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November 10, 2005

On Interface, Implementation, and Reuse

Filed under: architecture,integration,webarch,webservices — Mark Baker @ 2:58 pm

A common technique in software development, but particularly in distributed software development, is the “separation of interface from implementation”, whereby software components are developed to have dependencies only upon the interfaces (the connectors, in architectural terms) of other components, and not their implementations. The purpose of this technique is to isolate the impact of a change of implementation to just the changed component, thereby reducing coupling between components. The technique is so pervasively referenced and evangelized, that it’s practically axiomatic.

Unfortunately, it’s not commonly practiced.

Many who claim that they’re using this technique will tell you that the use of a description language, such as IDL or more commonly nowadays, WSDL, does this for them. It does not.

As mentioned, the purpose of the technique is to isolate the impact of implementation changes. Yet this WSDL contract for a new home buyers database places extreme restrictions on how the implementation can vary because it is so specific to, well, a new home buyers database. Need an interface to an historical database of home purchases, or one to a database of recent automobile sales? Too bad, you need a new interface, and therefore new code to use that service. So much for that separation of concerns!

The lesson here, I suggest, is the following;

True separation of interface from implementation requires designing broadly reusable interfaces.

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November 1, 2005

Principled Sloppiness

Filed under: integration,semanticweb,webarch,webservices — Mark Baker @ 11:52 am

Adam Bosworth has published a great article in ACM Queue titled Learning from THE WEB. As you might expect, it’s very much in the vein of previous missives from Adam, where he highlights the advantages of Web based development, much as we espouse here.

As surely comes as no surprise to my regular readers, I largely agree with the points Adam makes in this article. But rather than focus on those points, I’d like to instead make an observation about his writing style, and the descriptors he uses for the Web, since I feel this makes for an interesting contrast with my own style.

Adam uses words like “sloppy” to describe what I would describe as the principled application of must-ignore style extensibility (as seen throughout Web architecture). It’s not that either descriptor is “better” or “worse” necessarily, just different sides of the same coin … which isn’t recognized nearly enough IMO, by those who promote “better” alternatives to Web based development, e.g. Web services. But then that’s the same point Dick Gabriel was getting at with his schizophrenic and self-contradictory ramblings on Worse is better.

Update: the one part of the article I took issue with – the critique of the Semantic Web – is well countered by Danny Ayers.

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