the user (not unilaterally chosen - this one had real tradeoffs).

## Design (agreed with user before implementing)
Non-blocking dispatch is a hard constraint - events come from external systems whose APIs may change unexpectedly, so the dispatcher must never wait on the listener's real work. The agreed shape:

- Three states, driven by two nullable timestamps rather than a stored `notify_again_at`: `unprocessed` (`notified_at IS NULL`) → `notified` (`notified_at` set, `processed_at` still null) → `processed` (`processed_at` set).
- The dispatcher sets `notified_at` and calls the listener; it never sets `processed_at` itself.
- The listener signals completion via a callback passed into `Notify` itself: `Notify(ctx, e, ack func(context.Context) error) error`. `ack` can be called synchronously (fast listeners, like the real SSE one) or arbitrarily later from elsewhere (slow/async listeners) - exactly once is the contract, and extra calls are safe (see below).
- If an event sits in `notified` past a retry threshold without being acked, the dispatcher's normal loop re-notifies it - reusing the existing poll/reactive machinery, no new loop. The threshold (`notifyRetryAfter`) is a Go-side field with a sensible default, not a stored per-row timestamp, so the cadence can change without touching any row (explicit user preference over storing `notify_again_at`).

## Changes
- `database_migrations/000031_amazon_event_notify_ack.{up,down}.sql`: drops `processed`/`processed_successfully` booleans on `mock.shop_amazon_events`, adds nullable `notified_at`/`processed_at` timestamps, updates the partial index accordingly. Applied to both the test DB and the real dev DB.
- `domains/amazon/mock.go`:
  - `MockEventListener.Notify` gained the `ack` parameter.
  - `Mocks` gained a `notifyRetryAfter` field (default 30s) and `WithNotifyRetryAfter(d)` builder for tuning/tests.
  - `processUnprocessedEvents`'s query now selects events that are either brand new or notified-but-stale (`processed_at IS NULL AND (notified_at IS NULL OR notified_at < retry_after)`).
  - Restructured so the async dispatch to the listener happens **after** the transaction that recorded `notified_at` has committed, not from inside it - `ack`'s own independent write can never race a still-open transaction it implicitly depends on being visible.
  - `ack` is idempotent (`... AND processed_at IS NULL` in its UPDATE), since a late ack from an earlier notification and a fresh one from a retry can both eventually fire for the same event.
- `server/sse/db_event_publisher.go`: the real listener now acks inline right after a successful SSE publish - its work is synchronous, so there's no reason to defer completion.
- `domains/amazon/mock_test.go`: updated `notifySpy` for the new signature (records ack callbacks, supports disabling auto-ack to simulate a listener that doesn't finish), updated `isProcessed`/added `isNotified` to check the new columns, and added `TestProcessUnprocessedEvents_RetriesUnackedNotification` - the core new-behavior test: a non-acking listener gets re-notified after the retry window (not before), and once *any* recorded ack for that event fires (including a stale one from an earlier attempt, not just the latest), the event settles into `processed` and stops being retried.

## A test bug caught and fixed along the way (not production code)
First run: `TestProcessUnprocessedEvents_ProcessesAllEventsAcrossBatches` failed (8/150 events still unprocessed) and the new retry test hung/timed out. Both were bugs in my own `notifySpy`, not the production code:
- It signaled "notified" *before* calling the auto-ack, so a test could observe "all N notified" before all N acks had actually landed in the DB.
- `waitForCount` drained a fixed number of channel signals per call instead of tracking a cumulative total, so calling it twice (once for 1, once for 2) double-counted and hung waiting for a signal that would never come.

Fixed by replacing the channel with a cumulative `completedCount` incremented only after any auto-ack attempt returns, and made `waitForCount` poll that count (safe to call repeatedly with increasing thresholds).

## Verification
- `go build ./...` / `go vet ./...` clean.
- `go test ./domains/amazon/... -v -race`: all 5 tests pass (4 existing + 1 new).
- 10x repeated runs (`-count=1 -race`) with no flakes, ~1.3-1.5s each.
- `make test` and `make test-against-dev-db`: full suite green both ways; confirmed zero leftover rows in the dev DB afterward.
- Migration applied cleanly to both the test DB and the real dev DB; verified `\d mock.shop_amazon_events` matches the intended shape on both.

## Follow-ups / not done here
- Insights #3 (hardcoded 1-minute poll interval) and #4 (a LISTEN-connection error is fatal to the *entire application*, most consequential) remain open - continuing one at a time per the user's request.