Clustering

Contents

Clustering
Overview
Limitations
Requirements
Unique Cluster Node ID
Sync Delay
Removing Old Revisions
Journal Type
1. 1. File Journal
  2. Database Journal
Sample Cluster Configuration
1. Persistence Manager Configuration

Overview

Clustering in Jackrabbit works as follows: content is shared between all cluster nodes. That means all Jackrabbit cluster nodes need access to the SAME persistent storage (persistence manager and data store).

The persistence manager must be clusterable (eg. central database that allows for concurrent access, see PersistenceManagerFAQ); any DataStore (file or DB) is clusterable by its very nature, as they store content by unique hash ids. However, each cluster node needs its own (private) FileSystem and Search index.

Every change made by one cluster node is reported in a journal, which can be either file based or written to some database.

Limitations

Session scoped locks currently have no effect on other cluster nodes - see http://issues.apache.org/jira/browse/JCR-1173

Requirements

In order to use clustering, the following prerequisites must be met:

Each cluster node must have its own repository configuration.
Each cluster node needs its own (private) FileSystem and Search index.
Every cluster node must be assigned a unique ID.
A journal type must be chosen, either based on files or stored in a database.
The persistence managers must store their data in the same, globally accessible location (see PersistenceManagerFAQ).
A DataStore must always be shared between nodes, if used.

Unique Cluster Node ID

Every cluster node needs a unique ID. This ID can be either specified in the cluster configuration as id attribute or as value of the system property org.apache.jackrabbit.core.cluster.node_id. When copying repository configurations, do not forget to adapt the cluster node IDs if they are hardcoded. See below for some sample cluster configurations. A cluster id can be freely defined, the only requirement is that it has to be different on each cluster node.

Sync Delay

By default, cluster nodes read the journal and update their state every 5 seconds (5000 milliseconds). To use a different value, set the attribute syncDelay in the cluster configuration.

Removing Old Revisions

The journal in which cluster nodes write their changes can potentially become very large. By default, old revisions are not removed. This enables one to add a cluster node without much work: the new cluster node just replays the journal to get up to date (of course, if the journal contains data from two years of work then this might take a while...)

As of Jackrabbit 1.5 there is the possibility to automatically clean the database-based journal. The local revision counter is automatically migrated to a new table in the database called LOCAL_REVISIONS. To support a proper migration the "revision" parameter must be present in the configuration. After the migration it can be removed.

The clean-up task can be configured with three parameters:

janitorEnabled specifies whether the clean-up task for the journal table is enabled (default = false)
janitorSleep specifies the sleep time of the clean-up task in seconds (only useful when the clean-up task is enabled, default is 24 hours)
janitorFirstRunHourOfDay specifies the hour at which the clean-up task initiates its first run (default = 3, which means 3:00 at night)

The current solution has three known caveats:

If the janitor is enabled then you loose the possibility to easily add cluster nodes. (It is still possible but takes detailed knowledge of Jackrabbit.)
You must make sure that all cluster nodes have written their local revision to the database before the clean-up task runs for the first time because otherwise cluster nodes might miss updates (because they have been purged) and their local caches and search-indexes get out of sync.
If a cluster node is removed permanently from the cluster, then its entry in the LOCAL_REVISIONS table should be removed manually. Otherwise, the clean-up thread will not be effective.

Journal Type

The cluster nodes store information identifying items they modified in a journal. This journal must again be globally available to all nodes in the cluster. This can be either a folder in the file system or a database running standalone.

File Journal

The file journal is configured through the following properties:

revision: location of the cluster node's revision file
directory: location of the journal folder

Database Journal

The database journal is configured through the following properties:

revision: location of the cluster node's revision file
driver: JDBC driver class name
url: JDBC URL
user: user name
password: password

Sample Cluster Configuration

This section contains some sample cluster configurations. First, using a file based journal implementation, where the journal files are created in a share exported by NFS:

<Cluster id="node1">
  <Journal class="org.apache.jackrabbit.core.journal.FileJournal">
    <param name="revision" value="${rep.home}/revision.log" />
    <param name="directory" value="/nfs/myserver/myjournal" />
  </Journal>
</Cluster>

In the next configuration, the journal is stored in an Oracle database, using a sync delay of 2 seconds (2000 milliseconds):

<Cluster id="node1" syncDelay="2000">
  <Journal class="org.apache.jackrabbit.core.journal.OracleDatabaseJournal">
    <param name="revision" value="${rep.home}/revision.log" />
    <param name="driver" value="oracle.jdbc.driver.OracleDriver" />
    <param name="url" value="jdbc:oracle:thin:@myhost:1521:mydb" />
    <param name="user" value="scott"/>
    <param name="password" value="tiger"/>
  </Journal>
</Cluster>

In the following configuration, the journal is stored in an PostgreSQL database, accessed via "JNDI" (See Also UsingJNDIDataSource):

<Cluster id="node1" syncDelay="2000">
  <Journal class="org.apache.jackrabbit.core.journal.DatabaseJournal">
    <param name="revision" value="${rep.home}/revision.log" />
    <param name="driver" value="javax.naming.InitialContext"/>
    <param name="url" value="java:jdbc/Workspace"/>
    <param name="databaseType" value="postgresql"/>
  </Journal>
</Cluster>

Note: the journal implementation classes have been refactored in Jackrabbit 1.3. In earlier versions, journal implementations resided in the package org.apache.jackrabbit.core.cluster.

Persistence Manager Configuration

For performance reasons, only information identifying the modified items is stored in the journal. This implies, that all cluster nodes must have access to the items' actual content. The persistence manager needs to be transactional, and need to support concurrent access from multiple processes. When using Jackrabbit, one option is to use a database persistence manager, and use a database that does support concurrent access. The file system based persistence managers in Jackrabbit are not transactional and don't support concurrent access; Apache Derby doesn't support concurrent access in the embedded mode. The following sample shows a workspace's persistence manager configuration using an Oracle database:

<PersistenceManager class="org.apache.jackrabbit.core.persistence.db.OraclePersistenceManager">
  <param name="url" value="jdbc:oracle:thin:@myhost:1521:mydb" />
  <param name="user" value="scott"/>
  <param name="password" value="tiger"/>
  <param name="schemaObjectPrefix" value="${wsp.name}_"/>
  <param name="externalBLOBs" value="false"/>
</PersistenceManager>

The data store should be used to store large binaries (all cluster nodes need to access the same data store). When not using the data store, one need to set the parameter externalBLOBs to false so that large binaries are stored in the persistence manager. The file system blob store does not support clustering, because it uses a local directory.