Setting up Apache Karaf for high availability

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## Sample lock configuration
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karaf.lock=true
karaf.lock.class=org.apache.karaf.main.lock.SimpleFileLock
# specify path to lock directory
karaf.lock.dir=[PathToLockFileDirectory]
karaf.lock.delay=10

The previous configuration sample contains the essential entries for a file-based locking mechanism, that is, two or more Karaf instances attempt to gain exclusive ownership of a file over a shared filesystem.

Each Karaf instance will include the same lock directory location on a shared filesystem common to each Karaf installation. If a shared filesystem is not practical between systems, then a JDBC locking mechanism can be used. This is described in the following code:

karaf.lock=true
karaf.lock.class=org.apache.karaf.main.DefaultJDBCLock
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

The JDBC configuration is similar to the SimpleFileLock configuration. However, it is expanded to contain the JDBC url, driver, timeout, user, and password options. Two additional JDBC options are included to allow for multiple Master/Slave Karaf deployments to use a single database. These are the JDBC table and clustername options. The JDBC table property sets the database table to use for the lock, and the JDBC clustername property specifies which pairing group a Karaf instance belongs to (for example, hosts A and B belong to a cluster prod group, and hosts C and D belong to a cluster dev group).

When using the JDBC locking mechanism, you'll have to provide the relevant JDBC driver JAR file to Karaf's lib/ext folder. For specific database configurations, consult Karaf's user manual (http://karaf.apache.org/manual/latest/index.html).