Conditional GET

A conditional GET is an HTTP request that carries a precondition based on a cache validator. The standard preconditions are If-None-Match, which checks against an ETag, and If-Modified-Since, which checks against a Last-Modified timestamp. The server evaluates the precondition before producing a response body. When the precondition indicates the client's cached copy is still current, the server returns 304 Not Modified. The 304 response carries no body, only headers that may need refreshing such as Cache-Control or a new ETag. The client reuses the body it already has, which is the core efficiency win: a small round trip avoids retransmitting potentially large payloads. Conditional requests are the standard revalidation step in HTTP Caching. When a stored response becomes stale under its max-age, or when the response is marked no-cache and must be revalidated before every use, the cache issues a conditional GET rather than an unconditional one. If the validator matches, the cache extends the response's freshness without a new download; if it does not match, the server returns 200 with a fresh body that replaces the stored one. Conditional requests extend beyond GET. Writes can use If-Match to implement optimistic concurrency control: the client supplies the ETag it last observed, and the server applies the change only if the ETag still matches the current resource state. If it has been updated by someone else, the server returns 412 Precondition Failed, prompting the client to refetch and retry. This pattern is used heavily by REST APIs for safe updates. If-Unmodified-Since serves the same role with Last-Modified semantics.