Thursday, March 20, 2008

Hierarchical vs. Relational

Some Thoughts About Hierarchical vs. Relational Representation
Part 1 – Rationale

As a rule of thumb, I like to design things so that each instance has three possible representations: in-memory object, hierarchical (e.g. XML), and relational. This tends to work out well because these are the three media of exchange in most computer architecture. Thus, in-memory for quick manipulation, hierarchical for static storage and communication via remoting or clipboard, and relational for dynamic storage.

This tends to work seamlessly except in one particular area: referenced objects (i.e. indexed objects in relational systems). Which is unfortunate, because referenced objects are a key paradigm in most computer languages and are the backbone of relational databases. This problem of coordinating referenced objects comes up in virtually every large program or code library, from e-commerce databases, to 3D graphics systems, to spreadsheets, etc.

I’ve tried several generic solutions to this problem. Most worked, but most became unwieldy as the code grew and special cases were considered. Because of this, I’ve decided to accept the fact that generic solutions here are difficult, and decided that instead, hope lies in the design patterns approach.

So, over the course of the next few blog entries, I will be presenting a series of design patterns aimed at tackling this problem. I don’t presume to think that any of these techniques are novel. The point is to arrange a series of rules-of-thumb and ad-hoc solutions into a more coherent set of design patterns.