Editing Preservation Support (section)

== Complex-Object Export: METS and DIDL plugins ==

There are many ways to disseminate digital objects stored in an EPrints repository, depending on whether the request for an object comes from a human user or a machine (e.g. a search engine robot), and on what service is requested. The request may be for a full-text document, or for the data to be presented in some other format.

To increase the number of 'export' formats available in EPrints v3 it is possible to write  ''plugins'' - modular bits of code that are dynamically loaded into the system. Plugins are a new mechanism for customising and extending EPrints by writing code that interacts with the core software but is not part of it. In this way plugins can be written and implemented independently, and made available for others to use. These modules are written in Perl and can perform a number of roles: importing and exporting from and to arbitrary formats, converting documents from one format to another (for full-text indexing) and user interface widgets. Plugins sit in their own directory and are registered by EPrints when the system is run.

EPrints export plugins convert ''repository objects'' (things that contain metadata and files) into streamed data, in most cases XML. Objects can be exported through the EPrints Web interface (in search results, abstract pages or from the user's workspace), the [http://www.openarchives.org/ OAI-PMH] interface,  or from the command-line `export` script. 

There are already a number of object types in EPrints that are supported by export plugins, as shown in Figure 1. The OpenURL !ContextObject export plugin can also be used to export  ''!AccessLog objects'', which enables access logs to be harvested from EPrints. ''Document objects'' are exported as part of an EPrint object. (There are several other EPrint objects available in EPrints - ''Users, saved searches and History'' - but these aren't yet exportable, until export plugins have been written to support them.)

[[Image:eprints_exports.png]]

'''Figure 1. Export formats suported by EPrints plugins'''

If the request to export an object from a repository is from a preservation service provider, it will need to obtain all the files associated with the object. Objects that have more than one file are often referred to as 'complex objects'.

To support more efficient transfer of complex objects, two formats that may be used to disseminate objects for digital preservation are Metadata Encoding Transmission Standard [http://www.loc.gov/standards/mets/ METS] and  MPEG-21 Digital Item Declaration Language [http://www.chiariglione.org/mpeg/standards/mpeg-21/mpeg-21.htm DIDL]. Examples of the XML exports for these two formats are shown in Figures 2 and 3.

The METS export plugin is derived from work done by the [http://www.inf.aber.ac.uk/bridge/ Repository Bridge] project, who implemented a METS export for EPrints 2. This has been updated for the new plugin architecture and data model in EPrints 3.

[[Image:eprints_mets.png]]

'''Figure 2. METS export format example'''

DIDL support has been built by Chris Gutteridge (the lead developer on EPrints) as the result of collaboration with researchers from the [http://www.dlib.org/dlib/november03/bekaert/11bekaert.html Los Alamos National Laboratory] who have utilised MPEG-21 DIDL to build digital library systems. While DIDL is less well known in the digital preservation field, it serves a similar purpose and may well end up being more widely used (given the strength of backing of media companies in MPEG standards).

[[Image:eprints_didl.png]]

'''Figure 3. MPEG-21/DIDL export format example'''