1. Celery tasks and RabbitMQ
Start with the contract: make inputs, outputs and failure behavior explicit before adding infrastructure. This keeps the feature easy to reason about and gives tests a stable boundary.
2. Delivery guarantees and idempotent consumers
Apply the pattern through a small vertical slice. Keep framework wiring at the edge and business decisions in focused functions or services that can be tested without starting the whole application.
@celery.task(bind=True, autoretry_for=(TemporaryError,), retry_backoff=True, max_retries=5)
def send_welcome_email(self, user_id: int):
if email_log.already_sent(user_id):
return
mailer.send_welcome(user_id)3. Retries, dead-letter handling and the outbox pattern
Treat failure paths as part of the design. Add bounded resource usage, meaningful errors and a verification step so the behavior remains dependable under real production conditions.
4. Hands-on Exercise
Build the feature
Publish a task-created event and consume it in an idempotent Celery worker. Demonstrate retry behavior without sending duplicate notifications.
Definition of done
- The happy path works through the real HTTP boundary.
- At least one failure path is handled and tested.
- Configuration and secrets stay outside source code.
- The README explains how to run and verify the result.
5. Knowledge Check
What does at-least-once delivery require from a consumer?
Show answer
The consumer must be idempotent because the same message may legitimately be delivered more than once.