Space Management

This week I am working on the Space Manager module which will handle page allocation in containers. This module is not so exciting, it is tedious work with lots of little details all of which have to be right for it to work correctly. While working on this, I did have some interesting experiences though.

I am a firm believer in Checked Exceptions. I think Unchecked Exceptions should be used very very rarely, and only after considerable thought. It seems to me that those who champion Unchecked Exceptions are primarily looking for clean code that is aesthetically pleasing and is not cluttered with a lot of error handling code. While this may be desirable for improving the readability of the code, correct error handling is more important than improving readability, especially in any mission critical system. Unchecked Exceptions have the big problem that the Java compiler does not flag it if you ignore such exceptions. Therefore, you can write code that happily ignores exceptions, and if this is part of a large system, believe me, bad things will happen.

I will write more on this subject in a separate post. The reason I brought this up is that in SimpleDBM, almost all exceptions are checked exceptions, and while implementing the Space Manager module, I encountered a problem that would not have occurred had I used unchecked exceptions. The Space Manager module is transactional, and therefore needs to implement the transactional redo/undo interfaces defined by the Transaction Manager. While implementing these interfaces, I suddenly found that because I was using checked exceptions, the existing redo/undo interfaces did not allow the Space Manager to throw exceptions that are unknown to the Transaction Manager. At first I thought that maybe I should expand the interfaces to allow this, but then, I realized that doing this would break the modular structure of the system, as it would create a cycle in the dependency graph between the two modules. Also, the Transaction Manager has to be able to manage any number of transactional modules, and therefore must not know about the specifics of any module. If I allowed the TM to know about exceptions thrown by the Space Manager, then it would mean that the same would have to be done for every other module that is implemented in future!

The solution to this problem is to specify the redo/undo interfaces in such a way that modules that implement these interfaces can wrap any undeclared exceptions in a specific exception that is meant to wrap such exceptions. This way, we can continue to use and benefit from checked exceptions, without having to resort to unchecked exceptions.