TL;DROne line, sized for the Lock Screen.
Repeating the action leaves the same result as doing it once.
{Idempotency}
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An action is idempotent if repeating it changes nothing after the first time. Saving a file with the same text twice leaves the file exactly as it was after the first save.
{Idempotency}
AnalogyA comparison from everyday life.
Like setting a thermostat to 21 degrees: set it twice and it stays at 21. "Raise by one degree" is the opposite, since every repeat changes the result.
{Idempotency}
ProfessionalPrecise and technical.
A property of an operation: applying it once or many times has the same effect on state. HTTP defines PUT, DELETE and the safe methods as idempotent (RFC 9110), while POST is not.
{Idempotency}
InterviewHow you’d explain it to an interviewer.
An operation is idempotent when running it several times has the same effect as running it once. This matters because networks fail and clients retry, and a retry must not charge a card twice or create a duplicate order. HTTP defines GET, PUT and DELETE as idempotent but not POST, so APIs often add an idempotency key that lets the server recognise a repeat and return the first result. Idempotency concerns the final state, not the response, so a second DELETE can return 404 and still be idempotent. It is the usual way to live with at-least-once delivery.
{Idempotency}
Example
-- Idempotent: same result however often it runs UPDATE accounts SET balance = 100 WHERE id = 7; -- Not idempotent: each run adds 10 more UPDATE accounts SET balance = balance + 10 WHERE id = 7;
Related
Idempotency key, HTTP method, Retry policy, At-least-once delivery, Idempotent consumer, Exactly-once delivery
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