
- HTTP - Home
- HTTP - Overview
- HTTP - Parameters
- HTTP - Messages
- HTTP - Requests
- HTTP - Responses
- HTTP - Methods
- HTTP - Status Codes
- HTTP - Header Fields
- HTTP - Caching
- HTTP - URL Encoding
- HTTP - Security
- HTTP - Message Examples
- HTTP - Versions
- HTTP - Connection Management
- HTTP - Content Negotiation
- HTTP - Redirection
- HTTP - Authentication and Authorization
- HTTP - HTTP over TLS(HTTPS)
- HTTP - HTTP/2 and HTTP/3 Features
- HTTP - API Design Considerations
- HTTP - Troubleshooting
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
Cache Request Directive
The following cache request directives can be used by the client in its HTTP request:
Cache Request Directive | Description |
---|---|
no-cache | A cache must not use the response to satisfy a subsequent request without successful revalidation with the origin server. |
no-store | The cache should not store anything about the client request or server response. |
max-age = seconds | Indicates that the client is willing to accept a response whose age is not greater than the specified time in seconds. |
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. |
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. |
no-transform | Does not convert the entity-body. |
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. |
Cache Response Directive
The following cache response directives can be used by the server in its HTTP response:
Cache Response Directive | Description |
---|---|
public | Indicates that the response may be cached by any cache. |
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. |
no-cache | A cache must not use the response to satisfy a subsequent request without successful re-validation with the origin server. |
no-store | The cache should not store anything about the client request or server response. |
no-transform | Does not convert the entity-body. |
must-revalidate | The cache must verify the status of the stale documents before using it and expired ones should not be used. |
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. |
max-age = seconds | Indicates that the client is willing to accept a response whose age is not greater than the specified time in seconds. |
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. |