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<title>Shared Object Cache in Apache HTTP Server - Apache HTTP Server Version 2.4</title>
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<p class="apache">Apache HTTP Server Version 2.4</p>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.4</a></div><div id="page-content"><div id="preamble"><h1>Shared Object Cache in Apache HTTP Server</h1>
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<p>The Shared Object Cache provides a means to share simple data
across all a server's workers, regardless of <a href="mpm.html">thread
and process models</a>. It is used where the advantages of sharing
data across processes outweigh the performance overhead of
inter-process communication.</p>
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<div class="section">
<h2><a name="providers" id="providers">Shared Object Cache Providers</a></h2>
<p>The shared object cache as such is an abstraction. Four different
modules implement it. To use the cache, one or more of these modules
must be present, and configured.</p>
<p>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
<code class="directive"><a href="./mod/mod_cache_socache.html#cachesocache">CacheSocache</a></code>,
<code class="directive"><a href="./mod/mod_authn_socache.html#authncachesocache">AuthnCacheSOCache</a></code>,
<code class="directive"><a href="./mod/mod_ssl.html#sslsessioncache">SSLSessionCache</a></code>, and
<code class="directive"><a href="./mod/mod_ssl.html#sslstaplingcache">SSLStaplingCache</a></code>.</p>
<p>Currently available providers are:</p>
<dl>
<dt>"dbm" (<code class="module"><a href="./mod/mod_socache_dbm.html">mod_socache_dbm</a></code>)</dt>
<dd>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.</dd>
<dt>"dc" (<code class="module"><a href="./mod/mod_socache_dc.html">mod_socache_dc</a></code>)</dt>
<dd>This makes use of the <a href="http://distcache.sourceforge.net/">distcache</a>
distributed session caching libraries.</dd>
<dt>"memcache" (<code class="module"><a href="./mod/mod_socache_memcache.html">mod_socache_memcache</a></code>)</dt>
<dd>This makes use of the <a href="http://memcached.org/">memcached</a>
high-performance, distributed memory object caching system.</dd>
<dt>"shmcb" (<code class="module"><a href="./mod/mod_socache_shmcb.html">mod_socache_shmcb</a></code>)</dt>
<dd>This makes use of a high-performance cyclic buffer inside a
shared memory segment.</dd>
</dl>
<p>The API provides the following functions:</p>
<dl>
<dt>const char *create(ap_socache_instance_t **instance, const char *arg,
apr_pool_t *tmp, apr_pool_t *p);</dt>
<dd>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.</dd>
<dt>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)</dt>
<dd>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.</dd>
<dt>void destroy(ap_socache_instance_t *instance, server_rec *s)</dt>
<dd>Destroy a given cache instance object.</dd>
<dt>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)</dt>
<dd>Store an object in a cache instance.</dd>
<dt>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)</dt>
<dd>Retrieve a cached object.</dd>
<dt>apr_status_t remove(ap_socache_instance_t *instance, server_rec *s,
const unsigned char *id, unsigned int idlen,
apr_pool_t *pool)</dt>
<dd>Remove an object from the cache.</dd>
<dt>void status(ap_socache_instance_t *instance, request_rec *r, int flags)</dt>
<dd>Dump the status of a cache instance for mod_status.</dd>
<dt>apr_status_t iterate(ap_socache_instance_t *instance, server_rec *s,
void *userctx, ap_socache_iterator_t *iterator,
apr_pool_t *pool)</dt>
<dd>Dump all cached objects through an iterator callback.</dd>
</dl>
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