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HTTP Caching
HTTP Caching
Typically, HTTP is used for distributed information systems, where the use of response caches can improve performance. The HTTP/1.1 protocol contains a number of elements designed to make caching work.
The goal of caching in HTTP/1.1 is to eliminate the need to send requests in many cases, and in many other cases to eliminate the need to send full replies.
In HTTP/1.1 the basic cache mechanisms are implicit directives for caches where expiration times and validators are specified by the server. To this end, we use the header Cache-Control.
The header Cache-Control allows a client or server to convey a variety of directives in either requests or responses. Typically these directives override the standard caching algorithms. The directives for caching are set out in a comma-separated list. Take , for example:
Cache-control: no-cache
The client may use the following cache request directives in its HTTP request:
| Sr No | Cache Request Directive and Description |
|---|---|
| 1. | no-cache A cache must not use the answer to satisfy a subsequent request with the origin server without successful revalidation. |
| 2. | no-store Nothing about the client request or the server response should be stored in the cache. |
| 3. | max-age = seconds Indicates the client's willingness to accept a response whose age in seconds is no greater than the time specified. |
| 4. | max-stale [ = seconds ] Indicates that the client is prepared to accept a response that has exceeded its expiry time. If seconds are given, they should not expire by more than that time. |
| 5. | min-fresh = seconds Indicates that the client is willing to accept a response whose lifetime of freshness is no less than its current age plus the time in seconds specified. |
| 6. | no-transform Does not convert the entity-body. |
| 7. | only-if-cached Recovers no new data. The cache can only send a document if it is in the cache, and should not contact the origin-server to see if there is a newer copy. |
The server may use the following cache response directives in its HTTP response:
| Sr No | Cache Response Directive and Description |
|---|---|
| 1. | public Indicates that every cache can cache the response. |
| 2. | private Indicates that all or part of the response message is for a single user and must not be cached through a shared cache. |
| 3. | no-cache Without successful re-validation with the origin server, a cache must not use the response to satisfy a subsequent request. |
| 4. | no-store Nothing about the client request or the server response should be stored in the cache. |
| 5. | no-transform Does not convert the entity-body. |
| 6. | must-revalidate Before using it, the cache must check the status of stale documents, and should not use expired documents. |
| 7. | proxy-revalidate The proxy-revalidate directive has the same meaning as the directive on the need-revalidate except that it does not apply to caches of non-shared user agents. |
| 8. | max-age = seconds Indicates the client's willingness to accept a response whose age in seconds is no greater than the time specified. |
| 9. | s-maxage = seconds The maximum age specified by this directive overrides the maximum age specified by either the directive max-age or the header Expires. A private cache shall always ignore the s-maxage directive |