Wikipedia defines this principle as:



Today Google Alerts dumped me into two discussions where OSGi seems to be on the wrong side of the divide regarding the KISS principle. The first is an extensive debate in the dev@tomcat.apache.org mailing list, the second is the JSR 277 mailing list.

The Tomcat developers have been asked to osgify their popular web server. This resulted in a discussion that was very interesting to read. Several people seem to have a disdain for the OSGi zealots (which I guess is me) but there seems to be a general uneasiness with this request. However, my key trigger for this blog was that in this discussion I noticed that several people regard OSGi as overly complex and heavy. The JSR 277 discussion raised the same problem from another direction. From the JSR 291 interoperation discussion: "I wanted something in Java that was simpler and cleaner that OSGi-users could leverage if they wanted.". Is OSGi already ossified?

The key problem to decide this question is, as so often the case, requirements. Is Java complex? Hard to answer, if you want to write a "Hello World" it is humongous overkill. If you are writing a large scale web application it is nice to have a solid foundation. The key phrase in the definition of KISS is unnecessary complexity. Complexity is in the eye of the beholder.

First, the OSGi API is very small by any measure. There is only one mandatory package that only has 26 class files, including the security and exception classes. And you can already leverage OSGi without writing Java code. Can you call it intrusive when you can turn a JAR into a bundle by adding one or two manifest lines? All the complexity people complain about is optional. That said, I do agree that even optional things create some kind of conceptual complexity. So maybe there is a lot of cruft in the options?

Let us take a feature like import packages which is often considered hard. Most dependency mechanisms like Maven, Ivy, and also the original JSR 277 are based on the Require-Bundle concept. I.e. you declare a dependency on the wrapper.
Wrapper based dependency models are simple to understand for us humans. It would work actually very well in practice if JARs would be highly cohesive and not change their constituent packages over time. Which raises the question: Do you want the woman, or the dress?

We developers have discovered the wonders of refactoring and things do change. OSGi is a module system, it is about being able to change software while not breaking the deployments. Though import packages might be harder to understand initially than Require-Bundle, Import-Package does create more resilient systems because it minimizes coupling. That is why people that have used both systems usually decide to standardize on Import-Package (and bnd makes this quite easy).

Similar to the uses: directive. In the Tomcat discussion it was stated that the uses directives made the manifest unreadable. It was much nicer to create a manifest by hand, it looked better. I do agree from an aesthetic point of view but who reads a manifest? The manifest is intended to be read by the OSGi Framework. The uses directive is an incredible important concept to maintain classpace consistency when the same class can be loaded multiple times through different class loaders. If you have lived in a world where this model is supported, you understand that uses have a very high value because they minimize runtime problems.

I do not say that OSGi has no unnecessary complexity at all. Obviously, the specification has been evolving over almost ten years. We have to tried to keep it is simple as possible, but we are as fallible as all of us. However, requirements have always come first in OSGi and fulfilling requirements just forbid you to make things too simple because they force you to understand the problem.

Obviously the key question is, are many of these requirements highly specialized cases not useful for the majority. Or are they just solving problems that awaits anybody that ventures into the wonderful land of modularity?

Peter Kriens

Last evening I got back from a very pleasant trip to JAX 2008 in Wiesbaden. I was asked to give the best practice and the OSGi modularity presentations. Rod Johnson had already warmed the audience for OSGi with the result that we had to close the room when people where sitting, crawling, and standing everywhere even though there were still 40-50 people standing outside. Good news for OSGi, but I had to give the presentation for a second time that evening at 21.15, where I was surprised that there were still 50-60 people present that late in the evening. The next day was even better because I got a much bigger room that was also filled. Feels good after gotten used to preaching to audiences of 10 people for so many years.

I also got handed a copy of the first official OSGi book in the world. The book, written by Gerd Wütherich, Matthias Lübken, Nils Hartmann, and Bernd Kolb.

This is a thorough book on OSGi (though Equinox biased) that I recommend to anyone who is using OSGi or wants to know more about it. You can buy it at d.punkt. I hope they translate it in English soon. I already read most of the text during my Christmas vacation to help them with a review. Gerd had already shown me on the first day the thank you section. They had been really nice, and made the thank you in Dutch! Great work! It is great to finally being able to point to a good OSGi book.

JAX is a great conference, very well organized. Well, there is so much to do that I'll keep it short. Though there is one observation, unrelated to OSGi, I can't help myself making. On the way back with Ryan Air I observed that people can be incredibly rude at jumping the queue, and not just the one that look like it, even elegantly looking older people showed an incredible rudeness. Though I admire Ryan Air for many things, their queue management is non existent, seeming to call out the worst behavior in people. I figured people behaved so badly so that to get the best seats in the plane. However, when I boarded at the end of the queue the emergency seats in the middle were still mostly free. So why are people so much in a hurry that they are jumping the queue? One day I have to debug this, but now back to the build!

Peter Kriens

P.S. Talking about books, you can now also order the specification (core + compendium) in printed form for a special price.

Last week I had a great time in Stockholm working two days at ComActivity AB. They are reworking their applications so they can take advantage of OSGi in their server applications. In this process they are investigating lots of technologies. And as is so often the case, one runs in modularity problems while using legacy libraries. Ok, they are not all legacy libraries, they are often brand new and provide valuable functionality, and so often for free as well. What a world!

However, integrating libraries into an OSGi based system is often fraud with class loader problems. In this case we took an in-depth look at the Cinnamon library from www.spicefactory.org. This library is a Remote Procedure Call (RPC) library for Flex based applications using the AMF protocol. AMF is an Adobe (publicized) standard for serializing ActionScript objects from Flash.

At the start of the visit I was shown a very nice demo of the application. Fast, powerful, and looking good despite the austere theme they were using. As can be expected, the application took advantage of the OSGi architecture. A top level application used the service registry to discover modules that implemented a specific application functionality. This module provided the Flex flash SWF file and provided the receiving end of the RPC interface. It was kind of cool to see how they made new bundles with bnd, deployed them with File Install and during the whole morning never restarted their Tomcat that was hosting their OSGi WAR file.

During the demo we went through the design by looking at the source code, modifying things, and then testing it. One thing I noticed was the large amount of configuration and specialized classes to make this application work with Cinnamon. I once write a tiny webrpc application and did not think it had to be that hard. (I am an optimist).

One of the key issues was configuration. The Cinnamon library had two types of configuration: Spring based and a private XML configuration format. The key problem is that these text based configuration files are forced to use dynamic class loading. If these classes must come from other bundles, then OSGi (rightfully) barks. The whole purpose of the OSGi modularity is to have controlled class loading, taking care of versions and other constraints. Doing Class.forName blissfully ignores such mundane details. A large part of the custom code was necessary to find the right classes at the right time.

Obviously, this was working. However, cruft frustrates me and I convinced them we should try to get rid of it. So at the last two hours of the second day we created a new project and started from scratch. No Spring XML and no Cinnamon XML. It was already at the end of the second day so we had little time but it turned out to be surprisingly simple to use the Cinnamon library from Java only, taking advantage of the facilities of the service registry (and that without even reading the manual or javadoc). Still, by getting rid of the XML configuration things became significantly easier. All of the class loading problems disappeared as by magic because from the Java code we had direct access to the right types without requiring dynamic loading.

By single-stepping through the Cinnamon library I also noticed how much code was spent on making the library flexible and adaptable. As so many libraries today, it was doing impressive class loader gymnastics to implement yet another plugin system. I think the library could be significantly simplified if it had been using OSGi to provide this plugin mechanism with bundles and services. It is sad to see how many time the same wheels are invented over and over again.

Anyway, I also remembered how I had a similar experience with Hibernate last year. In an effort to use Hibernate from an OSGi based system, I switched from the XML configuration to do the configuration in Java code. Also in that case, a lot of things that caused class loading issues just disappeared because classes could be manipulated as classes (with the proper versioning) and not as strings.

Different, but still related, the day before we had another experience where we significantly simplified life. They had used Spring to define global configuration for database connections. This is obviously highly specific information for each of their customers. With Spring they could provide this configuration through a bundle. However, if they wanted to change the configuration, they had to update this bundle. Looking closer at the configuration file it was clear that they were specifying several connections to databases. This looked like a clear case for the Configuration Admin's Managed Service Factory. We modified their pool manager to use the Configuration Admin by registering a Managed Service Factory service. This was only a few lines of code. They were already using File Install, and File Install makes it easy to set new configurations by placing a property file in the watched directory. It was kind of sad to see how small and readable the property file was in comparison with the XML configuration it replaced.

Now do not get me wrong. I am very impressed with the Spring, Cinnamon, and Hibernate libraries and I do see the value of centralizing configuration files. Maybe OSGi changes the rules a bit and this XML based model works better in monolithic applications. Lets face it, what is the difference of changing XML or Java code in Eclipse? However, last weeks experience makes me wonder if we should not be a little bit more critical before using XML to solve problems that are much cleaner and better solved in plain old Java?

Peter Kriens

Alex Buckley posted a mail to the JSR 294 list with was more or less superpackages meeting kryptonite. Though in this case, Alex is in my opinion the good guy! He basically abandons all the unnecessary complexity of the earlier proposal, see my blog about this subject, and comes up with a guiding principle: ''simpler is better''.

The proposal definitely follows that principle. A source file is marked with the word ''module'', just like a package. And that is basically it.

package org.osgi.framework;
module org.osgi.core;

public class AdminPermission {
module void checkCertificate();
...
}

The compiler encodes the module membership in the class file and the VM can enforce that any member attached with the module keyword is not accessible from another member unless it is in the same module. I like it! This can actually be quite useful, also for some of the work we are currently doing in the OSGi Alliance concerning modularization. More detail is needed, but this significant simplification is absolutely in the right direction. There are some rough edges obviously, and I like to weigh my input on them.

Module membership should not be on the source but on the package because it is ''inherently'' safer. Alex argues in his blog that from a technical point of view the best place for module membership declaration is the package (package-info.java), but that from a ''moral'' point of view the class file is better because it allows one to introspect a class file and have all information together. Moral in this case should be seen as the Java philosophy, which he defines as focus on understandability by not letting the compiler infer information, but requiring that the programmer states his intentions clearly, even if this is redundant. I.e.

  
  List<List<List<CharSequece>>> list = new ArrayList<List<List<CharSequence>>>()

Is for me harder to read than:

  
  List<List<List<CharSequece>>> list = new ArrayList&lt>()

Because I can trust the compiler to do the right thing and I trust it more with these things than my reading capabilities.

I guess this was also attempted in the JLS, which is also highly redundant. Personally I think this decreases readability because you tend to glaze over sections you already have read 5 times in the previous page. However, this makes it too easy to miss small, highly important differences.

Anyway, I guess consistency is important (though I strongly believe Java would be more readable with ''less'' verbosity, less is more!) but I still very much prefer inherent safe models instead of tool enforced safe models. Why would you introduce a data structure that can be wrong if you can also do it in a way that does not allow those errors? Let me explain this a bit more.

Both Alex and I seem to agree wholeheartedly that packages are first class citizens and that you can not have modules criss-cross through packages. I.e. each package member must have the same module membership. That is, you can not have two classes in the same package that would not belong to the same module. So module membership is therefore not on the type but on its package. Placing the module membership in the source of the type just allows you to create invalid configurations when two sources in the same package would have a different module membership, which will just require more tools to fix them.

Putting the membership on the package would also be a good reason to overhaul the reflection methods of Package. Packages have always been second class citizens in the Java VM, for example you can not find its members. By placing the module membership on the package we have a perfectly good reason to fix these deficiencies.

Additionally, I do have some more wishes for the EDR2. I'll go through them shortly.

• Module private interfaces - Modules should have module private interfaces. Currently, when you implement an interface, you are forced to make your method public. It must be possible to implement interfaces without forcing you to declare the implementation methods public.
• Class Loaders - Modules should be able to cross class loaders (with the restriction that packages do not cross class loaders). This will multiple bundles to form one module.
  
• Dynamic Membership - I do hope the API will allow dynamic module membership. That is, it should be possible to add and remove packages to/from a module in runtime.
• Module-Export - I think there is a need to export a member or type to another module only. I.e. if module M1 imports M2, it should be possible for M2 to selectively exports members and types to an importer. This could be done with a keyword or an export on the module. for example:
  

        M1: Import-Module: M2
      public class Foo {
        void bar() {
          Ok ok = new Ok();           // no problem
          new Fail(); }       // not visible
      }

      M2:
      export module class Ok { }
      module class Fail { }

• Scoping - Currently type references are only encoded with their name. It will be necessary to add the module name to a type reference, and while we are at it, it would be terrific if this then also encoded the version it was compiled against. This will allow the module system to calculate dependencies instead of relying on the old model of the serendiptity of the classpath. Obviously the module file should be able to change the hard references from versions (1.3.2) to a range (1.3,1.4).
• Versions First class citizens - As show in the previous example, versions should become first class citizens. I do not think annotations are the right way to go for this.
  

The previous points are requirements, and not solutions. I do not envy Alex's task to come up with JLS changes. This is the same as open heart surgery.

From an OSGi perspective I think modules as proposed by Alex will be a welcome addition to
the language. I think it addresses some of our uses cases we are working on. Nice work!

Peter Kriens