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<modulesynopsis metafile="mod_mem_cache.xml.meta">

<name>mod_mem_cache</name>
<description>Content cache keyed to URIs</description>
<status>Extension</status>
<sourcefile>mod_mem_cache.c</sourcefile>
<identifier>mem_cache_module</identifier>

<summary>
    <p>This module <em>requires</em> the service of <module
    >mod_cache</module>. It acts as a support module for <module
    >mod_cache</module> and provides a memory based storage manager.
    <module>mod_mem_cache</module> can be configured to operate in two
    modes: caching open file descriptors or caching objects in heap storage.
    <module>mod_mem_cache</module> is most useful when used to cache locally
    generated content or to cache backend server content for <module
    >mod_proxy</module> configured for <directive module="mod_proxy"
    >ProxyPass</directive> (aka <dfn>reverse proxy</dfn>).</p>

    <p>Content is stored in and retrieved from the cache using URI based
    keys. Content with access protection is not cached.</p>
    
    <note><title>Note</title>In most cases <module>mod_disk_cache</module>
    should be the preferred choice. This is explained further in the <a
    href="../caching.html#inmemory">Caching Guide</a>.  In particular,
    this module's cache is <em>per-process</em>, which can be partially
    mitigated by configuring threaded MPMS to use fewer child processes
    via configuration of larger values for <directive 
    module="mpm_common">ThreadsPerChild</directive>.  This module's cache is 
    also limited to storing a single variant (see HTTP Vary: header) of each 
    resource in the cache.</note>
</summary>
<seealso><module>mod_cache</module></seealso>
<seealso><module>mod_disk_cache</module></seealso>

<directivesynopsis>
<name>MCacheSize</name>
<description>The maximum amount of memory used by the cache in
KBytes</description>
<syntax>MCacheSize <var>KBytes</var></syntax>
<default>MCacheSize 100</default>
<contextlist><context>server config</context></contextlist>

<usage>
    <p>The <directive>MCacheSize</directive> directive sets the maximum
    amount of memory to be used by the cache, in KBytes (1024-byte units).
    If a new object needs to be inserted in the cache and the size of the
    object is greater than the remaining memory, objects will be removed
    until the new object can be cached. The object to be removed is
    selected using the algorithm specified by <directive
    module="mod_mem_cache">MCacheRemovalAlgorithm</directive>.</p>

    <example><title>Example</title>
      MCacheSize 700000
    </example>

    <note><title>Note</title>
      <p>The <directive>MCacheSize</directive> value must be greater than
      the value specified by the <directive module="mod_mem_cache"
      >MCacheMaxObjectSize</directive> directive.</p>
    </note>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>MCacheMaxObjectCount</name>
<description>The maximum number of objects allowed to be placed in the
cache</description>
<syntax>MCacheMaxObjectCount <var>value</var></syntax>
<default>MCacheMaxObjectCount 1009</default>
<contextlist><context>server config</context></contextlist>

<usage>
    <p>The <directive>MCacheMaxObjectCount</directive> directive sets the
    maximum number of objects to be cached. The value is used to create the
    open hash table. If a new object needs to be inserted in the cache and
    the maximum number of objects has been reached, an object will be
    removed to allow the new object to be cached. The object to be removed
    is selected using the algorithm specified by <directive
    module="mod_mem_cache">MCacheRemovalAlgorithm</directive>.</p>

    <example><title>Example</title>
      MCacheMaxObjectCount 13001
    </example>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>MCacheMinObjectSize</name>
<description>The minimum size (in bytes) of a document to be allowed in the
cache</description>
<syntax>MCacheMinObjectSize <var>bytes</var></syntax>
<default>MCacheMinObjectSize 1</default>
<contextlist><context>server config</context></contextlist>

<usage>
    <p>The <directive>MCacheMinObjectSize</directive> directive sets the
    minimum size in bytes of a document for it to be considered
    cacheable.</p>

    <example><title>Example</title>
      MCacheMinObjectSize 10000
    </example>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>MCacheMaxObjectSize</name>
<description>The maximum size (in bytes) of a document allowed in the
cache</description>
<syntax>MCacheMaxObjectSize <var>bytes</var></syntax>
<default>MCacheMaxObjectSize 10000</default>
<contextlist><context>server config</context></contextlist>

<usage>
    <p>The <directive>MCacheMaxObjectSize</directive> directive sets the
    maximum allowable size, in bytes, of a document for it to be considered
    cacheable.</p>

    <example><title>Example</title>
      MCacheMaxObjectSize 6400000
    </example>

    <note><title>Note</title>
      <p>The value of <directive>MCacheMaxObjectSize</directive> must be
      greater than the value specified by the <directive
      module="mod_mem_cache">MCacheMinObjectSize</directive> directive.</p>
    </note>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>MCacheRemovalAlgorithm</name>
<description>The algorithm used to select documents for removal from the
cache</description>
<syntax>MCacheRemovalAlgorithm LRU|GDSF</syntax>
<default>MCacheRemovalAlgorithm GDSF</default>
<contextlist><context>server config</context></contextlist>

<usage>
    <p>The <directive>MCacheRemovalAlgorithm</directive> directive specifies
    the algorithm used to select documents for removal from the cache.
    Two choices are available:</p>

    <dl>
      <dt><code>LRU</code> (Least Recently Used)</dt>
      <dd><code>LRU</code> removes the documents that have not been accessed
      for the longest time.</dd>

      <dt><code>GDSF</code> (GreadyDual-Size)</dt>
      <dd><code>GDSF</code> assigns a priority to cached documents based
      on the cost of a cache miss and the size of the document. Documents
      with the lowest priority are removed first.</dd>
    </dl>

    <example><title>Example</title>
      MCacheRemovalAlgorithm GDSF<br />
      MCacheRemovalAlgorithm LRU
    </example>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>MCacheMaxStreamingBuffer</name>
<description>Maximum amount of a streamed response to buffer in memory
before declaring the response uncacheable</description>
<syntax>MCacheMaxStreamingBuffer <var>size_in_bytes</var></syntax>
<default>MCacheMaxStreamingBuffer the smaller of 100000 or MCacheMaxObjectSize</default>
<contextlist><context>server config</context>
</contextlist>

<usage>
    <p>The <directive>MCacheMaxStreamingBuffer</directive> directive
    specifies the maximum number of bytes of a streamed response to
    buffer before deciding that the response is too big to cache.
    A streamed response is one in which the entire content is not
    immediately available and in which the <code>Content-Length</code>
    may not be known. Sources of streaming responses include proxied
    responses and the output of CGI scripts. By default, a streamed
    response will <em>not</em> be cached unless it has a
    <code>Content-Length</code> header. The reason for this is to
    avoid using a large amount of memory to buffer a partial response
    that might end up being too large to fit in the cache.
    The <directive>MCacheMaxStreamingBuffer</directive> directive allows
    buffering of streamed responses that don't contain a
    <code>Content-Length</code> up to the specified maximum amount of
    space. If the maximum buffer space is reached, the buffered
    content is discarded and the attempt to cache is abandoned.</p>

    <note><title>Note:</title>
      <p>Using a nonzero value for <directive>MCacheMaxStreamingBuffer</directive> 
      will not delay the transmission of the response to the client.
      As soon as <module>mod_mem_cache</module> copies a block of streamed
      content into a buffer, it sends the block on to the next output
      filter for delivery to the client.</p>
    </note>

    <example>
      # Enable caching of streamed responses up to 64KB:<br />
      MCacheMaxStreamingBuffer 65536
    </example>
</usage>
</directivesynopsis>

</modulesynopsis>
