HTTP - Caching
HTTP is typically used for distributed information systems, where performance can be improved by the use of response caches. The HTTP/1.1 protocol includes a number of elements intended to make caching work.
The goal of caching in HTTP/1.1 is to eliminate the need to send requests in many cases, and to eliminate the need to send full responses in many other cases.
The basic cache mechanisms in HTTP/1.1 are implicit directives to caches where server-specifies expiration times and validators. We use the Cache-Control header for this purpose.
The Cache-Control header allows a client or server to transmit a variety of directives in either requests or responses. These directives typically override the default caching algorithms. The caching directives are specified in a comma-separated list. For example:
Cache-control: no-cache
The following cache request directives can be used by the client in its HTTP request:
| S.N. |
Cache Request Directive and Description |
| 1 |
no-cache A cache must not use the response to satisfy a subsequent request without successful revalidation with the origin server. |
| 2 |
no-store The cache should not store anything about the client request or server response. |
| 3 |
max-age = seconds Indicates that the client is willing to accept a response whose age is not greater than the specified time in seconds. |
| 4 |
max-stale [ = seconds ] Indicates that the client is willing to accept a response that has exceeded its expiration time. If seconds are given, it must not be expired by more than that time. |
| 5 |
min-fresh = seconds Indicates that the client is willing to accept a response whose freshness lifetime is not less than its current age plus the specified time in seconds. |
| 6 |
no-transform Does not convert the entity-body. |
| 7 |
only-if-cached Does not retrieve new data. The cache can send a document only if it is in the cache, and should not contact the origin-server to see if a newer copy exists. |
The following cache response directives can be used by the server in its HTTP response:
| S.N. |
Cache Response Directive and Description |
| 1 |
public Indicates that the response may be cached by any cache. |
| 2 |
private Indicates that all or part of the response message is intended for a single user and must not be cached by a shared cache. |
| 3 |
no-cache A cache must not use the response to satisfy a subsequent request without successful re-validation with the origin server. |
| 4 |
no-store The cache should not store anything about the client request or server response. |
| 5 |
no-transform Does not convert the entity-body. |
| 6 |
must-revalidate The cache must verify the status of stale documents before using it and expired ones should not be used. |
| 7 |
proxy-revalidate The proxy-revalidate directive has the same meaning as the must- revalidate directive, except that it does not apply to non-shared user agent caches. |
| 8 |
max-age = seconds Indicates that the client is willing to accept a response whose age is not greater than the specified time in seconds. |
| 9 |
s-maxage = seconds The maximum age specified by this directive overrides the maximum age specified by either the max-age directive or the Expires header. The s-maxage directive is always ignored by a private cache. |
