What is SOA? It is currently the hottest thing going on in corporate technology, and promises to simultaneously integrate disparate applications on multiple platforms as well as provide code reuse to all of those platforms. According to Juval Lowy, it is the culmination of a 20 year project to enable true component-based design — in other words, the fulfillment of COM, rather than merely its replacement. Others see it as a threat to object oriented programming. According to yet others, it is simply the wave of the future. Rocky Lhotka recently remarked at a users-group meeting that it reminds him of mainframe programming. In Windows Communication Foundation Unleashed, the authors write somewhat uncharitably:
"Thomas Erl, for instance, published two vast books ostensibly on the subject, but never managed to provide a noncircuitous definition of the approach in either of them."
This diversity of opinion, I believe, gives me an opening to offer my own definition of SOA. SOA is, put simply, the triumph of the Facade pattern.
In the 90’s, Erich Gamma, Ralph Johnson, John Vlissides and Richard Helm popularized the notion of the 23 fundamental design patterns of object oriented programming. I’ve often wondered why they came up with 23 patterns. Some, such as the Flyweight pattern, are simply never used. At the same time, one of the most popular patterns, MVP, doesn’t even make the canonical list. How did they come up with 23?
Here’s an article on the significance of the number 23 which may or may not shed light on the Gang of Four’s motivation. In Peter Greenaway’s A Zed and Two Noughts, the characters become obsessed with the number 23, and claim that there are 23 letters in the Greek alphabet and that Vermeer created 23 paintings (both false, by the way). Perhaps the Gang of Four are Discordians — Discordians are fascinated by what they call the 23 Enigma.
In any case, they came up with 23 canonical (or "fundamental" or "classic") design patterns, and in the past decade, knowing these patterns has become the unofficial dividing line between the common run of code monkeys (I use the term affectionately) and so-called "true" developers — the initiation rite that turns boy programmers into men. Anyone in development who wants to be anybody makes the attempt to learn them, but for whatever reason, the 23 patterns resist the attempt — sometimes because it is difficult to see how you would ever actually use them. It helps, however, to remember that the StringBuilder type in C# is based on the Builder pattern, and that the Clone method on most types implements the Prototype pattern. Delegates are built around the Observer pattern and collections are built around the Iterator pattern — but since these are both basically part of the C# language, among others, you don’t really need to learn them anymore. In my opinion, the most useful patterns are the Template and the Factory Method. The Singleton pattern, on the other hand, starts off seeming like a useful pattern but turns out not to be — a bit like a bad joke one eventually tires of. It is, however, easy to remember, if somewhat tricky to implement.
The one pattern no one ever fails to remember is the Facade pattern. It doesn’t do anything clever with abstract base classes or interfaces. It doesn’t have tricky implementation details. It simply takes the principle of encapsulation and goes crazy with it. Whatever complicated code you have, you place it behind a wall of code, called the Facade, which provides methods to manipulate your "real" code. It’s the sort of pattern which, like Monsieur Jordan, once you find out about it you realize you’ve been doing it all your life. The simplicity and ubiquity of the Facade makes it an unattractive pattern — it takes no programming acumen to learn it; it requires great effort to avoid it. It is the dumbest of the 23 canonical design patterns.
And Service Oriented Architecture is all built around it. In some sense, SOA marks the democratization of architecture. There are still tricks to planning a good SOA, and securing it may require some sophistication — but with SOA, anyone can be an architect. Well … anyone who can build a Facade.