What do safe and idempotent mean for HTTP methods, and why do those properties matter when clients retry requests?

Safe and idempotent HTTP method semantics

Explains HTTP safety and idempotency, then connects those guarantees to retries, side effects, and API design.

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A safe method is intended for retrieval and should not request a business-state change; GET, HEAD, and OPTIONS are common examples. An idempotent method can be repeated with the same intended effect as one request. PUT and DELETE are idempotent by definition, even though they are not safe, while POST is not generally idempotent. Idempotent does not mean every response is identical: a second DELETE can return 404 after the first one removed the resource.

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GET, HEAD, OPTIONS, PUT, DELETE, POST, safe methods, idempotent effects, retries, and application side effects.

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Retry behavior depends on what a method promises, not only on its name. Confusing safety with idempotency can turn ordinary network recovery into duplicate writes or hidden state changes.
A safe method is intended for retrieval and should not request a business-state change; GET, HEAD, and OPTIONS are common examples. An idempotent method can be repeated with the same intended effect as one request. PUT and DELETE are idempotent by definition, even though they are not safe, while POST is not generally idempotent. Idempotent does not mean every response is identical: a second DELETE can return 404 after the first one removed the resource.
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I design handlers so the method semantics remain true at the application boundary. A GET may still produce logs or metrics, but it must not charge a card or advance an order. A repeated PUT with the same representation should converge on the same resource state. DELETE should tolerate the resource already being absent according to the contract. For operations that naturally use POST, an idempotency key can give a client a safe retry path without pretending that all POST requests are inherently idempotent.

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Calling every non-POST method safe.
Assuming idempotent means the status code never changes.
Using GET for an operation that changes business state.
Making the database write idempotent while duplicating messages or emails.

What do safe and idempotent mean for HTTP methods, and why do those properties matter when clients retry requests?

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