Caching · Response header

ETag

A version identifier for the resource — the backbone of conditional GETs.

What is ETag?

ETag is an opaque string uniquely identifying a specific version of a resource. Clients send it back in If-None-Match on later requests; if unchanged, the server returns 304 Not Modified with no body — a huge bandwidth win.

Typical usage

Set on cacheable responses. Most frameworks generate ETags automatically.

Examples

Strong ETag (byte-identical)

ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"

Weak ETag (semantically equivalent)

ETag: W/"0815"

Common gotchas

Behind a load balancer, different instances must generate the same ETag for the same content, or caches will thrash. Base ETag on content hash, not memory address.

Specification

RFC 9110 §8.8.3

Related headers

If-None-Match

Send the resource only if its ETag has changed.

Cache-Control

Directives that control caching behavior.

Last-Modified

When the resource was last modified.

Expires

Legacy: an absolute HTTP date when the response becomes stale.

Age

How many seconds the response has been in a shared cache.

Vary

Which request headers cause different cached variants of the response.