Friday, August 28, 2009

Design versus Implementation

My last few postings have discussed the design of my wishlist service in very broad strokes: here's what I want to accomplish, here are a few of the issues that I've thought about. They have no detail; they don't even look at choice of platform. To me, this is the essence of the divide between design and implementation.

I see it as analogous to traditional building architecture, where the lead architect sketches broad forms, the rest of the architecture studio fleshes those out as models, and the mechanical engineers make it all work. It's also similar to a corporate mission statement — at least, a mission statement for a corporation that isn't floundering in indecision.

If the design is complete, then the implementation should flow naturally. My design specifies a RESTful interface using either XML or form-encoded data in the request; this implies that my implementation will have a front-end layer to transform these inputs into a common form. The design calls for separation between base product data and any application specific data; the data model will reflect that, and the data access code won't need to perform joins or multiple queries to load objects.

There are decisions that don't derive from the design: for example, my choice of Java and Servlets. This decision is one of convenience: it minimizes the amount of stuff that I have to learn. Other choices include Java/Spring, Python/Django, and Ruby/Rails. While I have some familiarity with Python, and could learn Ruby in short order, any code that I write with those would not be idiomatic — it would probably look like Java with a different syntax. Eventually, I plan to use the service as a basis for exploring Python and Ruby in an idiomatic way, but the first pass should leverage what I know.

This dividing line still leaves a lot of implementation decisions to be made, and the next several postings will look at those decisions and the implementation process itself.

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