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System

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Architecture: Storage Layer

In this section, we explain the storage layer: the database structure, maintenance, and the bistream store and configurations.

RDBMS / Database Structure

DSpace uses a relational database to store all information about the organization of content, metadata about the content, information about e-people and authorization, and the state of currently-running workflows. The DSpace system also uses the relational database in order to maintain indices that users can browse.

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Most of the functionality that DSpace uses can be offered by any standard SQL database that supports transactions. Presently, the browse indices use some features specific to PostgreSQL and Oracle, so some modification to the code would be needed before DSpace would function fully with an alternative database back-end.

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The database schema used by DSpace is created by SQL statements stored in a directory specific to each supported RDBMS platform:

• Wiki MarkupPostgreSQL schemas are in _\[dspace-source\]/dspace/etc/postgres/_
• Wiki MarkupOracle schemas are in _\[dspace-source\]/dspace/etc/oracle/_ The SQL
  The SQL (DDL) statements to create the tables for the current release, starting with an empty database, aer in _database_schema.sql_. The schema SQL file also creates the two required e-person groups (_Anonymous_ and _Administrator_) that are required for the system to function properly.

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Also in _\[dspace-source\]/dspace/etc/\[database\]_ are various SQL files called _database_schema_1x_1y_. These contain the necessary SQL commands to update a live DSpace database from version 1._x_ to 1._y_. Note that this might not be the only part of an upgrade process: see Updating a DSpace Installation for details.

The DSpace database code uses an SQL function getnextid to assign primary keys to newly created rows. This SQL function must be safe to use if several JVMs are accessing the database at once; for example, the Web UI might be creating new rows in the database at the same time as the batch item importer. The PostgreSQL-specific implementation of the method uses SEQUENCES for each table in order to create new IDs. If an alternative database backend were to be used, the implementation of getnextid could be updated to operate with that specific DBMS.

The etc directory in the source distribution contains two further SQL files. clean-database.sql contains the SQL necessary to completely clean out the database, so use with caution! The Ant target clean_database can be used to execute this. update-sequences.sql contains SQL to reset the primary key generation sequences to appropriate values. You'd need to do this if, for example, you're restoring a backup database dump which creates rows with specific primary keys already defined. In such a case, the sequences would allocate primary keys that were already used.unmigrated-wiki-markup

Versions of the _*.sql*_ files for Oracle are stored in _\[dspace-source\]/dspace/etc/oracle_. These need to be copied over their PostgreSQL counterparts in _\[dspace-source\]/dspace/etc_ prior to installation.

Maintenance and Backup

When using PostgreSQL, it's a good idea to perform regular 'vacuuming' of the database to optimize performance. This is performed by the vacuumdb command which can be executed via a 'cron' job, for example by putting this in the system crontab:

# clean up the database nightly
40 2 * * * /usr/local/pgsql/bin/vacuumdb --analyze dspace > /dev/null
       null 2>&1

The DSpace database can be backed up and restored using usual methods, for example with pg_dump and psql. However when restoring a database, you will need to perform these additional steps:

• The fresh_install target loads up the initial contents of the Dublin Core type and bitstream format registries, as well as two entries in the epersongroup table for the system anonymous and administrator groups. Before you restore a raw backup of your database you will need to remove these, since they will already exist in your backup, possibly having been modified. For example, use:

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  DELETE FROM dctyperegistry; DELETE FROM bitstreamformatregistry; DELETE FROM epersongroup;
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• After restoring a backup, you will need to reset the primary key generation sequences so that they do not produce already-used primary keys. Do this by executing the SQL in _\[dspace-source\]/dspace/etc/update-sequences.sql_, for example with:

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  psql -U dspace -f -U dspace -f [dspace-source]/dspace/etc/update-sequences.sql

  Future updates of DSpace may involve minor changes to the database schema. Specific instructions on how to update the schema whilst keeping live data will be included. The current schema also contains a few currently unused database columns, to be used for extra functionality in future releases. These unused columns have been added in advance to minimize the effort required to upgrade.

Configuring the RDBMS Component

The database manager is configured with the following properties in dspace.cfg:

Bitstream Store

DSpace offers two means for storing content. The first is in the file system on the server. The second is using SRB (Storage Resource Broker). Both are achieved using a simple, lightweight API.

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This cleanup can be invoked from the command line via the Cleanup class, which can in turn be easily executed from a shell on the server machine using /dspace/bin/cleanup. You might like to have this run regularly by cron, though since DSpace is read-lots, write-not-so-much it doesn't need to be run very often.

Backup

The bitstreams (files) in traditional storage may be backed up very easily by simply 'tarring' or 'zipping' the assetstore directory (or whichever directory is configured in dspace.cfg). Restoring is as simple as extracting the backed-up compressed file in the appropriate location.

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Of course, it isn't really ideal to 'freeze' the system while backing up to ensure that the database and files match up. Since DSpace uses the bitstream data in the database as the authoritative record, it's best to back up the database before the files. This is because it's better to have a bitstream in storage but not the database (effectively non-existent to DSpace) than a bitstream record in the database but not storage, since people would be able to find the bitstream but not actually get the contents.

With DSpace 1.7 and above, there is also the option to backup both files and metadata via the AIP Backup and Restore feature.

Configuring the Bitstream Store

Both traditional and SRB bitstream stores are configured in dspace.cfg.

Configuring

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Traditional Storage

Bitstream stores in the file system on the server are configured like this:

assetstore.dir =  [dspace]/assetstore

Wiki Markup(Remember that _\[dspace\]_ is a placeholder for the actual name of your DSpace install directory).

The above example specifies a single asset store.

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Then restart DSpace (Tomcat). New bitstreams will be written to the asset store specified by assetstore.dir.1, which is /mnt/other_filesystem/assetstore_1 in the above example.

Configuring SRB Storage

The same framework is used to configure SRB storage. That is, the asset store number (0..n) can reference a file system directory as above or it can reference a set of SRB account parameters. But any particular asset store number can reference one or the other but not both. This way traditional and SRB storage can both be used but with different asset store numbers. The same cautions mentioned above apply to SRB asset stores as well: The particular asset store a bitstream is stored in is held in the database, so don't move bitstreams between asset stores, and don't renumber them.

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