ETag

An entity tag (ETag) is an HTTP header used as a validator for web caches and conditional requests. It is an opaque identifier assigned by a server to a specific version of a resource, and is included in the representation of that resource, typically in the ETag response header.

How it works: When a server responds with a representation of a resource, it may attach an ETag that reflects the resource version. A client that caches the resource stores the ETag and later sends a conditional request using If-None-Match with the ETag value. If the resource has not changed, the server can respond with 304 Not Modified and no body, saving bandwidth. For updates, clients can use If-Match in requests such as PUT or PATCH to ensure the update applies only if the resource version matches, otherwise the server can return 412 Precondition Failed.

Strong and weak validation: ETags can be strong or weak. A strong ETag (no prefix) requires byte-for-byte identity for resource equivalence. A weak ETag is prefixed with W/ (for example, W/"12345") and allows semantically equivalent representations to share the same validator, even if the exact bytes differ.

Implementation and considerations: ETags are defined in RFC 7232 as part of HTTP/1.1. They aid efficient caching and concurrency control but require careful handling in shared caches and authentication scenarios. If content varies by user, or if metadata changes without content change, ETag management should reflect that (for example, using Vary headers or user-specific validators). ETags are typically generated from content hashes, version counters, or other versioning schemes, and must be updated whenever the represented resource changes.