DNS & Glossary

ETag (Entity Tag)

A per-version fingerprint of a resource that lets clients revalidate cheaply — the server answers a conditional request with 304 Not Modified when the ETag still matches.

What it is

The server sends an ETag with a response; the client later revalidates with If-None-Match: <etag>. If unchanged, the server returns 304 Not Modified with no body. Strong ETags mean byte-for-byte identity; weak ones (W/) mean semantic equivalence. ETags also enable optimistic concurrency via If-Match.

Why it matters

  • Revalidation with ETags turns a full download into a tiny 304 exchange.
  • If-Match with ETags prevents lost updates in concurrent writes.

Common mistakes

  • Unstable ETags that change on every request, defeating caching.
  • Inconsistent ETags across a cluster, causing needless cache misses.

How WebInspect checks this

  • WebInspect reports ETag and Last-Modified and evaluates whether conditional requests can revalidate efficiently.