Consuming arbitrary remote services with the OSGiELResolver (camunda BPM OSGi)

In my last blog post I promised to give a slightly more advanced example about how to use the new OSGiELResolver. And as I promised, here it is ;-)


The setup is quite simple. We have three bundles:

  1. API
  2. Service Provider
  3. Service Consumer

You can find all the sources here. (feel free to suggest improvements, possible bugs, etc.)
As runtime I used two Apache Karaf instances on my computer (version 2.3.5; I had some problems with 3.0.1).
For remoting we'll use Apache CXF 1.4 (single bundle release).
And of course we'll need camunda BPM platform OSGi, which you'll have to build yourself.
Before I tell you more about the three bundles I'd like to point the book "Enterprise OSGi in Action" out. Without that great book I couldn't have provided this example. It's definitely worth reading.

So, enough advertisement, let's take a look at the bundles.

API bundle

The API bundle is really simple. It only contains one interface with a method. We'll need the bundle in both runtimes.

Provider bundle

Now we're getting a little bit more serious. The provider bundle contains the service implementation we want to use.
The context.xml contains the important parts for remoting:

<entry key="service.exported.interfaces"   
 value="de.blogspot.wrongtracks.osgielresolver.api.SomethingService"/><entry key="service.exported.configs"  
<entry key=""  

"service.exported.interfaces" should be obvious.
"service.exported.configs" tells Distributed OSGi to look for implementation specific properties.
Lastly "" lets us define an alternative address. It is quite helpful if you don't want to type the fully qualified name of the class in your browser or other config files.

Consumer bundle

Let's take a look at the consumer. This bundle needs a little bit more information to work properly. To be able to consume remote services we need the OSGI-INF/remote-service/remote-services.xml. It doesn't have to be that name or that directory. You can specify the path inside the bundle with the "Remote-Service" header, which I set in the POM to:


I won't walk you through the remote-services.xml. I'm sure you'll find better explanations somewhere else. (e.g. in Enterprise OSGi in Action ;-) )

After we configured this we can use the reference tag in the context.xml to find the service.
To make the service work with the OSGiELResolver we have to add two things. In the remote-services.xml the property "processExpression" has to be set and in the context.xml we have to use a filter.
As you may know the ELResolver uses the filter to search for classes. Searching only worked when both, attribute and filter, were set.

The provider Karaf

Like I said, I used Karaf as runtime. The "provider" Karaf needs three bundles:

  1. API
  2. Provider
  3. Apache CXF

Just drop them into the deploy directory. It worked best for me when I started them in the order API, CXF and provider. Then everything should work as expected.

The consumer Karaf

The "consumer" Karaf needs a little bit more bundles (and if you run it on the same machine you'll have to change three ports). You have to add:

  1. API
  2. Consumer
  3. Apache CXF
  4. camunda BPM platform OSGi and dependencies

Drop API, consumer and CXF jars into deploy (again, starting API, CXF and then consumer works best). Adding camunda BPM platform OSGi isn't very difficult because there is a feature.xml (assumed it is installed in your local Maven repository).
To install it type: features:addurl mvn:org.camunda.bpm.extension.osgi/camunda-bpm-karaf-feature/1.0.0-SNAPSHOT/xml/features

and then:
features:install camunda-bpm-karaf-feature-minimal

This should resolve all you bundles. Now, If you start the consumer bundle you should see the log saying "Started process". Strangely the logger of the service implementation was quiet. But if you uncomment the exception you can see that the service was called.

So, as you can see, the new OSGiELResolver makes it possible to consume arbitrary remote services, which is quite an improvement. I hope my example is understandable and helps to see the possibilities.


When you encounter this exception:
java.lang.IllegalStateException: Invalid BundleContext just start the CXF bundle again, then it should work.

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