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AKKA-actor DynamicImport-Package: * header

When OSGi modules are fabricated so that they have limited dependencies to other modules, the development process is a breeze. Changes to any single module reflect to a limited set of other modules which enables easy deployment and update of the running software. In the Karaf based project I'm working on, this is mostly the case: updating for example a bundle which is responsible for a certain UI view, only the corresponding view goes down for a couple of milliseconds and the page is automatically refreshed after that.

We started seeing exceptions to this on places where there should not have been any "core" dependencies. When upgrading for example a bundle which exports message/model classes which are part of the Akka communication between the bundles, sometimes the whole karaf seems to go down. The message classes can be used by multiple bundles including the core ones but we started seeing this behavior happening when deploying also non-core Akka message bundles. What made matters worse, it seemed to happen at random - sometimes most bundles go down, sometimes not.

We tracked the reason down to the restarts to akka-actor bundle. The whole actor system (which we have only one in the whole OSGi container) restarted. Using Karaf command bundle:tree-list we saw that the Akka bundle had become dependent on the Akkamessage bundles + some others. This was something that was not supposed to happen. Akka-actor bundle is a core bundle which other bundles should depend on, not the other way around.

After more digging into the cause, we came across an interesting line in the akka-actor bundle manifest file.  DynamicImport-Package: *. Upon further investigation we confirmed that this indeed was causing the dependencies going the wrong way around. In other words, cyclic dependencies. More on dynamic imports in this blog post http://swexplorations.blogspot.fi/2014/07/osgi-dynamic-imports.html

All messages that went through Akka actors that were on a bundles export-package list were causing akka-actor bundle to become dependent on it. First thought was that this is normal, perhaps this is how Akka works. For some reason it might need to import the actual class. But what struck odd is that also messages that were not in a bundle export package list were working fine with the actor system.

Next action was to remove the dynamic import manifest header. That caused akka-actor bundle to not find some class thus preventing the actor system going up. The solution for that was to replace the DynamicImport-Package: * with DynamicImport-Package: akka,akka.* Now actor system went up and also all messages passed through it normally. We did not find anything that was out of the ordinary. We can now also deploy bundles more safely and no core bundles go down when Akka message containing bundles are deployed.
tl;dr akka-actor bundle has DynamicImport-Package: * which causes it to become dependent on bundles exporting messages passing through the actor system.

 EDIT: Found the "problem". When a message passes through akka serialization, it will get dynamically imported.
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