Object is not being cached
When an object that is defined in the origin and in the appropriate path is not cached by Akamai, you should contact Rackspace Support for assistance.
However, the following information, which is extracted from several articles from Akamai, provides some insight into why this situation might occur.
Most common reason why cacheable objects are not cached on Akamai
The most common reason that cacheable objects are not cached is the presence of a Vary header in the origin server response.
The Vary header informs Akamai servers that the content of the object might differ based on some characteristic, such as the client’s specified user-agent or language, and is therefore not cacheable. This behavior is valid based on RFC HTTP/1.1.
However, if the Vary header has a single value of Vary: Accept-Encoding, and the response is accompanied by Content-Encoding: gzip, then Akamai caches the content. If the Vary header contains any values other than Accept-Encoding, Akamai does not cache the object.
This restriction is necessary to prevent different versions of content from being cached and then later served to end users for whom the content is not intended. For example, this prevents pages in one language from being cached and served to users of another language, or pages designed specifically for a certain browser from being cached and then served to users on another browser.
An Accept-Encoding value to the Vary header is the exception to this restriction only when it relates to serving the object compressed. Because compression does not change the content of the object (only its size for transfer), an object that varies only by its compression can be safely cached.
Other common reasons why objects are not cached
In addition to the presence of a Vary header in the response, following are the most prevalent reasons why objects that are specified to be cached are not cached on an edge server:
- Another configuration setting overrides the cache setting.
- Query strings are making your objects unique.
- The request is a POST request.
- The origin server returns a 302 redirect.
- The origin server sends no-store Cache-Control and Expires headers and the configuration is setup to honor Cache-Control and Expires headers.
- The origin server is sending an Edge-Control header specifying no-store.
- The Content Length header from the origin does not match the actual content length.
- The Date header from the origin does not represent meaningful time because the origin clock is not synchronized.
- The Age header from the origin is inconsistent.
- Authentication or revalidation settings require all objects to check freshness with the origin every time.
- The object is Edge Side Included (ESI)-processed or is a fragment from an ESI include.
- The ETag is mismatched from the origin.
- ESSL is used, and even though a request goes to the same virtual IP (VIP) address, it resolves to a different physical IP address behind the VIP. As a result, it appears that the VIP doesn’t have it in cache.
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