ETagIfNoneMatch

ETagIfNoneMatch refers to the use of the ETag validator in conjunction with the If-None-Match HTTP request header to perform conditional requests. An ETag is a server-generated identifier that represents a specific version of a resource. The If-None-Match header lets a client ask the server to return the resource only if its current ETag differs from the one provided, enabling efficient caching and concurrency control.

How it works: The client sends If-None-Match with one or more ETag values (quoted strings) or the special value *. If the current ETag of the resource matches any of the provided values, the server can respond with 304 Not Modified (for safe methods like GET/HEAD) or apply its own policy for other methods. If there is no match, the server returns the requested representation (typically 200 OK) along with a new or updated ETag. The header supports both strong and weak validators (weak ETags are prefixed with W/), and multiple values can be listed comma-separated.

Usage and implications: This mechanism is primarily used for cache validation to avoid sending unchanged content. It also supports optimistic concurrency, allowing clients to detect changes before applying updates. The exact behavior for non-safe methods (such as POST or PUT) is implementation-dependent, but the general pattern is to prevent unnecessary writes when the client already holds the current version.

Example: A client requests GET /resource with If-None-Match: "abc123". If the server’s current ETag is "abc123", it responds 304 Not Modified; otherwise, it returns 200 OK with the resource and a new ETag.