JGroups - A Toolkit for Reliable Messaging

HSQLDB/R is an experimental alpha version of database replication for the HSQLDB open source Java database. It uses a modified copy of HSQLDB 1.7.1-alpha and provides the following:

• Replication between any number of databases. No master-slave concept, pure peer-to-peer
• Changes to one database will be replicated to all others
• New nodes acquire full database from existing nodes or - if no node exists yet - initializes itself from file

This implementation will be integrated into one of the next versions of HSQLDB. The current implementation can be used to replicate changes to one database to all other replicas. This is mainly used to failover to a hot-standby dabatase server in case the primary database server crashes.
It is possible but not recommended to modify both replicas at the same time (see documentation below for details). We will provide this capability in the (near) future. Other items on the todo list are:

• Transfer of large state: instead of transferring the entire contents of the database (including e.g. all table definitions, users, and table contents), we will dump the existing contents into a file, load the new replica from file (offline) and then only request the update log from the existing member
• Binary format for replication. Currently we ship the SQL statements as strings, in the future we will use an efficient internal format. This also avoids having to parse the SQL statements again before applying them to the database
• Distributed locking: this will allow 2 applications to modify any of the replicas (even the same data), and JGroups will prevent conflicts.

The documentation is available in MS Word and PDF format and as HTML.

The hsqldbr.zip file can be downloaded from the regular SourceForge download page for JGroups. It contains all the necessary JARs and configuration files to run HSQLDB/R, including a minimal JGroups and a modified HSQLDB (based on HSQLDB-1.7.1). We will shortly integrate our changes into the HSQLDB code base, so the modified version of HSQLDB will not be needed any longer.