What can capabilities do for your processes?

Before we release camunda BPM OSGi 2.0 I want to do a little bit more of advertisement for it and show what is possible with the new version. One change in the new version will be, that it depends on OSGi 4.3 and no longer 4.2. One change, besides the fact that I can now use generics in the code (yay!) is that with OSGi 4.3 the capabilities headers will work. So, what's so impressive about them?

Capability headers

The capability headers are two header Provide-Capability and Require-Capability. They are a further abstraction of the Import-Package and Export-Package headers we all (should ;)) know. But with the capability headers you are not as limited as with the package headers. Arbitrary things can be defined, e.g.

Provide-Capability: sensor; type=gyro

would be a valid statement. But you are not limited to one attribute:

Provide-Capability: sensor; type=heat; minTemp=0; maxTemp=100

is also possible. And the bundle that requires such capabilities can use an LDAP filter expression:

Require-Capability: sensor; filter:="(&(type=type=heat)(minTemp=0)(maxTemp=100))"

That ways it is possible to find exactly what is needed in a way that allows to specify more than just packages and versions.
How can you use this for your business processes?

Capability headers for processes

One use-case that came quickly to my mind were process definitions that depend on each other, e.g. if you have a process with a call activity. An example could look like this (please excuse that I didn't prepare an exhaustive example):

Let's call this one the "Hunger process". And the callee process, the "Phone process" can be as simple as this:

The last time I checked there is nothing that would stop you to try to start the Hunger process although the Phone process hasn't been deployed yet. If the Hunger process would be something that you want to start automatically you would run into a nasty exception. Here, the headers can help. You could simply describe in your MANIFEST that you require the Phone process before your bundle can be started:

Require-Capability: process; filter:="(key=Phone_process)"

You could also add a version number or whatever seems useful. The bundle containing the Phone process should then of course contain the appropriate part:

Provide-Capability: process; key=Phone_process

So, when you deploy the bundle with the Hunger process it cannot be started without the bundle containing the Phone process. That ways you can manage your process interdependencies without running into exceptions.
Finally, if you use the maven-bundle-plugin I want to give you a short example.

Setting the headers with the maven-bundle-plugin

With the maven-bundle-plugin it is really easy to set the headers. I'll suppose that you use <packaging>bundle</packaging> in your POM. Here's how you can set the headers:

<plugin>
   <groupId>org.apache.felix</groupId>
   <artifactId>maven-bundle-plugin</artifactId>
   <extensions>true</extensions>
   <configuration>
     <instructions>
       <Provide-Capability>process; key=Phone_process</Provide-Capability>
     </instructions>
   </configuration>
</plugin>

See, piece of cake ;)

I hope I could give you some idea how you could use the capability headers that OSGi 4.3 introduced. This was just a quick example but I think it shows nicely, how OSGi can support your BPMN processes.