D is for Defensive design

This year, for the A to Z Blogging Challenge I'm posting alphabetically on topics related to software development...


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So far, we've been pretty high up in the stratosphere, talking about architecture, business requirement, and core concepts. In contrast, this post can apply at all levels of design and development, but it is most at home down in the weeds of the code.

You reach a point in your program where you have a decision to make. A fork in the road. If the answer to a certain question is "Yes" you do one thing, if it's "No" you do something else.

The average programmer will likely code something that says: If "Yes" do one thing, otherwise do something else.

Notice the difference?

Where did "No" vanish to?

"What's the difference?" Average Joe-Coder protests. "It can only be yes or no, so if it isn't yes, it must be no. I'm just being efficient."

Sure, but you're not being defensive.

A defensive programmer will write code along the lines of: If "yes" do one thing, if "No" do something else, otherwise tell a bloody human that someone's f****d up!

"That's mad," Average Joe-Coder says. "Why code for something that can never happen?"

Fair point, if that were a true statement. But, despite Joe's insistence, it will happen ... some day. Trust me. You're relying on logic to ensure the only possible answers are yes or no, and that logic has been written by human beings - with all that implies. And computers are physical machines. They glitch sometimes. What went onto the disk as a nice clean "Yes" or "No" might overnight morph into "Whatever".

And, you might remind Joe, the more unlikely it is to happen, the more important it is to tell someone when it does.

If a stranger wanders into a public library, nobody cares. It's expected behavior, not a problem.

If a stranger wanders unchecked into the loading dock of the local supermarket, it's not expected, but plausible. There's no locked doors to stop it happening. A member of staff will likely ask, "Can I help you?" and it's not a big deal.

If a stranger walks unchallenged into the Queen's bedroom at Buckingham Palace, you can bet it will be a big deal and the authorities will want to know about it. Because that should be impossible.

But it has happened.

Defensive design makes no assumptions.

Face the unthinkable. Decide how to handle it, and act accordingly.