Overview

An ETag (Entity Tag) is an opaque string identifying a specific version of a resource representation. When the content changes, the ETag changes, so client and server compare it to decide whether a cache is still valid or to perform optimistic concurrency control.

The value is wrapped in double quotes and may carry a W/ prefix indicating a weak validator.

Details

Cache revalidation: when a client caches a resource it stores the ETag and sends it back on the next request as If-None-Match: "<etag>". If the current ETag matches, the server replies 304 Not Modified (no body) to save bandwidth; if not, it returns 200 with a fresh body.

Optimistic locking: sending If-Match: "<etag>" on an update makes the server apply the change only if the resource hasn't changed since, otherwise rejecting with 412 Precondition Failed. This prevents lost-update conflicts when multiple users edit concurrently.

A strong ETag matches only on byte-for-byte identity, while a weak ETag (W/) matches on semantic equivalence, ignoring trivial differences like compression. If-Range for range requests can only safely use a strong validator.

Syntax

ETag: [W/]"<opaque-validator>"

e.g. ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"

Notes

• The server is free to generate ETags however it likes — a content hash, a version number, or an inode+size+mtime combo are common. If several load-balanced servers emit different ETags for the same resource, revalidation keeps missing, so keep ETag generation consistent across servers.
• Some servers (e.g. default Apache) emit inode-based ETags, which can leak server details; switching to a content-hash scheme is recommended.
• Providing both ETag and Last-Modified lets clients revalidate more precisely. When both are present, the ETag (If-None-Match) takes precedence.

Related headers

Related status codes

Specification