A.4. Variable Substitution

When defining properties for a profile, you can use a variable substitution mechanism, which has the general syntax ${ResourceReference}. This variable substitution mechanism can be used in any profile resource, including the agent properties, PID properties, and other resources—for example, the mq-base profile's broker.xml resource references the ${broker.name} and ${data} variables.

Simple variables can be substituted in a profile resource using the following syntax:

${PropName}

Where PropName can be the name of any Java system property. In particular, Java system properties can be defined in the following locations:

Some of the more useful system properties defined in the etc/system.properties file are, as follows:

The profile URL enables you to substitute properties from a specific PID. A variable substitution using a profile URL has the following format:

${profile:PID/Property}

Where PID is the OSGi Config Admin persistent ID (which corresponds to the profile resource PID.properties) and Property is the name of the property you want to substitute.

For example, the fabric profile's org.fusesource.fabric.maven.properties resource includes the following property setting:

remoteRepositories=${profile:org.fusesource.fabric.agent/org.ops4j.pax.url.mvn.repositories}

So that the remoteRepositories property is set to the value of the org.ops4j.pax.url.mvn.repositories agent property (org.fusesource.fabric.agent is the PID for the agent properties).

You can substitue the contents of a ZooKeeper node using the zk URL. The URL can be specified either as an absolute node:

${zk:/PathToNode}

Or you can reference configuration properties from a specific container using the following syntax:

${zk:ContainerName/Property}

The preceding syntax is effectively a short cut to the following URL reference:

${zk:/fabric/registry/containers/config/ContainerName/Property}

It is also possible to use a variable within a variable (recursive substitution). For example, the dosgi profile's org.fusesource.fabric.dosgi.properties resource defines the following property:

exportedAddress=${zk:${karaf.name}/ip}

A potential problem arises with ZooKeeper URLs if you need to reference a ZooKeeper node that has a version number embedded in it. For example, suppose you want to reference the my-profile profile's my-resource resource, which can be done using the following ZooKeeper URL:

 ${zk:/fabric/configs/versions/1.0/profiles/my-profile/my-resource}

Notice that the profile version number, 1.0, is embedded in this path. But if you decide to upgrade this profile to version 1.1, this means you must manually edit all occurrences of this ZooKeeper URL, changing the version number to 1.1 in order to reference the upgraded resource. To avoid this extra work, and to ensure that the URL always references the current version of the resource, you can use the following trick which exploits recursive variable substitution:

 ${zk:/fabric/configs/versions/${zk:/fabric/configs/containers/${karaf.name}}/profiles/my-profile/my-resource}

This works because the /fabric/configs/containers/${karaf.name} ZooKeeper node contains the current profile version deployed in the container.