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.. _admin-plugins-cache-promote:

Cache Promote Plugin
********************

The :program:`cache_promote` plugin provides a means to control when an object should
be allowed to enter the cache. This is orthogonal from normal Cache-Control
directives, providing a different set of policies to apply. The typical use
case for this plugin is when you have a very large data set, where you want to
avoid churning the ATS cache for the long tail content.

All configuration is done via :file:`remap.config`, and the following options
are available:

.. program:: cache-promote

.. option:: --policy

   The promotion policy. The values ``lru`` and ``chance`` are supported.

.. option:: --sample

   The sampling rate for the request to be considered

If :option:`--policy` is set to ``lru`` the following options are also available:

.. option:: --label

   An optional label for this LRU, to allow sharing an LRU across multiple remap
   rules. Note: In order for an LRU to be used by multiple remap rules, not only
   must the label match, both the :option:`--hits` and  :option:`--buckets`
   options must be identical.

.. option:: --hits

   The minimum number of hits before promotion.

.. option:: --buckets

   The size (number of entries) of the LRU.

.. option:: --stats-enable-with-id

   Enables collecting statistics.  The option requires an argument, the
   remap-identifier.  The remap-identifier is a string that is concatenated
   to the stat name.  The following stats are collected.

*  **plugin.cache_promote.${remap-identifier}.cache_hits** - Cache hit total, available for all policies.
*  **plugin.cache_promote.${remap-identifier}.freelist_size** - Size of the freelist when using the LRU policy.
*  **plugin.cache_promote.${remap-identifier}.lru_size** - Size of the LRU when using the LRU policy.
*  **plugin.cache_promote.${remap-identifier}.lru_hit** - LRU hit count when using the LRU policy.
*  **plugin.cache_promote.${remap-identifier}.lru_miss** - LRU miss count when using the LRU policy.
*  **plugin.cache_promote.${remap-identifier}.lru_vacated** - count of LRU entries removed to make room for a new request.
*  **plugin.cache_promote.${remap-identifier}.promoted** - count requests promoted, available in all policies.
*  **plugin.cache_promote.${remap-identifier}.total_requests** - count of all requests.

These two options combined with your usage patterns will control how likely a
URL is to become promoted to enter the cache.

Examples
--------

These two examples shows how to use the chance and LRU policies, respectively::

    map http://cdn.example.com/ http://some-server.example.com \
      @plugin=cache_promote.so @pparam=--policy=chance @pparam=--sample=10%

    map http://cdn.example.com/ http://some-server.example.com \
      @plugin=cache_promote.so @pparam=--policy=lru \
      @pparam=--hits=10 @pparam=--buckets=10000

Note :option:`--sample` is available for all policies and can be used to reduce pressure under heavy load.
