Last week at OSGi DevCon, I attended Peter Krien’s Advanced OSGi tutorial. The tutorial focused on OSGi services, and I had just come off a rather lengthy e-mail discussion with Peter on a similar topic. So this was pretty good timing. I want to share with you one aspect of the discussion, and its relationship to architecture. It’s quite simple, though makes an important point.
In the past, I’ve talked about the importance of designing the joints of a system. OSGi services represent these joints, since services are essentially a module’s published interface. In general, if the implementation details of a module are well encapsulated, then implementation details can change without impacting the rest of the system. In other words, we minimize that nasty ripple affect of change if we confine change to a single module. But changes that span modules is nasty. Let’s take a few visuals to illustrate the point here.
Where’s the Architecture
Commonly, we visualize the module structure of a software system using a simple diagram similar to what’s shown at right (click to enlarge). We see the modules and the relationships between them. Of course, the relationships between modules represent the joints. Pretty simple stuff! Unfortunately, our attention is immediately drawn to the modules themselves, but not so much the joints.
And we really must be much more concerned about the joints than we are the implementation details because changes in the joints of the system are more complex and costly. This is where we need flexibility and it’s also where we need stability. Emphasizing module implementation is important, but flexibility in the joints is more important.
Illustrating the Joints
In the session, Peter annotated the diagram using a new convention, which I’ve depicted in the diagram at right (click to enlarge). The red triangles more clearly illustrate the joints. Here, our attention is drawn more toward the design constructs that span module boundaries. No longer is emphasis placed on the implementation details, but instead has moved to focus on the relationships between the modules. And the direction of the red triangle also helps illustrate the direction of the dependency relationship between the modules. In other words, these red triangles are the services.
The Real Architecture
Now, because the module itself represents behavior that is encapsulated, we can remove the modules from the diagram altogether and show only the joints, as shown at right (click to enlarge). This is where the impact and cost of change is most significant. And this is where we really need to ensure we have the greatest flexibility. These are the decisions that are most architecturally significant.
A Short Digression
Now I don’t think it’s necessarily important to start drawing architecture diagrams this way. In fact, I’ve never felt that it’s critically important to draw these pretty diagrams in the first place. While diagrams (and documentation) can be useful, they are little more than a pimple on software development’s ass. There are much more important things to worry about than diagrams. But I digress, and in general, this simple exercise helps recognize the importance of designing the joints of the software system.
The Real Point
Of course, while this example illustrates the point using modules, it’s not just the joints between modules that are important, but the joints at many different levels of the system, including applications, services, packages, and classes. In other words, the point where two entities connect is where the real architecture lives.