How would you test a background worker or message consumer that may receive the same job more than once and can crash partway through processing?

Test background workers and message consumers

Tests durable background processing across duplicate delivery, retries, acknowledgement timing, crashes, poison messages, and observable completion.

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I would unit-test the handler's business decisions, then add integration tests around durable state and delivery behavior. The main cases are successful processing, the same message delivered twice, a transient failure followed by retry, a permanent invalid message, and cancellation during shutdown. Assertions would verify the business result and the stored processing state so duplicate delivery cannot create a second charge, record, or notification. Time, identifiers, and retry delays would be controllable rather than based on wall-clock sleeps.

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Idempotency, retry behavior, broker semantics, durable state, fault injection, eventual assertions, dead-letter handling, and observability.

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Background processing fails across time and process boundaries. Useful tests need to prove the durable outcome, not merely that a handler method returned without throwing.
I would unit-test the handler's business decisions, then add integration tests around durable state and delivery behavior. The main cases are successful processing, the same message delivered twice, a transient failure followed by retry, a permanent invalid message, and cancellation during shutdown. Assertions would verify the business result and the stored processing state so duplicate delivery cannot create a second charge, record, or notification. Time, identifiers, and retry delays would be controllable rather than based on wall-clock sleeps.
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Where broker behavior matters, I would use an isolated real broker or a test container instead of a mock that can only call the handler once. The tests would cover acknowledgement after commit, visibility or lock expiry, concurrent delivery, ordering assumptions, retry limits, and dead-letter metadata. They would wait on an observable state with a bounded timeout rather than sleeping for a guessed duration. Message contracts and database constraints would participate in the test because idempotency often depends on a durable message id or business key.

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Calling the consumer method directly and treating that as broker coverage.
Using fixed sleeps to wait for eventual work.
Testing retry count without checking duplicate business effects.
Ignoring acknowledgement timing and process termination.

How would you test a background worker or message consumer that may receive the same job more than once and can crash partway through processing?

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