OOD and concurrency


LOOP: Lego Object Oriented Programming
LOOP: Lego Object Oriented Programming (Photo credit: jurvetson)

Some background first

Object Oriented Development is still the dominant programming model. It has replaced Structured Development something like 20 years ago. New languages made interesting breakthrough recently (Erlang, Python), but none of them is in a position to get the throne: Functional programming is powerful, but hard to master, and dynamic typing is expressive but leads to code mess.

Its success mainly comes from the accessibility of C++. Many interesting proposals were available at the time, including Smalltalk, but C++ ruled them all simply by being pragmatic.

Thanks to OOD, developers made significant progress towards the Holy Grail: code reuse. To that effect, it offered three major paradigms:

  • inheritance: reuse of code by having a class inherit from other(s). Permit changes done on the super class to trickle to the derived classes
  • encapsulation: the concept of interface: some code can manipulate entities thanks to some contract. The implementation details can change without impacting the code (as long as the contract does not change)
  • polymorphism: those contracts can be implemented by many classes, each of them having various side effects

Initially, there was a strong emphasis on inheritance, but, years of experience as taught us that the value lied in encapsulation and polymorphism, because they permit low coupling. Inheritance turned out to be more an embarrassment than anything else and is often used by opponents.

What about concurrency?

Concurrency is orthogonal to OOD. And as OOD implied linear execution, concurrency wreaks havoc in that perfect world. Methods are transactions that change or consume the state of the object (this/self). If two methods are executed simultaneously, the object sate will shift unexpectedly, e.g: the key found is some internal dictionary may be removed by a delete command, there is an error message but there is no error code, etc…

The traditional workaround for this is to rely on exclusive access, i.e. using synchronized in Java and locks for other languages.
But, we know that locks contain so many pitfalls that you cannot expect to produce durable code using them.
That pushed many highly skilled developers to reject OOP altogether and embrace a more radical approach: functional programing.
Clojure is their flagship, allowing to implement easily scalable code thanks to a share nothing paradigm.

I have a different view on this, I strongly believe that any solution will a steep learning curve will fail, as most of the developers would be kept out of it. That being said, I am very strong supporter of immutability, message passing and asynchronous programming. It is just that I am sure you can combine this with good old OOD.

Well, I have been doing that for more than 7 years now.

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