Lab: idempotency for background jobs and consumers¶
Redis 7 runs in a disposable container. Queue redelivery is modeled in memory, and a local mail provider counts sends: this lab starts neither Kafka nor an SMTP server.
Acknowledgement fails after the first action and its result have been stored. The same invoice arrives with a new delivery ID. Assertions verify two deliveries and one successful ACK: without coordination there are two emails, with it there is one and the saved email ID is returned.
The key identifies tenant, order and invoice version. A new version sends another email; changing the recipient of an existing version raises IdempotencyKeyReuseError. The failure is injected at ACK, not by killing an OS process. The separate keys lab covers a failure before result storage and concurrent callers.
Requires Docker and uv. Run from docs/blog/lab/2026-09-07-idempotency-for-jobs-and-consumers:
Dependencies are pinned in requirements.txt: idempotency-kit 0.4.1, redis-py 8.1.0, Pydantic 2.13.5, testcontainers 4.15.0, HTTPX 0.28.1 and aiohttp 3.14.3.
Keep the sibling 2026-09-07-idempotency-keys directory: this lab imports its shared code and requirements.