Friday, September 29, 2006

Alternative OSGi Styles

When you spent most of your daily life worrying about extremely detailed Java class loader peculiarities then you tend to start thinking that what you’re doing is really important. This feeling is amplified when you see all those important people taking it really serious. And then you suddenly get confronted by a group of people that completely violate our specification but still do something very interesting.

I am talking about osxa. They looked at our specification and decided that the goal was lofty but that it contained too much unnecessary cruft for them. They just took the interesting 80% (well for them interesting) and ignored the rest. I almost took it personal, but they just put all bundles on the class path! Ok, it meant that they did not need class loaders anymore and that opened a larger range of deployment scenarios. For example, they can deploy their applications in a JNLP web start without the need for signing.

Obviously this implies they have no proper isolation between bundles; bundles can conflict in their packages. However, this is isolation is not always needed, especially when the set of bundles that gets deployed together is fixed. These conflicts can then be resolved ahead of time.

And this is the different, surprising, perspective that they took. They love the service registry and do not care about the module layer. They needed a model where they could use many different components, bunch them up together, and deploy them as a whole in a JNLP deployment or inside web app server. What me worry about bundle life cycles?

Actually, when I realized how much they liked the service registry, I could not stop myself from warming up to them. I am often told that the real important stuff of the OSGi is the module layer. The service registry is just a hanger on. In JSR 291 we actually tried to remove the service registry to simplify the specification. We did not because the module layer and the service registry were very hard to separate. However, I always liked the services most; for me, the module layer was the necessity but the services were interesting.

Obviously the osxa people trample over our carefully crafted specifications. However, I think their perspective is interesting. I think it demonstrates the power of the specification that you can write bundles that will run in a compliant framework but also in something that is wildly different. I have always felt that bundles are the important parts; as long as bundles can write against a good specification, then framework developers should be allowed a lot of leeway. It is interesting to see how they use quite a few equinox bundles.

Anyway, the osxa will not receive compliance anytime soon because they completely discard the module layer and take a lot of liberty with other aspects. They can therefore obviously not call themselves an OSGi framework implementation and developers should be very careful not to target any of the peculiarities of this framework. Still it is interesting to see that the success of the OSGi specifications is enabling these initiatives. Initiatives that can have value in certain circumstances and clearly prove the viability of the OSGi model.

Now, if they only could get their demo working again …

     Peter Kriens

Tuesday, September 26, 2006

OSGi UIs and the Web

The OSGi user interface has been a problematic aspect from the beginning, despite the fact that there are so many to chose from: awt, swt, lcdui, swing, thinlet, flash, svg, p3ml, proprietary, and html based. We tried many, many times to come up with a model that all could live with, so far to no avail.

However, one model that seems to emerge is web based UIs. The advent of DHTML, Javascript, and SVG provide an extremely rich environment for application development. Unfortunately, Javascript is not a nice language to handle large and complex problems. Many companies solve this by using webservices to distribute the work between the graphic user interface (GUI) and a server. Web services are remote procedure calls done over the HTTP(S) protocol. This model is reasonably well supported by Javascript using the XMLHttpRequest object.

Th web services model maps very well to an OSGi service platform. In a way, you just need a way to easily call the services in the OSGi service registry from Javascript. So how would such an application look like?

Let us first layout the bundles I like to think in bundles because it is nicely concrete and down to earth. We obviously need a web server, which is easy to fulfill with the OSGi Http Service. We then require a bundle to handle the grunts of the remote procedure calling and another bundle to prove that it works.

The rpc calling bundle has the responsibility to handle the remote procedure calls from Javascript and map them to calls on an OSGi service. I’ve called this bundle webrpc. The second bundle is the demo application. This summer I emotionally blackmailed my son Thomas to learn how to write a program; he caved in and wrote a working Sudoku game in Java after a false start in C++. This exercise gave me more experience with the Sudoku domain than I ever wanted to have so the demo will be a sudoku bundle.

The webrpc bundle should not indiscriminately export services. There are obviously security issues but also not all services are well suited to be exported. However, we’d like to keep the service as simple as possible. The choice was made to look for the public property on the service registration. This is easy to set with declarative services and it means we can use a POJO as our service; the service itself is not involved with the remote procedure calling. The value of the property is the name under which the service should be known to the outside world.

I could have used a complete web services stack, there are an amazing amount of those applications available, almost all written in Java. However, I wanted to run the demo on my Nokia E70 phone that runs an OSGi service platform. Memory is only abundant on PCs and services, so a complete web services stack can add up. It also adds up on the Javascript side because a compliant web services stack is non-trivial. I therefore decided to optimize space and implement a very lite model of RPC over HTTP. Requests are done using the HTTP protocol. The path provides the service and its method, while the query parameters provide the arguments. I used 0 for the first argument, 1 for the next, and so on. For example:


This request will invoke the newGame method on the service that is registered with the property public set to “sudoku”.

The webrpc bundle depends on the Http Service. When there is an Http Service present, it registers itself under the /rpc alias. The servlet will get called for all requests that start with /rpc. For such a request, the webrpc bundle looks up the service name (in the example sudoku) in the service registry using a simple query string with the OSGi filter language to do this. The next part is calling the right method in this service; this is non-trivial because it is necessary to coerce all the arguments from strings into the method argument types. I wrote a separate Invoker class to handle this coercion. Read it and weep. Most of the work is handling Java’s silly primitives.

When the method has been called it returns an Object. This object must be shipped back to the Javascript in the browser. Now the gut reaction of most programmers is to think XML. Not me this time, there is another standard called JSON that is so much easier for Javascript. It is a representation of strings, numbers, arrays, and maps that is compatible with the Javascript syntax. Converting an object into JSON is quite straightforward for values like numbers, strings, maps, and arrays; references to objects are a bit harder but that is a nice subject for another article. The JSON value is then returned as the body of the HTTP response. The Javascript will receive this text body and compile it to Javascript arrays, values, and dictionaries. From the Javascript point of view it can not be simpler.

The webrpc bundle maps requests that do not look like a method name to a getResource() call on the service object’s class. A service implementer can place all its web resources in the www directory and they are automatically mapped. That is, if someone requests /rpc/sudoku/prototype.js, it will load the resource from aQute/sudoko/www/prototype.js. Assuming the implementation class is in the aQute.sudoku package.

All this effort in the webrpc bundle has makes the sudoku bundle almost trivial. Well, I guess it was trivial in the first place because the server only calculates the board and this is not rocket science. The Soduko service only implements a newGame method. This method returns an array of 81 integers (remember how 9x9 was?). Negative integers mark positions that can be shown from the beginning; positive integers are to be guessed by the player. For the HTML side I decided to make a splash screen in SVG. Hey! We now have those fantastic tools, so let us use them. Though splash screen sounds more dramatic than it turned out, I admit, I am a lousy artist.

For the handling of the HTML and http requests calls I am using prototype.js. The Javascript and HTML is actually quite small. The Sudoku board is completely drawn in HTML and formatted using XML. When the Javascript starts up, it requests a new game from the Sodoku service. When this information (asynchronously) arrives, it formats the board with the received information. Rather cool in this model is the use of Cascading Style Sheets (CSS). CSS makes the mundane detail of formatting all the little details nicely manageable; however, remember I am not an artist so the looks can be improved.

Both the webrpc and sudoku bundles used declarative services. The webrpc bundle was hard to test standalone because it relies heavily on the OSGi service registry. The Sudoku service was however a POJO and could therefore easily be tested.

Try it out! If you think it looks too big, just make the window smaller. Notice how nicely it all scales because CSS and SVG are used to adjust all parameters to the window size.

You can download the bundles from the OSGi Bundle Repository (OBR). The JAR’s contain the source code of the bundles in the OPT-INF/src directory. See how extremely little code is required to achieve the goals. Enjoy!

     Peter Kriens

Wednesday, September 13, 2006

Enterprise Workshop

I am typing this on my way back home from a very successful enterprise workshop. Over 30 people from many different companies had taken the trouble to show up for this requirements gathering session; many of them OSGi members but also a number of new companies that are interested in using the OSGi service platform in less resource constrained environments than home gateways, cars, or mobile phones. After this busy day I can safely say that there is sufficient interest to start working on enterprise OSGi! We will create the charter for the EG in the coming weeks.

How do you gather the requirements from 30 people at the same time? Not an easy task. We decided to first get a short introduction from all participatns and then let volunteers present their position statement. We got a few interesting presentations from Siemens, SAP, and others about where they see the opportunities for the OSGi specifications to play a role. We then spent some time doing a round table discussion to capture the ideas and requirements from the different participants. These ideas were categorized and then sorted by voting. After several failed voting system attempts we decided to vote with post-it notes glued to the projector screen, which reminded Jon Bork from Intel of “hanging chads”. Fortunately, we did not need the supreme court to intervene.

“Stay modular and small”. This was one of the first things Hal Hildebrand from Oracle said, and nobody disagreed. People like the OSGi technology because it was created for a constrained environment, which has given it a lean appearance. Unanimously, we liked to keep it that way. The new specifications required for the enterprise group should not fragment the world like J2ME and J2SE and neither should they bloat like J2EE. New features should be added in a modular fashion so that the basic framework is still usable in embedded environments.

A key requirement we identified was distribution. The strength of the OSGi specifications are that they specify a very efficient model for multiple applications to share a single VM in a single process. However, when moving to the enterprise it becomes crucial to provide mechanisms to scale to multiple processes, multiple VMs, multiple machines, and multiple languages. So far, we have not addressed outside VM issues because inter-process communications requires very CPU intensive mechanisms that are just no viable in resource constrained environments. It is, however, clear that the enterprise world is more than willing to pay the extra price for the increased scaling, availability, reliability, and access to applications written in other languages than Java that a distributed model can provide. As was remarked, most new code is just the fur on a giant hairball of legacy applications. From an implementation point of view, the OSGi service registry seems the perfect candidate to implement these requirements.

Another requirement was more focus on the whole life cycle of applications. The OSGi Alliance provides good specifications for the service platform but development, debugging, deployment, management, auditing, monitoring, patching, education, and certifications of applications have so far been out of scope. These were considered crucial aspects for “universal middleware” standard to succeed in the enterprises.

There was also a very interesting discussion about more comprehensive dependency management. Can we model the aspects that are outside the VM? More extensive metadata can go a long way to make deployments more accurate and offers possibilities in grid computing like models.

A number of new services were proposed. An interesting one was the network awareness service. I remember discussing such a service at least 5 years ago. Time flies. Also, people would like to see even more security mechanisms, an enhanced configuration admin, data synchronization, web services stacks, and several more.

There were many more topics raised and all of them deserve further discussions. However, the previously sketched areas seem to cover the most urgent needs. The next steps we have to take is chart a charter for the expert group, select a chair (if you are interested, you should propose yourself as a candidate), get board approval and then have our kick off meeting so we can start working. Most companies volunteered people to work on different aspects of the specifications..

The general scope of the EG is clear: We want to make OSGi feasible in the more distributed enterprise world and address the non-Java oriented aspects of enterprise applications. If your company is working in this scope, I would seriously consider joining. Looking at the current adoption rate of OSGi in his area and the participating companies, I would say that we will probably develop some pretty important standards in the coming time; specifications that will impact the work of many. Though we have a sufficient number of companies to get started, we can always use more!

     Peter Kriens