How would you respond if someone says the broker gives exactly-once delivery?

Explain the exactly-once messaging caveat

Explains why exactly-once claims do not remove the need for idempotent side effects and consistency boundaries.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

I would ask exactly-once for what boundary. A broker may provide strong guarantees inside its own log or producer-consumer protocol, but the application still writes to databases, sends emails, calls payment providers, and updates caches. Those side effects can succeed or fail around the broker acknowledgement. I would still design handlers to be idempotent.

Full answer below

Practice focus

Exactly-once semantics, broker boundaries, side effects, idempotent consumers, transactions, and practical guarantees.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Exactly-once guarantees apply only inside a defined broker or transactional boundary. Database writes, payments, email, caches, and acknowledgements still create failure windows that application idempotency must handle.
I would ask exactly-once for what boundary. A broker may provide strong guarantees inside its own log or producer-consumer protocol, but the application still writes to databases, sends emails, calls payment providers, and updates caches. Those side effects can succeed or fail around the broker acknowledgement. I would still design handlers to be idempotent.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I would avoid dismissing broker guarantees, but I would not extend them beyond their scope. If a stream processor writes back to the same transactional log, exactly-once semantics may be meaningful for that pipeline. If the consumer updates SQL and then commits an offset, there is still a failure window. The practical design uses idempotency keys, unique constraints, inbox/outbox records, and retry-safe handlers.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Believing exactly-once removes all duplicate side effects.
Rejecting broker guarantees without understanding their scope.
Ignoring database writes and external calls.
No crash testing around acknowledgement windows.

How would you respond if someone says the broker gives exactly-once delivery?

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue the section

Tech stacks

Skill areas

Suggest an improvement