Tuesday, July 31, 2007

OSGi Bundle Repository Indexer Open Sourced

The OSGi Bundle Repository (OBR) is one of the most popular pages on the OSGi developer web site (www2.osgi.org), it receives almost as many hits as its main page. Surprising, because OBR is hardly advertised.

We developed OBR last year to provide a single place where OSGi developers can find bundles for their projects. One of the key drivers of the OSGi Alliance is to provide a universal middleware platform, reuse is therefore crucial. To reuse, one has to find appropriate components. OBR was developed to address this need.

How does it work technically? Well, the current OBR was derived from Richard Hall's Oscar Bundle Repository, and it is based on the same idea of a federated repository. A repository is a web server that allows access to the meta data and location of bundles. This information can be used to install bundles on a framework directly. For example, Richard developed a shell extension for Oscar/Felix that downloads a bundle from OBR, including its dependencies. Last year, I developed a web interface that allows for browsing the repository interactively.

At the bottom of OBR you find the repository.xml file. This file contains the meta data of the bundles and can link to other repositories as well. OBR is therefore a federated repository. This was an important aspect because we needed to make it as easy as possible for people to provide their own bundles to the repository. In the OBR model, any company or person can maintain its own repository and link it to others. This also enables repositories to be partly private on an Intranet while it still provides access to external repositories. In this model, the point where OBR is accessed can provide a trust model.

The information you find in the repository.xml is equivalent but not identical, to the information in the bundle. The reason it is not identical is that it has become more clear to us in the last few years that the OSGi technology needs a more generic dependency model. Over time, we have added more and more dependencies like Import-Package, Require-Bundle, Bundle-RequiredExecutionEnvironment, Bundle-NativeCode, etc. These dependencies had their own header and their own intricacies. Though this has some advantages, I am afraid that over time it will become too complex. For this reason, we used a generic model for OBR consisting of Capabilities and Requirements. This model resembles JSR 124 but we made it more powerful by making requirements OSGi filters instead of simple properties. Filter based requirements make it possible to create complex expressions and check magnitudes. For example, a requirement can be that there is a screen with a size of more than 100x100 pixels. JSR 124 can only handle exact matches.

An OSGi Environment, at a certain moment in time, has a set of capabilities. Each bundle (or hardware) requires a set of capabilities but can also provide a set of new capabilities when installed (and its requirements are met). This is a wonderfully elegant model. For example, if you have a mobile phone and insert a headset, the phone will be extended with a headset capability. This enables the installation of a bundle that requires a headset, which in its turn provides new capabilities. This model worked so well, that Richard Hall changed his framework resolver in Apache Felix (the program that wires bundles before they can be started) to work in this generic mode. Though we found a few nasty cases related to constraints that are currently not modeled, the idea performed very well.

This model is described in a document available from our website. Please note that this document does not mean that the OSGi Alliance will develop this concept further, we are testing the waters only!

Maintaining the repository.xml by hand is not an option. Eclipse alone provides over 1100 bundles. Fortunately, the OSGi specifications define lots of manifest headers that provide meta data. We therefore developed a program that reads a set of bundles and creates the repository.xml. Last month we got permission of the OSGi Alliance Board to open source this program. This makes it possible for organizations to include a bundle indexer. The sources are hosted on a Subversion server at http://www2.osgi.org/svn/public/trunk/org.osgi.impl.bundle.bindex/.

So how can we make OBR a success. Well, only if you help us do we have a chance. In my work I see so many companies that develop lots of bundles that are not strategic for their company. If we want to make the idea of a universal middleware work, we must share these non-strategic bundles with each others so that we do not get an umpteenth version of a serial port bundle.

If you have those bundles that you want to share, then setup your own repository. It is quite easy. Absolutely crucial is the meta data in the bundles. It is sad, but there are way to many bundles (and JARs) out there that have no name, no description, no license information, no nothing. It takes 5 minutes to add this information to a manifest and it will make bundles more reusable. So you have no excuse to skip this! Then if you have documented bundles, you just create a directory on your company's webserver and save the bundles there. Then once a day you run bindex on this directory. Bindex creates the repository.xml. The only thing left to do is the URL to me. I will link it into the main OSGi Bundle Repository. So, do not ask what bundle you can get from OBR, ask what you can mean for OBR!

Looking forward to seeing all your repositories,

Peter Kriens

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