Thursday, June 16, 2011

Distributed OSGi in Fabric

The Remote Services OSGi specification describes how services registered in an OSGi framework can be transparently accessed from another OSGi framework. This is in essence a remoting capability for OSGi services. Fabric has a very fast implementation of this specification, leveraging ZooKeeper for the discovery of services.

From the user point of view, registering a service with a property service.exported.interfaces (with a value of ‘*’ or a list of classes to expose) is the only thing to do to make a service available from the outside. The Fabric DOSGi implementation will automatically detect which services have to be imported and will automatically create a proxy in the OSGi registry for those needed. This services to be imported are found through the use of Service Hooks which enable the implementation to be aware of which services are required by existing bundles. For example, if a bundle registers a ServiceListener (directly or indirectly by using Blueprint for example), the Fabric DOSGi implementation will check if there is a local service satisfying the listener and if there’s none, it will look into the ZooKeeper registry and import a matching service.

The Fabric implementation is based on the insanely fast HawtDispatch library which has a very nice support for NIO. The result is a very fast remoting mechanism for OSGi with more than 25.000 requests per second on my laptop. This remoting mechanism is actually not dependant on OSGi and external clients can also connect to a remote OSGi service provided they can find the service identifier from ZooKeeper or some other place.

A complete example can be found in the Fabric source tree at on github.

Tuesday, June 14, 2011

Provisionning with Fabric

Already, one month since my last blog entry, but time as been flying with the birth of my son Arthur a few weeks ago. I'd like to talk about the provisioning mechanism of Fabric a bit.

In my last post, I explained how configuration was done through the use of ZooKeeper as the repository for configurations and profiles, propagated to various bundles using the OSGi ConfigurationAdmin service. The provisioning mechanism in Fabric is based on this mechanism: the fabric-agent bundle receives its configuration through the org.fusesource.fabric.agent pid and will automatically update bundles as needed. The nice thing is that the configurations are computed using an overlay mechanism which means that the list of bundles or features to deploy will come from the various profiles a give node has been assigned.

The agent itself supports several kinds of informations:
  • bundles that need to be installed
  • karaf features that need to be installed
  • repositories for karaf features
  • a url pointing to the osgi framework to use
  • system properties
  • osgi framework properties

An example of defining such profiles can be found in the fabric example.

The agent will use those informations and delegate to an internal OSGi resolver to compute the actual list of bundles to deploy. It will then compare this list to the list of bundles already installed and resolve the differences by installing / updating / uninstalling bundles. The agent is able to upgrade all bundles, including core Karaf bundles and even the OSGi framework itself and framework properties.

The profile notion I explained in my previous blog entry allow the use of profiles for provisioning and configuration. For example, a profile can be defined for deploying ActiveM or Camel. And remember one can override profiles, so you can define a profile for your application and change the IP of the ActiveMQ broker in overridden profiles based on the geographic location to connect to the nearest broker.

Due to the configuration bridge between the OSGi Configuration Admin and ZooKeeper, any change made into the zookeeper tree will be reflected to the provisionning agent and the bundles updated immediately. This may or may not be a desired behavior and that's one of the reason why the ZooKeeper tree used by Fabric has a notion of versions. But that'll be explained in a follow up entry.