What is a cache key?

A cache key is the identifier a cache stores a response under and looks it up by. In HTTP it is at least the request method and target URI, and `Vary` adds request headers to it. CDNs let you add or remove parts, such as query parameters, cookies, headers or the device type.

GET https://example.com/pricing?utm_source=mail
  key without query string: example.com/pricing
  key with query string:    example.com/pricing?utm_source=mail

Every distinct key is a separate entry with its own hit or miss. Extra parts in the key, such as tracking parameters, split one page into many entries, and lower the cache hit ratio. Amazon CloudFront's guide to raising the hit ratio recommends forwarding only the query string parameters, cookies and headers the origin actually uses.

• Remove from the key anything that does not change the response.
• Keep in it everything that does, or the cache serves the wrong variant. That includes the language and anything else listed in `Vary`.
• Cache warming fills one key per request, so warm the URL and header combinations your visitors actually use.