OSGi and Enterprise Java dynamic modularity after Spring dm Server

The key message to be taken from SpringSource's decision to hand last month their Spring OSGi projects (Spring DM and dm Server) to the Eclipse foundation is the following: that OSGi is not ready for adoption by the mainstream of enterprise Java. Reality has finally dawned!

According the announcement from Adrian Colyer, CTO of SpringSource:

For a mainstream development team though, who just want to build an enterprise application as quickly as possible, and with as little hassle as possible, the costs currently associated with adopting enterprise OSGi can outweigh the short-term benefits. This situation needs to be addressed before enterprise OSGi can become the de-facto approach for mainstream enterprise application development.

While SpringSource will no doubt continue to support OSGi, the extent to which it will continue to provide resources to the donated projects is far from certain. What is more clear, though, is it will no longer be wielding it's own considerable influence within the industry to shepherd its user base en masse towards OSGi.

This is a good thing. Firstly, the obsession with OSGi has distracted the community away from practical efforts which could work more comfortably within the existing enterprise Java model. Hopefully that obsession will go away, or at least be limited to those already converted to the OSGi way. Secondly, from a purely selfish, egotistical point of view, I feel vindicated. For years, I have been saying that OSGi is too complex for the Java mainstream. For years, I have been beaten over the head by a small group of hardcore OSGi evangelists who have persistently used the argument that if SpringSource is betting on OSGi, then it must be a good thing for everyone else, and that any alternative approach does not warrant any consideration. The bet has failed, and this argument has now fallen rather flat. If SpringSource, with all the credibility it has generated through the Spring Framework, all its intellectual and marketing muscle, and all the effort and expense it has incurred, cannot make work OSGi for mainstream enterprise Java, then no one else can.

But all of this is also points to a real missed opportunity. The fact remains that Spring users still need a modularity solution. They need a way of being able to take advantage of the best of what dynamic modularity has to offer - dynamically composable software with genuine decoupling between parts of the application, and rapid build/deploy/test cycles during development - without having to take on all the complexity of the full-blown OSGi approach. This need still exists, and will not go away. As Adrian suggests, this need will not be served by OSGi without some fundamental simplifications, something I could not imagine happening any time soon.

In the meantime, I have invited SpringSource to engage more with Impala. While they have flirted with the idea in the last few months, perhaps even seriously, they have backed away from it for now. In view of recent history, it is not that surprising. Imagine the uproar that would ensue among the OSGi zealots if SpringSource were to go down that route. It doesn't bear thinking about.

Impala 1.0 RC2 released

I am pleased to announce the release of Impala 1.0 RC2. With this release, support for Maven is introduced for the first time, along with various other enhancements, refactorings and bug fixes, including:

• simplifications to the default project structure, making it more intuitive and Maven-friendly
• a dynamic properties Spring namespace, making it easier to support properties which can be injected and dynamically updated without requiring module or application reloads.
• further refinements to the mechanisms for supporting multi-module web applications.
• initial support for Spring 3.0.

For more information on this release see: http://code.google.com/p/impala/wiki/Release1_0RC2Announcement.

Cheers,
Phil Zoio

Don't let OSGi put you off modularity

If you are interested in the ideas of modularity, there is quite a good chance you have looked at OSGi. But there is also quite a good chance that you have been put off by the extra development overhead required to set up and maintain OSGi-based applications.

If this is the case for you, my message is this: don't let fact that you are put off by OSGi put you off the ideas of modularity. Modularity is a bigger, more important and valuable idea than any particular technology. The ability to modularise an application can massively improve your application's ability to absorb complexity as new features and components are added, which in turn means that you remain much more productive as your application grows. It reduces the rate of entropy in your system, extending its potential life span, which of course if great for business.

This kind of modularity is one of the fundamental benefits of Impala. Also, because Impala's modules are dynamically reloadable, and you also get dynamic redeployment of small parts of your application, allowing you to greatly accelerate application development, maintain high levels of productivity, and retain high levels of code and functionality reuse as you application grows.

Impala does not solve all of the problems solved by OSGi. Notably, it does not provide versioning of third party libraries. However, it solves most of the important ones relevant for day to day development. Fundamentally, it allows you to modularise your application, without having to worry about micro-managing the dependencies of third party libraries in your application. Don't get me wrong - there are times when this capability can be important, especially for very large projects with large budgets and teams. But certainly not for every project, and probably not even for the typical one.

One way to think of choice is to look at the graph below.


The basic idea behind modularity is that the growth in complexity of your application slows as your application grows in size, compared to applications without modules. This applies both for Impala and OSGi-based applications. The difference is that because the barriers to entry are lower for Impala-based applications, the benefits kick in sooner, and accumulate for longer over the duration of the project, greatly reducing the overall cost of complexity over the lifetime of a project.

So, don't shy away from modularity just because OSGi looks complex. The benefits of modularity are too valuable, and an alternative like Impala make these attainable with fewer headaches.

The granularity of change in dynamic Java web applications

When writing Java web applications, you are continually making changes to your application, and to be productive you need to be able to deploy and test these changes quickly. The kinds of changes you make are of all sorts: from changes to resources to changes to markup templates to changes in the way your application is wired to changes in the code itself.

The point of this article is that not all changes are equal, both in their frequency and in the difficulty in applying them dynamically in a running web application. Let's go through some examples, and how the changes might be applied in different types of frameworks:

1. Static resources
The simplest kinds of changes for any dynamic web framework to apply are those of static resources - images, JavaScript files, page templates etc. That's because these kinds of artifacts are inherently free of dependents.

2. Application configuration
Configuration consists of things such as settings for data sources, mail servers, etc, as well as switches in your application itself. While it is in general possible to reflect these kinds of changes dynamically, there is a cost. For example, if you are using database connection pooling, pointing to a different data source dynamically is a non-trivial exercise. Also, if your application checks particular application settings at source each time the affected functionality is used, then the system is more dynamic, but also less performant. By contrast, if you only load particular settings at startup (for example using wired in property placeholders in a Spring application context), the application is more efficient but is more likely to require reloads to reflect changes.

3. Application wiring
Application wiring is a configuration of sorts, but relates more to how parts of the system are composed or wired together to form the whole application. In a Spring application, the application wiring is simply your Spring configuration.

In general, changes to application wiring requires reloads. There are special cases where this doesn't apply. For example, you could introduce new Spring beans without reloading the application context. Changes to existing beans and their collaborators are harder to make.

4. Scripts
Scripts are programs in your application which by definition can be altered without having to reload your entire application or even significant parts of it. However, a scripting infrastructure needs to be in place to allow changes to scripts to be introduced, recognised and reflected in the system.

5. Application code
Application code in a Java application are your Java classes. Actually, considering this group as a single category is an oversimplification, especially in a dynamic module system, where making changes to core interfaces and domain classes will impose much greater requirements in terms of reloading than peripheral or implementation classes with fewer dependants.

6. Third party libraries
The libraries in your application are the jar files containing all the third party dependencies.

The challenge for frameworks

The key productivity challenge with a dynamic application framework is to make it as easy as possible to make the kinds of changes you need to make, while at the same time keeping the framework as lightweight as possible. In the next section I take a look a number of technology stacks, what they do to make different types of reloading possible, what they get wrong, and what they get right.

A. Traditional Java web application
A traditional Java web application might consist, for example, of a Struts or JSF front end, Hibernate back end, all wired together using Spring.

The traditional Java web application has no problem reloading static resources without having to reload any other part of the application. Most web containers are able to reload an entire web application, including third party libraries, Java code, application wiring etc.

The problem is they are not much good at reloading any finer grained changes. Any changes you make to your application wiring or code will normally require a full application reload.

B. Scripted applications
Scripted applications are based on scripting languages such as Groovy (Grails) and JRuby (Rails). As well as explicitly providing support for reloading capabilities, these frameworks rely on the fact that all application functionality is in scripts rather than in compiled Java code, making fine grained reloading of parts of the application possible. The downside (if you think of it this way) is that you have to work with scripted code without any of the type safety checking of a statically typed language such as Java.

C. OSGi applications
OSGi applications offer a fairly comprehensive solution to the reloading problem. All artifacts within the application, from resources to application code to libraries are contained within modules which are treated in a more or less uniform manner by the OSGi container. This is a strength, but it is also a weakness. The strength is that it does allow third party libraries to be reloaded in a fine grained way. The weakness is that in your high level view of the application, OSGi doesn't really allow you to easily distinguish between the parts of your application which should be easy to reload - e.g. resources - and the parts which are harder to reload, but are changed much less frequently during the lifetime of the application (third party libraries).

What about Impala?

Impala tries to find the right balance in the strengths of the various approaches. Resource reloading works as with traditional Java applications - nothing special needs to take place. Impala includes mechanisms which make it easier to change configurations dynamically without requiring any module reloading. For changing static configuration and application wiring, Impala allows you to reload parts of your application at exactly the right granularity. If only a single module is affected, then only that module needs to be reloaded. If the change affects core interfaces, then the reload will automatically ripple through to the right dependent modules.

Impala even allows you to dynamically change the structure of modules within the application. Unlike OSGi, it doesn't support reloading of third party libraries. For this, an entire application reload is required. However, Impala's approach does your application modules in central focus, which is important as these are the normally parts of your application which change most frequently.

Slides for Impala talk at the Server Side Europe in Prague

I'm pleased to say that my talk at The Server Side Java Symposium in Prague went well and was apparently well received. There definitely is a growing interest in the ideas of modularity and how the benefits of modularity can be achieved in practice.

I've posted a copy of my slides for the talk here in PDF format. The document contains the slides that I presented, as well as quite a few which were held in reserve but weren't actually presented during the talk.

How to make your Spring wirings more manageable

When Spring first came along it was a breath of fresh air - a clever way to wire up applications which did not rely on the use of all sorts of singletons all over the place. (I still remember what it was like working that way, and shudder at the thought.) The idea was simple: let classes in the application just do their own job, but leave the business of figuring out how to get collaborators to the IoC container, with the help of some XML configuration. No longer did your application code did not have to deal the messy business of having to resolve their own dependencies.

OK, we've solved the problem with the code, but the job isn't completely done. Actually, the problem has shifted onto managing Spring wirings.

The problem with managing Spring wirings

Applications necessarily get big. You end up having to write a lot of XML. So while your code may stay nice and clean, you end up with some tricky questions about how to manage this part of your application. Of course, here you try to be as "modular" as you can, putting DAOs together, infrastructure related beans together, etc. You try to identify vertical slices for your application and put beans relating to particular vertical slices together.

The problem is that for all the bean definitions that exist in your application, some groups are inherently coupled, while others are inherently free of coupling. In a vanilla Spring application, there is no way to express these dependencies at a system level. So it is very easy for your application wiring to become an unnecessarily fragile collection of invisible dependencies, liable to break in unexpected ways when any rearrangement takes place.

Autowiring, namespaces and class path scanners don't necessarily help

Then of course there is the drudgery of editing XML configuration files by hand. Personally, I think that is less of a problem, but Spring has gone to great lengths to free developers from some of this pain over the years, through the introduction of autowiring, class path scanners, and XML namespaces. I happily embrace all of the above as they reduce the amount of code I need to write, but they don't address the fundamental problem. They don't enhance one's ability to express dependencies between parts of your application at a system level, and where possible, to reduce these dependencies.

So how does Impala help?

Remember how easy Spring seemed when we were working with just small applications? Impala allows you to keep your applications small, or at least keep them feeling small. This is done through modules. You can think of a module as a mini-application which is able to communicate with other mini-applications in the system through well defined mechanisms and through sharing common interfaces.

The Spring configuration for each module remains pretty small. If it starts getting too big, then its a good sign that some of it's functionality needs to be split off into another module. So within the module, you only need to deal with small configurations - bite size chunks.

You can configure beans within a module however you like - through plain Spring XML, custom namespaces or through annotations. If your module needs to use functionality from other parts of the system (as most will), then you can import services directly from the shared service registry (as long as the service has been exported using an interface or class visible to the current module). If necessary, you can allow your module to depend directly on another module, either as a dependent or as a direct child.

If you need to compose your application in different ways according to environment or customer, that's easy too. Simply change which modules your deploy, or you can even vary the configuration of within a module according to requirements.

You no longer need to wait ages for integration tests to load, because you can easily create integration tests which consist simply of the modules you need to use.

And you get the benefits of much more productive, responsive development environment because each of these modules can be reloaded on the fly, either individually, in groups, or as a whole - and this applies whether you are running integration tests or running your application on a web container.

Talk at the Server Side Europe in Prague

I am doing a talk on Impala at The Server Side Europe's conference in Prague, which is taking place on October the 27th and 28th. Really looking forward to it, especially as I used to be a regular visitor of Prague in the early 90s when I was living in Germany.