What is idempotency in HTTP methods?

Idempotency is a key concept in HTTP methods, meaning that executing an operation multiple times should yield the same result. In the HTTP protocol, this implies that identical requests can be sent multiple times, but subsequent requests beyond the first should not alter the server state.

HTTP methods that are idempotent include GET, PUT, DELETE, HEAD, OPTIONS, and TRACE. These methods share the characteristic that repeated execution of the same request should not change the resource state.

• GET: Used to retrieve resources; multiple executions of the same GET request will not modify the server resource, only returning the data.
• PUT: Used to update the resource state to match the request body or create a new resource. Repeated execution of the same PUT request on the same URL should result in the resource state matching the last request, with intermediate requests having no effect.
• DELETE: Used to delete a resource; the initial request may remove the resource, but subsequent requests will have no effect since the resource is already gone.

For example, consider an API for managing blog articles, with an endpoint at /articles/{id}:

• For GET requests, multiple executions of GET /articles/123 will always return the same content, provided article 123 has not been modified or deleted.
• For PUT requests, repeated identical requests PUT /articles/123 with the same body containing new content will result in the first request updating article 123, while subsequent requests have no effect since the content is already current.
• For DELETE requests, the first DELETE /articles/123 may delete article 123, but subsequent identical requests will have no effect since the article is already gone.

In summary, understanding which HTTP methods are idempotent is crucial for developers to design stable and predictable API interfaces.