ETags

ETags, or entity tags, are opaque validators assigned by a web server to a specific version of a resource. They enable efficient cache validation and support conditional requests, allowing clients to determine whether a cached representation matches the current version without transferring the full content.

In HTTP/1.1, an ETag is returned in the ETag response header. Values are strings enclosed in quotes; a prefix of W/ denotes a weak validator. For example: ETag: "abc123" or ETag: W/"abc123". A strong validator indicates byte-for-byte equality, while a weak validator may signal semantic equivalence and can be considered less strict for caching purposes.

Clients use ETags to perform conditional requests. With If-None-Match, a client asks for the resource only if it has changed; if unchanged, the server can respond with 304 Not Modified and no body, saving bandwidth. Conversely, If-Match is used when updating a resource, ensuring the operation proceeds only if the ETag still matches; if not, the server may return 412 Precondition Failed. ETags are often used alongside other cache headers like Cache-Control to manage freshness and validation.

Generation and interpretation: ETags are generated by the server, commonly as a hash of the resource contents or a version token that changes when the resource updates. They are opaque to clients and should be considered identifiers that change when the resource version changes. Weak versus strong validators determine how strictly changes are validated during cache comparisons.

Limitations: ETags can complicate caching in distributed environments and may reveal internal details if not managed carefully. Not all intermediaries handle them uniformly. They are typically used together with other HTTP caching mechanisms to ensure robust and predictable behavior. RFC 7232 formalizes their use in HTTP.