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Old School, But Still Relevant

“A distributed system is one in which the failure of a computer you didn’t even know existed can render your own computer unusable.” – Leslie Lamport

I’ve always loved that quote. But that’s only one reason why I was overjoyed when I stumbled upon this article written by Turing award winner Leslie Lamport: “Why We Should Build Software Like We Build Houses”. The other reason is that what he wrote is old school, but still relevant in many contexts:

Most programmers regard anything that doesn’t generate code to be a waste of time. Thinking doesn’t generate code, and writing code without thinking is a recipe for bad code. Before we start to write any piece of code, we should understand what that code is supposed to do. Understanding requires thinking, and thinking is hard.  – Leslie Lamport

I recently modified some code I hadn’t written to add one tiny feature to a program. Doing that required understanding an interface. It took me over a day with a debugger to find out what I needed to know about the interface — something that would have taken five minutes with a spec. To avoid introducing bugs, I had to understand the consequences of every change I made. The absence of specs made that extremely difficult. Not wanting to find and read thousands of lines of code that might be affected, I spent days figuring out how to change as little of the existing code as possible. In the end, it took me over a week to add or modify 180 lines of code. And this was for a very minor change to the program. – Leslie Lamport

New age software gurus and hard-core agilistas have always condescendingly trashed the “building a house” and “building a bridge” metaphors for software development. The reasoning is that houses and bridges are made of  hard-to-reconfigure atoms, whilst software is forged from simple-to-reconfigure bits. Well, yeah, that’s true, but… size matters.

In small systems, if you discover you made a big mistake three quarters of the way through the project, you can rewrite the whole shebang in short order without having to bend metal or cut wood. But as software systems get larger, at some point the “rewrite-from-scratch” strategy breaks down – and often spectacularly. Without house-like blueprints or bridge-like schema to consult, finding and reasoning about and fixing mistakes can be close to impossible – regardless of which state-of-the-art process you’re using.

rebuild

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