ETag

An ETag (entity tag) is an HTTP response header that identifies a specific version of a resource — typically a hash of its content. It lets browsers and CDNs ask "has this changed?" instead of re-downloading the whole thing: the client sends the ETag back in an If-None-Match request header, and if the version still matches, the server answers 304 Not Modified with an empty body.

How ETag revalidation works

The first response carries the tag:

HTTP/2 200
etag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
cache-control: max-age=3600

Once the cached copy's TTL expires, the client revalidates instead of refetching:

GET /style.css
if-none-match: "33a64df551425fcc55e4d42a148795d9f25f89d4"

HTTP/2 304 Not Modified

A 304 costs one round trip but no body transfer — for a large asset that's the difference between milliseconds and a full re-download. CDNs use the same mechanism upstream: an edge server revalidates its stale copy against your origin and refreshes the entry's freshness without moving bytes.

Strong vs. weak ETags

A strong ETag ("abc123") promises byte-for-byte identity. A weak ETag (W/"abc123") promises only semantic equivalence — the content is "the same enough" (say, a timestamp in a footer changed). Weak ETags can't be used for byte-range requests, but both work for revalidation.

The pitfalls worth knowing

• Multi-server setups: Apache's default ETag once included inode numbers, so identical files on different servers produced different tags — breaking revalidation behind load balancers. Derive ETags from content only.
• Compression changes bytes: some servers emit different ETags for gzip, Brotli and identity variants of the same resource (or append -gzip suffixes). Combined with Vary: Accept-Encoding this is correct behavior, but it surprises people diffing headers. Check what your stack sends with our compression test.
• ETag ≠ caching policy: an ETag alone doesn't make a response cacheable — it only enables revalidation. Freshness still comes from Cache-Control.

ETags and CDN caching

Revalidation makes expiry cheap, but the visitor still waits for the origin round trip whenever the edge copy has expired. That's why a healthy setup combines sensible ETags with a long s-maxage — and re-warms edge caches after purges, so real visitors hit fresh copies instead of triggering revalidations. See our cache hit ratio guide for where revalidation fits into the bigger picture.