Multiple Apache Karaf containers

Natively, Apache Karaf provides a high availability mechanism based on a locking system. It's a master-slaves configuration, following an active/passive pattern. Apache Karaf supports two kinds of locks, which are as follows:

When the first Apache Karaf instance starts, if the lock is available, the instance acquires the lock and becomes the master.

If another instance starts, as the lock is not available (held by the master), the instance is in standby (slave) mode and periodically checks the lock.

When you use a lock on a filesystem, all instances have to share the same filesystem. The lock is a simple file. If the Apache Karaf instances are located on different machines, it means that the filesystem storing the lock has to be available for all machines (using NFS, CIFS, SAN, and so on).

In order to enable the filesystem locking system, you have to update the etc/system.properties configuration file as follows:

karaf.lock=true
karaf.lock.class=org.apache.karaf.main.SimpleFileLock
karaf.lock.dir=/path/to/lockfile
karaf.lock.delay=10

When a shared filesystem is not an option (for security or infrastructure reasons, for instance), you can use a database to store the lock. With database locking, Apache Karaf uses a lock on a table (the KARAF_LOCK table by default). Any database that supports JDBC can be used.

The configuration is also defined in the etc/system.properties configuration file as follows:

karaf.lock=true
karaf.lock.class=org.apache.karaf.main.DefaultJDBCLock
karaf.lock.level=50
karaf.lock.delay=10
karaf.lock.jdbc.url=jdbc:derby://dbserver:1527/sample
karaf.lock.jdbc.driver=org.apache.derby.jdbc.ClientDriver
karaf.lock.jdbc.user=user
karaf.lock.jdbc.password=password
karaf.lock.jdbc.table=KARAF_LOCK
karaf.lock.jdbc.clustername=karaf
karaf.lock.jdbc.timeout=30

You have to copy the JDBC driver JAR file into the lib/ext folder. Apache Karaf provides the JDBC lock implementation dedicated to some specific databases (DefaultJDBCLock is the generic one, OracleJDBCLock for Oracle databases, DerbyJDBCLock for Derby databases, MySQLJDBCLock for MySQL databases, PostgreSQLJDBCLock for PostgreSQL databases, and SQLServerJDBCLock for Microsoft SQLServer databases).

The Apache Karaf locking mechanism provides a good solution for high availability. However, only one Apache Karaf instance is active (the master); all other instances are inactive (standby/master).

In order to provide both high availability and performance scalability, having multiple active Apache Karaf instances is a great advantage.