Goal
This document explains how the MRUMemoryStore and Swapping under
Cocoon executes.
Overview
The MRUMemoryStore was developed to provide a standard algorithm to
store data in memory. For web-based applications the MRU (Most Recently Used) algorithm
is very suitable, because the object most frequently accessed is always on "top".
If configured, the objects are also swapped to the filesystem if the
MRUMemoryStore reached his configured max. object limit.
Implementation
MRUMemoryStore
The heart of the MRUMemoryStore ist combination of a LinkedList and a HashMap:
The LinkedList provides the queue mechanism, and the entries in the LinkedList
contain the key to the data in the HashMap. When caching a new entry in to the list,
the entry is inserted to the front of the list.
If the list is already full, the oldest data entry is removed from the Cache.
When requesting a entry, the store returns the object by key and inserts the requested key
on the top of the Cache.
This implementation keeps the most recent used objects in the store and provides the best
use of the machines memory.
Swapping
When the MRUMemoryStore is full or the JVM is at the heap size limit the
objects in the MRUMemoryStore are swapped to the Filesystem. The default
directory is "cache-dir" in the work-directory.
NOTE: The keys under Cocoon are Strings at the moment. Therefor the
filenames of the swapped objects can be very long. Especially Windows OS
flavours have problems with long filenames. Use the JispFilesystemStore to
get rid of it.
Configuration of the MRUMemoryStore in the cocoon.xconf
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<store class="org.apache.cocoon.components.store.MRUMemoryStore"
logger="core.store">
<parameter name="maxobjects" value="100"/>
<parameter name="use-persistent-cache" value="true"/>
</store>
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Explanation of the paramters:
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<parameter name="maxobjects" value="100"/>:
Indicates how many objects will be hold in MRUMemoryStore. When the number
of maxobjects has been reached, the last object in the MRUMemoryStore will be
thrown out.
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<parameter name="use-persistent-cache" value="true"/>:
If this switch is set on true, objects are swapped out to the filesystem,
if the MRUMemoryStore has reached his maximum object limit, or the JVM
memory consumption is over the heap size limit. See StoreJanitor user
docs for more information.
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