What is Idempotency in REST APIs, and how should HTTP methods and status codes be structured?

An HTTP method is idempotent if making multiple identical requests has the same intended effect on the server state as a single request (i.e. $f(f(x)) = f(x)$).

• GET, HEAD, OPTIONS: Safe and idempotent (read-only, no server state mutation).
• PUT: Idempotent (replaces entire resource; calling PUT 10 times results in the same state).
• DELETE: Idempotent (deleting resource once or 5 times leaves it deleted; retu s 200/204 or 404).
• POST: Not idempotent by default (each POST call creates a new sub-resource or transaction, e.g. charging a payment card).

Standard HTTP Status Codes:

• 200 OK: Successful query or update.
• 201 Created: Successful resource creation (with Location header).
• 204 No Content: Successful action with no response body (common in DELETE).
• 400 Bad Request: Malformed payload or validation error.
• 401 Unauthorized: Missing or invalid authentication credentials.
• 403 Forbidden: Authenticated user lacks permission.
• 409 Conflict: State collision (e.g. duplicate unique key).

• 500 Inte
  al Server Error

  : Uncaught server-side failure.