CDN cache keys and purging: what identifies a cached object and how to invalidate it

What it is

RFC 9111 defines the cache key as the information a cache uses to choose a stored response, composed at a minimum from the request method and the target URI; when a stored response carries Vary, the cache may only reuse it if the request header fields named there match those of the original request. A CDN makes this concrete. Cloudflare's documentation lists what its default cache key includes: the scheme, the host, the URI with query string, the Origin header and a set of method- and host-override headers; custom cache keys can add or drop parts, and the documentation warns that they may reduce the hit rate and result in cache sharding. Purging is the reverse operation: removing objects by URL, by hostname, by prefix, by everything, or by tag. Fastly's surrogate keys are the tag model: the origin sends a Surrogate-Key header naming one or more keys, and a purge of a key removes all objects associated with it; other CDNs offer equivalent tag headers.

Why it matters

Two bugs come from the key. If it is too coarse (query string ignored, Accept-Language not in Vary), users receive each other's variants. If it is too fine (every tracking parameter, a session cookie), the hit rate collapses. Purging fails silently when the purge URL does not match the stored key: a different scheme, a trailing slash, an encoded character or a query string leaves the object in place.

How to apply

• Decide per path which parts of the request may change the response, and put exactly those in the key: strip known tracking parameters, sort the query string, keep Vary short.
• Tag every cacheable response from the origin with the identifiers of the data it was built from (article ID, product ID, template version), and purge by tag when that data changes.
• Reserve purge-everything for template or code changes, and expect origin load to spike afterwards.
• For fingerprinted assets, never purge; a new name is a new key.
• After a purge, verify with a request that shows the cache status header and the object's age.

Pitfalls

Purge APIs commonly have rate limits and a propagation delay; a script that purges one URL per change on a busy site runs into both. Vary: * or Vary: Cookie makes responses effectively uncacheable. Tags are attached at store time, so objects cached before tagging was introduced cannot be purged by tag.