Pipes & Filters, Layered Systems, Iterative Refinement

So far the pattern descriptions from the Berkeley wiki seem to provide a reasonably good summary. By no means is it comprehensive but if one is familiar with the Pipe-n-Filter & Layered patterns, then it's straight forward. The Iterative Refinement pattern, though, was a hard read. By rereading this article twice, one can get a high-level understanding. VHDL was the first thing that came to mind for applying iterative refinement but I was a bit disappointment that not enough known use examples were given.

It seems to me that it would be easier to implement reentrant applications in Pipe-n-Filter and Layered architectures as opposed to some others, simply because they promote more local state dependencies and less shared state dependencies between the filters/layers (because they promote the modular ability to interchange filters/layers with ease). And this reentrancy would allow additional parallelism on top of the parallelism that could be achieved through pipelining. As the article states, pipeline parallelism can be achieved along a linear branch while task parallelism can be done for tasks along parallel branch.

On a side note, the description of how a filter cannot invoke/control another filter up-stream or down-stream is a great example of showing how it's data abstraction but not necessarily object-oriented programming. Most students, including myself, have a hard time thinking about data abstraction without thinking about objects encapsulating data.

Finally, the Layered systems article says "Increasing abstraction increases code readability and project organization" and I'm not sure I agree with this. Usually when you increase abstraction, it makes it harder for one to read the code because you're constantly going through all these redirections in order to search for functions that do actual work. Smalltalk and the Adaptive Object Model are examples that come to mind. Although one can argue that Smalltalk makes up for all these redirections by promoting methods with a few lines of readable code.