IfNoneMatchETag

IfNoneMatchETag is an HTTP request header field used in conditional requests. It allows a client to specify one or more entity tags (ETags) that represent versions of a resource it has previously stored. The server then compares the provided ETags with the current ETag of the requested resource.

If the ETag provided by the client matches the current ETag of the resource on the server, it indicates that the client already possesses the latest version of the resource. In this scenario, the server will typically respond with a 304 Not Modified status code and an empty response body, saving bandwidth and processing time. This is a key optimization for caching.

Conversely, if the ETag provided by the client does not match the current ETag, or if the client has not seen the resource before (i.e., no ETags are provided), the server will respond with a 200 OK status code and the full content of the requested resource, along with a new ETag in the ETag response header.

ETags are opaque identifiers assigned by the server to a specific version of a resource. They can be based on various factors, such as a hash of the resource's content, a version number, or a modification timestamp. The IfNoneMatch header can contain a single ETag, a comma-separated list of ETags, or a wildcard asterisk (*). The wildcard asterisk specifically requests the resource only if it does not exist on the server, effectively checking for the existence of a resource without downloading its content. This header works in conjunction with the ETag response header and the IfMatch request header for comprehensive conditional request handling.