How Is an Asset's TTL Calculated?

The amount of time that an asset will be cached on our edge servers is determined by the response headersThis type of header describes the HTTP response provided by a server. Among their many uses, certain response headers may be used to define the cache policy for the requested content. returned by the origin server when a client requests it. By default, our CDN honors the following response headers:

Response Header	Description
Cache-Control: private	Prevents our edge servers from caching the response.
Cache-Control: no-store	Prevents our edge servers from caching the response.
Cache-Control: no-cache	Prevents our edge servers or the client from caching the response.
Pragma: no-cache	Prevents our edge servers or the client from caching the response.
Cache-Control: s-maxage	Determines the requested content's TTL on our edge serversThe length of time that the requested content will be cached on our network . This response header is ignored if the response contains any of the above response headers.
Cache-Control: max-age	This response header is ignored if the response contains any of the above response headers. Determines the requested content's TTL on the user agent and our edge servers. By default, caching is disabled on the ADN platform.
Expires	Defines an expiration date for the requested content's TTL. The requested content will be considered stale after the specified date/time. If this directive has been assigned an invalid value, including zero, then it will be ignored for the request in question. This response header is ignored if the response contains any of the above response headers.

Key information:

• By default, content will only be cached after a POP receives two GET requests that result in a 200 OK response.
• If none of the above headers are included with the response from the origin server, then the requested content will be cached for 7 days, by default, on the POP handling the request and by the requesting client's user agent.
• The above directives are ordered according to precedence. Higher directives take precedence over lower directives. In other words, if an asset contains both a Cache-Control and an Expires header, then the Cache-Control header will take precedence.
• Please refer to your web server’s documentation for more information on how to configure these settings.

• Rules Engine can be used to override a web server's cache policy.

  Key features that override cache policy are:

  • Bypass Cache
  • Cache-Control Header Treatment
  • External Max-Age

More Information

• Response (Response Headers)