Apache Module mod_cache

Summary

mod_cache implements anRFC 2616 compliantHTTP content caching filter, with support for the caching of content negotiated responses containing the Vary header.

RFC 2616 compliant caching provides a mechanism to verify whether stale or expired content is still fresh, and can represent a significant performance boost when the origin server supportsconditional requests by honouring theIf-None-Match HTTP request header. Content is only regenerated from scratch when the content has changed, and not when the cached entry expires.

As a filter,mod_cache can be placed in front of content originating from any handler, includingflat files (served from a slow disk cached on a fast disk), the output of aCGI script ordynamic content generator, or contentproxied from another server.

In the default configuration,mod_cache inserts the caching filter as far forward as possible within the filter stack, utilising thequick handler to bypass all per request processing when returning content to the client. In this mode of operation,mod_cache may be thought of as a caching proxy server bolted to the front of the webserver, while running within the webserver itself.

When the quick handler is switched off using theCacheQuickHandler directive, it becomes possible to insert theCACHE filter at a point in the filter stack chosen by the administrator. This provides the opportunity to cache content before that content is personalised by themod_include filter, or optionally compressed by themod_deflate filter.

Under normal operation,mod_cache will respond to and can be controlled by theCache-Control andPragma headers sent from a client in a request, or from a server within a response. Under exceptional circumstances,mod_cache can be configured to override these headers and force site specific behaviour, however such behaviour will be limited to this cache only, and will not affect the operation of other caches that may exist between the client and server, and as a result is not recommended unless strictly necessary.

RFC 2616 allows for the cache to return stale data while the existing stale entry is refreshed from the origin server, and this is supported bymod_cache when theCacheLock directive is suitably configured. Such responses will contain aWarning HTTP header with a 110 response code. RFC 2616 also allows a cache to return stale data when the attempt made to refresh the stale data returns an error 500 or above, and this behaviour is supported by default bymod_cache. Such responses will contain aWarning HTTP header with a 111 response code.

mod_cache requires the services of one or more storage management modules. The following storage management modules are included in the base Apache distribution:

mod_cache_disk

Implements a disk based storage manager. Headers and bodies are stored separately on disk, in a directory structure derived from the md5 hash of the cached URL. Multiple content negotiated responses can be stored concurrently, however the caching of partial content is not supported by this module. Thehtcacheclean tool is provided to list cached URLs, remove cached URLs, or to maintain the size of the disk cache within size and inode limits.

mod_cache_socache

Implements a shared object cache based storage manager. Headers and bodies are stored together beneath a single key based on the URL of the response being cached. Multiple content negotiated responses can be stored concurrently, however the caching of partial content is not supported by this module.

Further details, discussion, and examples, are provided in theCaching Guide.

Topics

• Related Modules and Directives
• Sample Configuration
• Avoiding the Thundering Herd
• Fine Control with the CACHE Filter
• Cache Status and Logging

Directives

• CacheDefaultExpire
• CacheDetailHeader
• CacheDisable
• CacheEnable
• CacheHeader
• CacheIgnoreCacheControl
• CacheIgnoreHeaders
• CacheIgnoreNoLastMod
• CacheIgnoreQueryString
• CacheIgnoreURLSessionIdentifiers
• CacheKeyBaseURL
• CacheLastModifiedFactor
• CacheLock
• CacheLockMaxAge
• CacheLockPath
• CacheMaxExpire
• CacheMinExpire
• CacheQuickHandler
• CacheStaleOnError
• CacheStoreExpired
• CacheStoreNoStore
• CacheStorePrivate

Bugfix checklist

• httpd changelog
• Known issues
• Report a bug

See also

• Caching Guide
• Comments

Related Modules and Directives

Sample Configuration

## Sample Cache Configuration#LoadModule cache_module modules/mod_cache.so<IfModule mod_cache.c>    LoadModule cache_disk_module modules/mod_cache_disk.so    <IfModule mod_cache_disk.c>        CacheRoot "c:/cacheroot"        CacheEnable disk  "/"        CacheDirLevels 5        CacheDirLength 3    </IfModule>    # When acting as a proxy, don't cache the list of security updates    CacheDisable "http://security.update.server/update-list/"</IfModule>

Avoiding the Thundering Herd

When a cached entry becomes stale,mod_cache will submit a conditional request to the backend, which is expected to confirm whether the cached entry is still fresh, and send an updated entity if not.

A small but finite amount of time exists between the time the cached entity becomes stale, and the time the stale entity is fully refreshed. On a busy server, a significant number of requests might arrive during this time, and cause athundering herd of requests to strike the backend suddenly and unpredictably.

To keep the thundering herd at bay, theCacheLock directive can be used to define a directory in which locks are created for URLsin flight. The lock is used as ahint by other requests to either suppress an attempt to cache (someone else has gone to fetch the entity), or to indicate that a stale entry is being refreshed (stale content will be returned in the mean time).

Initial caching of an entry

When an entity is cached for the first time, a lock will be created for the entity until the response has been fully cached. During the lifetime of the lock, the cache will suppress the second and subsequent attempt to cache the same entity. While this doesn't hold back the thundering herd, it does stop the cache attempting to cache the same entity multiple times simultaneously.

Refreshment of a stale entry

When an entity reaches its freshness lifetime and becomes stale, a lock will be created for the entity until the response has either been confirmed as still fresh, or replaced by the backend. During the lifetime of the lock, the second and subsequent incoming request will cause stale data to be returned, and the thundering herd is kept at bay.

Locks and Cache-Control: no-cache

Locks are used as ahint only to enable the cache to be more gentle on backend servers, however the lock can be overridden if necessary. If the client sends a request with a Cache-Control header forcing a reload, any lock that may be present will be ignored, and the client's request will be honored immediately and the cached entry refreshed.

As a further safety mechanism, locks have a configurable maximum age. Once this age has been reached, the lock is removed, and a new request is given the opportunity to create a new lock. This maximum age can be set using theCacheLockMaxAge directive, and defaults to 5 seconds.

Example configuration

## Enable the cache lock#<IfModule mod_cache.c>    CacheLock on    CacheLockPath "/tmp/mod_cache-lock"    CacheLockMaxAge 5</IfModule>

Fine Control with the CACHE Filter

Under the default mode of cache operation, the cache runs as a quick handler, short circuiting the majority of server processing and offering the highest cache performance available.

In this mode, the cachebolts onto the front of the server, acting as if a free standing RFC 2616 caching proxy had been placed in front of the server.

While this mode offers the best performance, the administrator may find that under certain circumstances they may want to perform further processing on the request after the request is cached, such as to inject personalisation into the cached page, or to apply authorization restrictions to the content. Under these circumstances, an administrator is often forced to place independent reverse proxy servers either behind or in front of the caching server to achieve this.

To solve this problem theCacheQuickHandler directive can be set tooff, and the server will process all phases normally handled by a non-cached request, including theauthentication and authorization phases.

In addition, the administrator may optionally specify theprecise point within the filter chain where caching is to take place by adding theCACHE filter to the output filter chain.

For example, to cache content before applying compression to the response, place theCACHE filter before theDEFLATE filter as in the example below:

# Cache content before optional compressionCacheQuickHandler offAddOutputFilterByType CACHE;DEFLATE text/plain

Another option is to have content cached before personalisation is applied bymod_include (or another content processing filter). In this example templates containing tags understood bymod_include are cached before being parsed:

# Cache content before mod_include and mod_deflateCacheQuickHandler offAddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html

You may place theCACHE filter anywhere you wish within the filter chain. In this example, content is cached after being parsed bymod_include, but before being processed bymod_deflate:

# Cache content between mod_include and mod_deflateCacheQuickHandler offAddOutputFilterByType INCLUDES;CACHE;DEFLATE text/html

Cache Status and Logging

Oncemod_cache has made a decision as to whether or not an entity is to be served from cache, the detailed reason for the decision is written to the subprocess environment within the request under thecache-status key. This reason can be logged by theLogFormat directive as follows:

LogFormat "%{cache-status}e ..."

Based on the caching decision made, the reason is also written to the subprocess environment under one the following four keys, as appropriate:

cache-hit

The response was served from cache.

cache-revalidate

The response was stale and was successfully revalidated, then served from cache.

cache-miss

The response was served from the upstream server.

cache-invalidate

The cached entity was invalidated by a request method other than GET or HEAD.

This makes it possible to support conditional logging of cached requests as per the following example:

CustomLog "cached-requests.log" common env=cache-hitCustomLog "uncached-requests.log" common env=cache-missCustomLog "revalidated-requests.log" common env=cache-revalidateCustomLog "invalidated-requests.log" common env=cache-invalidate

For module authors, a hook calledcache_status is available, allowing modules to respond to the caching outcomes above in customised ways.

CacheDefaultExpireDirective

TheCacheDefaultExpire 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 theCacheMaxExpire directive doesnot override this setting.

CacheDefaultExpire 86400

CacheDetailHeaderDirective

When theCacheDetailHeader directive is switched on, anX-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 whetherCache-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 headerCacheDetailHeader on

CacheDisableDirective

TheCacheDisable directive instructsmod_cache tonot cache urls at or belowurl-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>

Theno-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

CacheEnableDirective

TheCacheEnable directive instructsmod_cache to cache urls at or belowurl-string. The cache storage manager is specified with thecache_type argument. TheCacheEnable directive can alternatively be placed inside either<Location> or<LocationMatch> sections to indicate the content is cacheable.cache_typedisk instructsmod_cache to use the disk based storage manager implemented bymod_cache_disk.cache_typesocache instructsmod_cache to use the shared object cache based storage manager implemented bymod_cache_socache.

In the event that the URL space overlaps between differentCacheEnable 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 theCacheEnable directives in the configuration file.CacheEnable directives within<Location> or<LocationMatch> sections are processed before globally definedCacheEnable 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.orgCacheEnable  disk  "http://*example.org/"# Match www.example.org, but not fooexample.orgCacheEnable  disk  "http://.example.org/"

Theno-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

CacheHeaderDirective

When theCacheHeader directive is switched on, anX-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 headerCacheHeader on

X-Cache: HIT from localhost

CacheIgnoreCacheControlDirective

Ordinarily, requests containing aCache-Control: no-cache or Pragma: no-cache header value will not be served from the cache. TheCacheIgnoreCacheControl 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 willnever be cached.

CacheIgnoreCacheControl On

See also

• CacheStorePrivate
• CacheStoreNoStore

CacheIgnoreHeadersDirective

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 inany case regardless of the setting ofCacheIgnoreHeaders:

• 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 toNone.

CacheIgnoreHeaders Set-Cookie

CacheIgnoreHeaders None

CacheIgnoreNoLastModDirective

Ordinarily, documents without a last-modified date are not cached. Under some circumstances the last-modified date is removed (duringmod_include processing for example) or not provided at all. TheCacheIgnoreNoLastMod 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 theCacheDefaultExpire directive will be used to generate an expiration date.

CacheIgnoreNoLastMod On

CacheIgnoreQueryStringDirective

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. TheCacheIgnoreQueryString 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

CacheIgnoreURLSessionIdentifiersDirective

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 cacheable 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 cacheable 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

CacheKeyBaseURLDirective

When theCacheKeyBaseURL 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/"

CacheLastModifiedFactorDirective

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. TheCacheLastModifiedFactor directive specifies afactor 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, andfactor 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 byCacheMaxExpire, then the latter takes precedence.

CacheLastModifiedFactor 0.5

CacheLockDirective

TheCacheLock 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 lockCacheLock on

CacheLockMaxAgeDirective

TheCacheLockMaxAge 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.

CacheLockPathDirective

TheCacheLockPath 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.

CacheMaxExpireDirective

TheCacheMaxExpire directive specifies the maximum number of seconds for which cacheable 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

CacheMinExpireDirective

TheCacheMinExpire directive specifies the minimum number of seconds for which cacheable 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

CacheQuickHandlerDirective

TheCacheQuickHandler 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 cachebolts 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 authorization.

# Run cache as a normal handlerCacheQuickHandler 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 theCACHE filter to the chain.

# Cache content before mod_include and mod_deflateCacheQuickHandler offAddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html

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

CacheStaleOnErrorDirective

When theCacheStaleOnError 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

CacheStoreExpiredDirective

Since httpd 2.2.4, responses which have already expired are not stored in the cache. TheCacheStoreExpired 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

CacheStoreNoStoreDirective

Ordinarily, requests or responses withCache-Control: no-store header values will not be stored in the cache. TheCacheStoreNoStore 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 willnever be cached.

CacheStoreNoStore On

See also

• CacheIgnoreCacheControl
• CacheStorePrivate

CacheStorePrivateDirective

Ordinarily, responses withCache-Control: private header values will not be stored in the cache. TheCacheStorePrivate 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 willnever be cached.

CacheStorePrivate On

See also

• CacheIgnoreCacheControl
• CacheStoreNoStore