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<manualpage metafile="socache.xml.meta">

  <title>Shared Object Cache in Apache HTTP Server</title>

  <summary>
    <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>
  </summary>

  <section id="providers">
    <title>Shared Object Cache Providers</title>
    <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
    <directive module="mod_cache_socache">CacheSocache</directive>,
    <directive module="mod_authn_socache">AuthnCacheSOCache</directive>,
    <directive module="mod_ssl">SSLSessionCache</directive>, and
    <directive module="mod_ssl">SSLStaplingCache</directive>.</p>
    <p>Currently available providers are:</p>
    <dl>
    <dt>"dbm" (<module>mod_socache_dbm</module>)</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" (<module>mod_socache_dc</module>)</dt>
    <dd>This makes use of the <a href="http://www.distcache.org/">distcache</a>
    distributed session caching libraries.</dd>
    <dt>"memcache" (<module>mod_socache_memcache</module>)</dt>
    <dd>This makes use of the <a href="http://memcached.org/">memcached</a>
    high-performance, distributed memory object caching system.</dd>
    <dt>"shmcb" (<module>mod_socache_shmcb</module>)</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 paramater 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>

  </section>

</manualpage>
