Chapter 8. Deploying Features

Abstract

Because applications and other tools typically consist of multiple OSGi bundles, it is often convenient to aggregate inter-dependent or related bundles into a larger unit of deployment. Red Hat JBoss Fuse therefore provides a scalable unit of deployment, the feature, which enables you to deploy multiple bundles (and, optionally, dependencies on other features) in a single step.

8.1. Creating a Feature

Overview

Essentially, a feature is created by adding a new feature element to a special kind of XML file, known as a feature repository. To create a feature, perform the following steps:

Create a custom feature repository

If you have not already defined a custom feature repository, you can create one as follows. Choose a convenient location for the feature repository on your file system—for example, C:\Projects\features.xml—and use your favorite text editor to add the following lines to it:
<?xml version="1.0" encoding="UTF-8"?>
<features name="CustomRepository">
</features>
Where you must specify a name for the repository, CustomRepository, by setting the name attribute.
Note
In contrast to a Maven repository or an OBR, a feature repository does not provide a storage location for bundles. A feature repository merely stores an aggregate of references to bundles. The bundles themselves are stored elsewhere (for example, in the file system or in a Maven repository).

Add a feature to the custom feature repository

To add a feature to the custom feature repository, insert a new feature element as a child of the root features element. You must give the feature a name and you can list any number of bundles belonging to the feature, by inserting bundle child elements. For example, to add a feature named example-camel-bundle containing the single bundle, C:\Projects\camel-bundle\target\camel-bundle-1.0-SNAPSHOT.jar, add a feature element as follows:
<?xml version="1.0" encoding="UTF-8"?>
<features name="MyFeaturesRepo">
  <feature name="example-camel-bundle">
    <bundle>file:C:/Projects/camel-bundle/target/camel-bundle-1.0-SNAPSHOT.jar</bundle>
  </feature>
</features>
The contents of the bundle element can be any valid URL, giving the location of a bundle (see Appendix A, URL Handlers). You can optionally specify a version attribute on the feature element, to assign a non-zero version to the feature (you can then specify the version as an optional argument to the features:install command).
To check whether the features service successfully parses the new feature entry, enter the following pair of console commands:
JBossFuse:karaf@root> features:refreshUrl
JBossFuse:karaf@root> features:list
...
[uninstalled] [0.0.0                 ] example-camel-bundle                 MyFeaturesRepo
...
The features:list command typically produces a rather long listing of features, but you should be able to find the entry for your new feature (in this case, example-camel-bundle) by scrolling back through the listing. The features:refreshUrl command forces the kernel to reread all the feature repositories: if you did not issue this command, the kernel would not be aware of any recent changes that you made to any of the repositories (in particular, the new feature would not appear in the listing).
To avoid scrolling through the long list of features, you can grep for the example-camel-bundle feature as follows:
JBossFuse:karaf@root> features:list | grep example-camel-bundle
[uninstalled] [0.0.0                 ] example-camel-bundle                 MyFeaturesRepo
Where the grep command (a standard UNIX pattern matching utility) is built into the shell, so this command also works on Windows platforms.

Add the local repository URL to the features service

In order to make the new feature repository available to Apache Karaf, you must add the feature repository using the features:addUrl console command. For example, to make the contents of the repository, C:\Projects\features.xml, available to the kernel, you would enter the following console command:
features:addUrl file:C:/Projects/features.xml
Where the argument to features:addUrl can be specified using any of the supported URL formats (see Appendix A, URL Handlers).
You can check that the repository's URL is registered correctly by entering the features:listUrl console command, to get a complete listing of all registered feature repository URLs, as follows:
JBossFuse:karaf@root> features:listUrl
file:C:/Projects/features.xml
mvn:org.apache.ode/ode-jbi-karaf/1.3.3-fuse-01-00/xml/features
mvn:org.apache.felix.karaf/apache-felix-karaf/1.2.0-fuse-01-00/xml/features

Add dependent features to the feature

If your feature depends on other features, you can specify these dependencies by adding feature elements as children of the original feature element. Each child feature element contains the name of a feature on which the current feature depends. When you deploy a feature with dependent features, the dependency mechanism checks whether or not the dependent features are installed in the container. If not, the dependency mechanism automatically installs the missing dependencies (and any recursive dependencies).
For example, for the custom Apache Camel feature, example-camel-bundle, you can specify explicitly which standard Apache Camel features it depends on. This has the advantage that the application could now be successfully deployed and run, even if the OSGi container does not have the required features pre-deployed. For example, you can define the example-camel-bundle feature with Apache Camel dependencies as follows:
<?xml version="1.0" encoding="UTF-8"?>
<features name="MyFeaturesRepo">
  <feature name="example-camel-bundle">
    <bundle>file:C:/Projects/camel-bundle/target/camel-bundle-1.0-SNAPSHOT.jar</bundle>
    <feature version="6.3.0.redhat-187">camel-core</feature>
    <feature version="6.3.0.redhat-187">camel-spring-osgi</feature>
  </feature>
</features>
Specifying the version attribute is optional. When present, it enables you to select the specified version of the feature.

Add OSGi configurations to the feature

If your application uses the OSGi Configuration Admin service, you can specify configuration settings for this service using the config child element of your feature definition. For example, to specify that the prefix property has the value, MyTransform, add the following config child element to your feature's configuration:
<?xml version="1.0" encoding="UTF-8"?>
<features name="MyFeaturesRepo">
  <feature name="example-camel-bundle">
    <config name="org.fusesource.fuseesb.example">
      prefix=MyTransform
    </config>
  </feature>
</features>
Where the name attribute of the config element specifies the persistent ID of the property settings (where the persistent ID acts effectively as a name scope for the property names). The content of the config element is parsed in the same way as a Java properties file.
The settings in the config element can optionally be overridden by the settings in the Java properties file located in the InstallDir/etc directory, which is named after the persistent ID, as follows:
InstallDir/etc/org.fusesource.fuseesb.example.cfg
As an example of how the preceding configuration properties can be used in practice, consider the following Blueprint XML file that accesses the OSGi configuration properties:
<?xml version="1.0" encoding="UTF-8"?>
<blueprint xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xmlns:cm="http://aries.apache.org/blueprint/xmlns/blueprint-cm/v1.1.0">

    <!-- osgi blueprint property placeholder -->
    <cm:property-placeholder id="placeholder"
                             persistent-id="org.fusesource.fuseesb.example">
        <cm:default-properties>
            <cm:property name="prefix" value="DefaultValue"/>
        </cm:default-properties>
    </cm:property-placeholder>

    <bean id="myTransform" class="org.fusesource.fuseesb.example.MyTransform">
      <property name="prefix" value="${prefix}"/>
    </bean>

</blueprint>
When this Blueprint XML file is deployed in the example-camel-bundle bundle, the property reference, ${prefix}, is replaced by the value, MyTransform, which is specified by the config element in the feature repository.

Automatically deploy an OSGi configuration

By adding a configfile element to a feature, you can ensure that an OSGi configuration file gets added to the InstallDir/etc directory at the same time that the feature is installed. This means that you can conveniently install a feature and its associated configuration at the same time.
For example, given that the org.fusesource.fuseesb.example.cfg configuration file is archived in a Maven repository at mvn:org.fusesource.fuseesb.example/configadmin/1.0/cfg, you could deploy the configuration file by adding the following element to the feature:
<configfile finalname="etc/org.fusesource.fuseesb.example.cfg">
  mvn:org.fusesource.fuseesb.example/configadmin/1.0/cfg
</configfile>