Apache Module mod_cache

CacheDefaultExpire Directive

The CacheDefaultExpire directive specifies a default time, in seconds, to cache a document if neither an expiry date nor last-modified date are provided with the document. The value specified with the CacheMaxExpire directive does not override this setting.

CacheDefaultExpire 86400

CacheDetailHeader Directive

When the CacheDetailHeader directive is switched on, an X-Cache-Detail header will be added to the response containing the detailed reason for a particular caching decision.

It can be useful during development of cached RESTful services to have additional information about the caching decision written to the response headers, so as to confirm whether Cache-Control and other headers have been correctly used by the service and client.

If the normal handler is used, this directive may appear within a <Directory> or <Location> directive. If the quick handler is used, this directive must appear within a server or virtual host context, otherwise the setting will be ignored.

# Enable the X-Cache-Detail header
CacheDetailHeader on

CacheDisable Directive

The CacheDisable directive instructs mod_cache to not cache urls at or below url-string.

CacheDisable "/local_files"

If used in a <Location> directive, the path needs to be specified below the Location, or if the word "on" is used, caching for the whole location will be disabled.

<Location "/foo">
    CacheDisable on
</Location>

The no-cache environment variable can be set to disable caching on a finer grained set of resources in versions 2.2.12 and later.

See also

• Environment Variables in Apache

CacheEnable Directive

The CacheEnable directive instructs mod_cache to cache urls at or below url-string. The cache storage manager is specified with the cache_type argument. The CacheEnable directive can alternatively be placed inside either <Location> or <LocationMatch> sections to indicate the content is cacheable. cache_type disk instructs mod_cache to use the disk based storage manager implemented by mod_cache_disk. cache_type socache instructs mod_cache to use the shared object cache based storage manager implemented by mod_cache_socache.

In the event that the URL space overlaps between different CacheEnable directives (as in the example below), each possible storage manager will be run until the first one that actually processes the request. The order in which the storage managers are run is determined by the order of the CacheEnable directives in the configuration file. CacheEnable directives within <Location> or <LocationMatch> sections are processed before globally defined CacheEnable directives.

When acting as a forward proxy server, url-string must minimally begin with a protocol for which caching should be enabled.

# Cache content (normal handler only)
CacheQuickHandler off
<Location "/foo">
    CacheEnable disk
</Location>

# Cache regex (normal handler only)
CacheQuickHandler off
<LocationMatch "foo$">
    CacheEnable disk
</LocationMatch>

# Cache all but forward proxy url's (normal or quick handler)
CacheEnable  disk  /

# Cache FTP-proxied url's (normal or quick handler)
CacheEnable  disk  ftp://

# Cache forward proxy content from www.example.org (normal or quick handler)
CacheEnable  disk  http://www.example.org/

A hostname starting with a "*" matches all hostnames with that suffix. A hostname starting with "." matches all hostnames containing the domain components that follow.

# Match www.example.org, and fooexample.org
CacheEnable  disk  "http://*example.org/"
# Match www.example.org, but not fooexample.org
CacheEnable  disk  "http://.example.org/"

The no-cache environment variable can be set to disable caching on a finer grained set of resources in versions 2.2.12 and later.

See also

• Environment Variables in Apache

CacheHeader Directive

When the CacheHeader directive is switched on, an X-Cache header will be added to the response with the cache status of this response. If the normal handler is used, this directive may appear within a <Directory> or <Location> directive. If the quick handler is used, this directive must appear within a server or virtual host context, otherwise the setting will be ignored.

HIT

The entity was fresh, and was served from cache.

REVALIDATE

The entity was stale, was successfully revalidated and was served from cache.

MISS

The entity was fetched from the upstream server and was not served from cache.

# Enable the X-Cache header
CacheHeader on

X-Cache: HIT from localhost

CacheIgnoreCacheControl Directive

Ordinarily, requests containing a Cache-Control: no-cache or Pragma: no-cache header value will not be served from the cache. The CacheIgnoreCacheControl directive allows this behavior to be overridden. CacheIgnoreCacheControl On tells the server to attempt to serve the resource from the cache even if the request contains no-cache header values. Resources requiring authorization will never be cached.

CacheIgnoreCacheControl On

See also

• CacheStorePrivate
• CacheStoreNoStore

CacheIgnoreHeaders Directive

According to RFC 2616, hop-by-hop HTTP headers are not stored in the cache. The following HTTP headers are hop-by-hop headers and thus do not get stored in the cache in any case regardless of the setting of CacheIgnoreHeaders:

• Connection
• Keep-Alive
• Proxy-Authenticate
• Proxy-Authorization
• TE
• Trailers
• Transfer-Encoding
• Upgrade

CacheIgnoreHeaders specifies additional HTTP headers that should not to be stored in the cache. For example, it makes sense in some cases to prevent cookies from being stored in the cache.

CacheIgnoreHeaders takes a space separated list of HTTP headers that should not be stored in the cache. If only hop-by-hop headers not should be stored in the cache (the RFC 2616 compliant behaviour), CacheIgnoreHeaders can be set to None.

CacheIgnoreHeaders Set-Cookie

CacheIgnoreHeaders None

CacheIgnoreNoLastMod Directive

Ordinarily, documents without a last-modified date are not cached. Under some circumstances the last-modified date is removed (during mod_include processing for example) or not provided at all. The CacheIgnoreNoLastMod directive provides a way to specify that documents without last-modified dates should be considered for caching, even without a last-modified date. If neither a last-modified date nor an expiry date are provided with the document then the value specified by the CacheDefaultExpire directive will be used to generate an expiration date.

CacheIgnoreNoLastMod On

CacheIgnoreQueryString Directive

Ordinarily, requests with query string parameters are cached separately for each unique query string. This is according to RFC 2616/13.9 done only if an expiration time is specified. The CacheIgnoreQueryString directive tells the cache to cache requests even if no expiration time is specified, and to reply with a cached reply even if the query string differs. From a caching point of view the request is treated as if having no query string when this directive is enabled.

CacheIgnoreQueryString On

CacheIgnoreURLSessionIdentifiers Directive

Sometimes applications encode the session identifier into the URL like in the following Examples:

• /someapplication/image.gif;jsessionid=123456789
• /someapplication/image.gif?PHPSESSIONID=12345678

This causes cachable resources to be stored separately for each session, which is often not desired. CacheIgnoreURLSessionIdentifiers lets define a list of identifiers that are removed from the key that is used to identify an entity in the cache, such that cachable resources are not stored separately for each session.

CacheIgnoreURLSessionIdentifiers None clears the list of ignored identifiers. Otherwise, each identifier is added to the list.

CacheIgnoreURLSessionIdentifiers jsessionid

CacheIgnoreURLSessionIdentifiers None

CacheKeyBaseURL Directive

When the CacheKeyBaseURL directive is specified, the URL provided will be used as the base URL to calculate the URL of the cache keys in the reverse proxy configuration. When not specified, the scheme, hostname and port of the current virtual host is used to construct the cache key. When a cluster of machines is present, and all cached entries should be cached beneath the same cache key, a new base URL can be specified with this directive.

# Override the base URL of the cache key.
CacheKeyBaseURL "http://www.example.com/"

CacheLastModifiedFactor Directive

In the event that a document does not provide an expiry date but does provide a last-modified date, an expiry date can be calculated based on the time since the document was last modified. The CacheLastModifiedFactor directive specifies a factor to be used in the generation of this expiry date according to the following formula: expiry-period = time-since-last-modified-date * factor expiry-date = current-date + expiry-period For example, if the document was last modified 10 hours ago, and factor is 0.1 then the expiry-period will be set to 10*0.1 = 1 hour. If the current time was 3:00pm then the computed expiry-date would be 3:00pm + 1hour = 4:00pm. If the expiry-period would be longer than that set by CacheMaxExpire, then the latter takes precedence.

CacheLastModifiedFactor 0.5

CacheLock Directive

The CacheLock directive enables the thundering herd lock for the given URL space.

In a minimal configuration the following directive is all that is needed to enable the thundering herd lock in the default system temp directory.

# Enable cache lock
CacheLock on

CacheLockMaxAge Directive

The CacheLockMaxAge directive specifies the maximum age of any cache lock.

A lock older than this value in seconds will be ignored, and the next incoming request will be given the opportunity to re-establish the lock. This mechanism prevents a slow client taking an excessively long time to refresh an entity.

CacheLockPath Directive

The CacheLockPath directive allows you to specify the directory in which the locks are created. By default, the system's temporary folder is used. Locks consist of empty files that only exist for stale URLs in flight, so is significantly less resource intensive than the traditional disk cache.

CacheMaxExpire Directive

The CacheMaxExpire directive specifies the maximum number of seconds for which cachable HTTP documents will be retained without checking the origin server. Thus, documents will be out of date at most this number of seconds. This maximum value is enforced even if an expiry date was supplied with the document.

CacheMaxExpire 604800

CacheMinExpire Directive

The CacheMinExpire directive specifies the minimum number of seconds for which cachable HTTP documents will be retained without checking the origin server. This is only used if no valid expire time was supplied with the document.

CacheMinExpire 3600

CacheQuickHandler Directive

The CacheQuickHandler directive controls the phase in which the cache is handled.

In the default enabled configuration, the cache operates within the quick handler phase. This phase short circuits the majority of server processing, and represents the most performant mode of operation for a typical server. The cache bolts onto the front of the server, and the majority of server processing is avoided.

When disabled, the cache operates as a normal handler, and is subject to the full set of phases when handling a server request. While this mode is slower than the default, it allows the cache to be used in cases where full processing is required, such as when content is subject to authorisation.

# Run cache as a normal handler
CacheQuickHandler off

It is also possible, when the quick handler is disabled, for the administrator to choose the precise location within the filter chain where caching is to be performed, by adding the CACHE filter to the chain.

# Cache content before mod_include and mod_deflate
CacheQuickHandler off
AddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html

If the CACHE filter is specified more than once, the last instance will apply.

CacheStaleOnError Directive

When the CacheStaleOnError directive is switched on, and when stale data is available in the cache, the cache will respond to 5xx responses from the backend by returning the stale data instead of the 5xx response. While the Cache-Control headers sent by clients will be respected, and the raw 5xx responses returned to the client on request, the 5xx response so returned to the client will not invalidate the content in the cache.

# Serve stale data on error.
CacheStaleOnError on

CacheStoreExpired Directive

Since httpd 2.2.4, responses which have already expired are not stored in the cache. The CacheStoreExpired directive allows this behavior to be overridden. CacheStoreExpired On tells the server to attempt to cache the resource if it is stale. Subsequent requests would trigger an If-Modified-Since request of the origin server, and the response may be fulfilled from cache if the backend resource has not changed.

CacheStoreExpired On

CacheStoreNoStore Directive

Ordinarily, requests or responses with Cache-Control: no-store header values will not be stored in the cache. The CacheStoreNoStore directive allows this behavior to be overridden. CacheStoreNoStore On tells the server to attempt to cache the resource even if it contains no-store header values. Resources requiring authorization will never be cached.

CacheStoreNoStore On

See also

• CacheIgnoreCacheControl
• CacheStorePrivate

CacheStorePrivate Directive

Ordinarily, responses with Cache-Control: private header values will not be stored in the cache. The CacheStorePrivate directive allows this behavior to be overridden. CacheStorePrivate On tells the server to attempt to cache the resource even if it contains private header values. Resources requiring authorization will never be cached.

CacheStorePrivate On

See also

• CacheIgnoreCacheControl
• CacheStoreNoStore