Editing API:EPrints/Storage (section)

==DESCRIPTION==
Unless you are directly manipulating how data streams are stored you should use the methods for retrieving and writing data available from [[API:EPrints/DataObj/File|EPrints::DataObj::File]].

This module is the storage control layer which uses [[API:EPrints/Plugin/Storage|EPrints::Plugin::Storage]] plugins to support various storage back-ends. It enables the storage, retrieval and deletion of data streams. The maximum size of a stream is dependent on the back-end storage mechanism.

Storage works on [[API:EPrints/DataObj/File|EPrints::DataObj::File]] objects, which record the size of the data stream and where it has been stored. Unless a specific storage back-end is used, where a file is stored depends on the storage policy which is located in ''lib/storage/default.xml'':

<source lang="xml"><store xmlns="http://eprints.org/ep3/storage" xmlns:epc="http://eprints.org/ep3/control">
<epc:choose>
	<epc:when test="datasetid = 'document'">
		<plugin name="Local"/>
	</epc:when>
	<epc:otherwise>
		<plugin name="Local"/>
	</epc:otherwise>
</epc:choose>
</store></source>

The storage policy is a EP-script file that resolves to a list of one or more "plugin" elements. When storing, all matching plugins will be given the data stream to store (i.e. multiple copies). When retrieving, the first plugin to be successfully opened for reading will be used. The item context used when the storage policy is evaluated is the [[API:EPrints/DataObj/File|EPrints::DataObj::File]].

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===Callbacks===
Writing to and retrieving from data streams makes use of callback functions. This allows data to be streamed (to avoid memory overheads) and for flexibility in where data is going.

For details on the callback API see [[API:EPrints/DataObj/File#get_file|EPrints::DataObj::File/get_file]] and [[API:EPrints/DataObj/File#set_file|EPrints::DataObj::File/set_file]] for retrieving and writing respectively.

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