Saturday, September 23, 2006

The Flawed Interface Principle - Part 2

After re-reading my last post, I felt quite disappointed: the main point could have been squeezed up in
  1. the more you add stuff to a system, the more you’re likely to bring mess

  2. if you do things by yourself you’re probably getting a better result (but in more time)

  3. interfaces are dangerous.
Ok, point number one, is right, but really sounds too obvious to deserve a post (I'll never win a Nobel with that...). We’ll get on it later.

Point number two is simply dangerous: you simply don’t start a project developing everything from scratch, but start choosing the available components on the market (let’s say… JBoss, Hibernate, Log4J etc.) No way I am telling that you should re-invent everything from scratch! If time is a constraint (and in software development always is) then the better trade off is somewhere else, remember that we moved from a two-minutes micro waved junk food to an 6/8 hours preparation delicacy. Moreover, there is normally no reuse purpose in cooking. While reusable components are the keys to successful projects.

Point three. The real meaning is that interfaces are tricky. In OOP theory, an interface is just a substitution point. As a client class, I declare the need of an object instance able to perform certain operations, namely the public methods of the interface. I can’t make no assumptions about the implementing class, I just expect that somebody (a factory method or the classloader) will provide the needed runtime implementation. What I don’t know is just what’s behind the interface. Which is both powerful and dangerous.

What lies behind
In complex systems, the value of interfaces is their ability to simplify the overall picture. A complex subsystem can be masked by an elegant Fa├žade allowing designers to ignore implementation details. I’ll make another example with a pretty simple interface: the electricity plug on your wall. It’s a pretty simple interface: 3 holes, 220V, 50Hz. Every electronic device can work everywhere (at least in one single country). The plug mask the whole complexity that lies behind so you can happily ignore about tri-phase cables, power plants, and so on. Unless they start building a nuclear power plant near your home, or stocking plutonium in your garden…
We got closer to the point. Complexity is still there. Interfaces make it more manageable, by allowing us to ignore details, not to delete them.

Crossing competence boundaries
While developing complex J2EE applications, a common problem to face is a version clash between (indirectly) required libraries. Xerces is a common suspect in such circumstances, cause it’s often referenced by the application server, the application itself and one or more used libraries often use it as well. Normally the solution is nontrivial and has to do with fine tuning of the application server classloading mechanism. Still you need an expert J2EE architect to drill and solve the problem. Sometimes is not so easy to find a good one, but it’s a well defined professional profile.

Things are not that easy when you start crossing competence boundaries. One of the first borders is the Java-SQL. How many applications have you seen that could be told to be optimized both on DB and Java performances?

If you start adding more layers, like running on a virtual environment instead of a real one, such as VMWare. How many details are you missing in the meanwhile?

Problem determination in Babylon
Here we go. We built our beautiful tower, by putting freely available components together. The tower is high and majestic. Then we have a problem. If the problem is in the glue we used to put the things together, it’s an easy task. After all gluing is all we did. If the problem is in one brick, we have to determine where the problem is (which can start to be a nontrivial task), and update, refactor or substitute the faulty component.
If the problem is in the interaction between two or more components, or in what happens behind the scenes (or the public interfaces) then you’re in deeper trouble: cause you need cross concerns competence and you probably won’t have it. And it’s not easy to be found as well.

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