ETagbased

ETagbased refers to a caching mechanism in HTTP that relies on entity tags (ETags) as validators to determine whether a cached representation of a resource is current. An ETag is a server-generated identifier that represents a specific version of a resource. The server sends an ETag value in the response header along with the resource, and a Cache-Control header describing freshness rules.

On subsequent requests, a client may include the If-None-Match header with the stored ETag value. The server compares the provided ETag with the current version. If they match, the server responds with 304 Not Modified and no body, allowing the client to reuse the cached copy. If they do not match, the server returns the updated representation along with a new ETag.

ETags may be strong or weak. Strong ETags require byte-for-byte equivalence, while weak ETags (prefixed with W/) allow semantically equivalent versions to share a cache entry. The choice affects correctness in the presence of content transformations like compression, or reordering.

ETag generation schemes vary: it can be a hash of the response body, a version counter, or a combination. The important property is that the ETag changes when the resource content changes. In distributed environments, all caches and origin servers must agree on the same ETag semantics to ensure coherence.

Usage considerations include coordinating with Cache-Control for freshness, the impact on multi-server caches, and the need to avoid sensitive data leakage or user-specific variation in a shared cache. ETags should be invalidated when the content changes, and vary headers may be required if different representations exist for the same resource.

Limitations include potential cache fragmentation in load-balanced setups and the risk of stale validation if ETags are not updated properly. Alternatives include Last-Modified validators or explicit versioning strategies.