Fabric8 Documentation

Property Resolving

When using properties files in Fabric8 for implementing a distributed OSGi Config Admin; we support a number of different property resolvers.

So there is normal variable expansion for system properties. e.g.

myDir = ${runtime.data}/cheese

would default to the data directory of the current container plus "/cheese". However there are a number of additional property resolvers available using the ${...:...} format.


When you wish to refer to the current containers network settings; such as its ip, local host name or bind address:

localhostname = ${container:localhostname}
ip = ${container:ip}
bindaddress = ${container:bindaddress}


When you wish to calculate the checksum of a file or URL:

mychecksum = ${checksum:profile:\foo.xml}


When you wish to decrypt an encrypted value (such as a password):

mypassword = ${crypt:ABCDEF}


For accessing an environment variable.


You can use a the elvis operator a little like with groovy to specify a default value if an environment variable is not set


Where the text after the ?: token is used as the default value if the environment variable is not set


This property resolver allows you to use Groovy script expressions to dynamically resolve values; particularly using the API calls on the ZooKeeperFacade interface.

e.g. to dynamically lookup the available Cassandra hosts you can use this expression


Where the matchingDescendantStringData() function on the zk object looks up all the listen hosts in the ZooKeeper registry (using '*' to indicate a wildcard path) and then joins the values with a comma.

You can also do things like use the elvis operator to use an environment variable or system property value or use a default value if not using expressions such as

something=${groovy:env.MY_ENV_VAR ?: 'someDefaultValue'}
something=${groovy:sys['foo.bar'] ?: 'anotherThing'}

Helper objects

Environment Variable Description
env A Map of all the environment variables available
sys A Map of all the system properties available
zk An instance of the ZooKeeperFacade class</> for making it easy to query and navigate around ZooKeeper and extract values


When running multiple child containers on a machine, you need to associate ports to JVMs. Fabric8 supports port allocation using a property resolver of the form...

fooPort = ${port:1234-4567}

Where the 2 numbers represent the port range to use to pick a port. The expression will then evaluate to the port allocated to the current JVM.


This allows you to refer to a value in a config admin PID in a profile. e.g.

foo = ${profile:myPid/myKey}

would resolve to the config admin PID of "myPid" and key "myKey". e.g. if the current container's profile was "foo" it would look in /fabric/profiles/foo/myPid.properties and find the myKey value.


This resolves to a version number for a given key. Its often use to refer to a specific bundle or feature

bundle.mything = mvn:groupId/artifact/${version:camel}

What happens is the resolver looks up in the current profile the property file called io.fabric8.version.properties and then looks for the key "camel" (in this case) and uses that for the version value.

This means that there's a single place to define all the versions of things; which can be changed on a per profile (or version) basis easily - to avoid littering your profiles with version numbers and making it really easy to do a patch upgrade of versions of things.


This resolver evaluates the ZooKeeper path: