API Design · Senior · Question 70 of 75

Why do event-driven APIs usually promise 'at-least-once' rather than 'exactly-once' delivery?

1At-least-once is faster to type
2Exactly-once is forbidden by HTTP
✓Correct answer: Exactly-once across networks and failures is impractical; at-least-once plus idempotent consumers gives the same effective result

Explanation

Document the guarantee and give each event a unique ID for deduplication.

Further reading

Test yourself

Take the API Design · Senior quiz: questions and options are shuffled, and you get a review of your mistakes at the end.