HTTP – Caching
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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. |
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