Shared Object Cache in Apache HTTP Server

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The Shared Object Cache provides a means to share simple data + across all a server's workers, regardless of thread + and process models. It is used where the advantages of sharing + data across processes outweigh the performance overhead of + inter-process communication.

Shared Object Cache Providers

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The shared object cache as such is an abstraction. Five different + modules implement it. To use the cache, one or more of these modules + must be present, and configured.

The only configuration required is to select which cache provider + to use. This is the responsibility of modules using the cache, and + they enable selection using directives such as + CacheSocache, + AuthnCacheSOCache, + SSLSessionCache, and + SSLStaplingCache.

Currently available providers are:

"dbm" (mod_socache_dbm): This makes use of a DBM hash file. + The choice of underlying DBM used may be configurable + if the installed APR version supports multiple DBM implementations.
"dc" (mod_socache_dc): This makes use of the distcache + distributed session caching libraries.
"memcache" (mod_socache_memcache): This makes use of the memcached + high-performance, distributed memory object caching system.
"redis" (mod_socache_redis): This makes use of the Redis + high-performance, distributed memory object caching system.
"shmcb" (mod_socache_shmcb): This makes use of a high-performance cyclic buffer inside a + shared memory segment.

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The API provides the following functions:

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const char *create(ap_socache_instance_t **instance, const char *arg, + apr_pool_t *tmp, apr_pool_t *p);: Create a session cache based on the given configuration string. + The instance pointer returned in the instance parameter will be + passed as the first argument to subsequent invocations.
apr_status_t init(ap_socache_instance_t *instance, const char *cname, + const struct ap_socache_hints *hints, + server_rec *s, apr_pool_t *pool): Initialize the cache. The cname must be of maximum length 16 + characters, and uniquely identifies the consumer of the cache + within the server; using the module name is recommended, e.g. + "mod_ssl-sess". This string may be used within a filesystem + path so use of only alphanumeric [a-z0-9_-] characters is + recommended. If hints is non-NULL, it gives a set of hints for + the provider. Return APR error code.
void destroy(ap_socache_instance_t *instance, server_rec *s): Destroy a given cache instance object.
apr_status_t store(ap_socache_instance_t *instance, server_rec *s, + const unsigned char *id, unsigned int idlen, + apr_time_t expiry, + unsigned char *data, unsigned int datalen, + apr_pool_t *pool): Store an object in a cache instance.
apr_status_t retrieve(ap_socache_instance_t *instance, server_rec *s, + const unsigned char *id, unsigned int idlen, + unsigned char *data, unsigned int *datalen, + apr_pool_t *pool): Retrieve a cached object.
apr_status_t remove(ap_socache_instance_t *instance, server_rec *s, + const unsigned char *id, unsigned int idlen, + apr_pool_t *pool): Remove an object from the cache.
void status(ap_socache_instance_t *instance, request_rec *r, int flags): Dump the status of a cache instance for mod_status.
apr_status_t iterate(ap_socache_instance_t *instance, server_rec *s, + void *userctx, ap_socache_iterator_t *iterator, + apr_pool_t *pool): Dump all cached objects through an iterator callback.

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Shared Object Cache in Apache HTTP Server

Shared Object Cache Providers

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