ETag header
If the cache expires, the client requests with the ETag = fingerprint (as sent by the server in the past). The server checks the fingerprint and returns Not_modified if not changed.
If the file changes before it expires in the cache, that cannot be communicated to the browser.
Thus, file versioning (by index / fingerprinting) is recommended for invalidation control.
Recommended expiry
1 week to 1 year for resources.
Updated:
2026 Aug 19
