Every time our system fails, and we go to analyze the failure, we find ourselves saying things like “We didn’t know X was happening,” “we didn’t know Y could happen,” and so on. And it’s true: we didn’t know those things.

We can never predict with certainty what the next system failure will be. But we can predict, because painful experience has taught us, that some or all of the causes of that failure will be surprising.

We can use that!

When we go looking at data (and by “data” I mostly mean logs, traces, metrics, and so on, but data can be many things), sometimes we see something weird, and we go like, Huh!. That Huh! is a signal. If we follow that Huh! – get to the bottom of it, figure it out, make it not surprising anymore – two things happen. First, we get a chance to correct a latent problem which might some day contribute to a failure. And second, we make our mental model that much better.

Of course, any individual Huh! could turn out to be nothing. Perhaps there’s a bug. Perhaps circumstances have shifted, and our expectations no longer line up with reality. Or perhaps it’s just a monitoring blip. We won’t know until we run it down.

But, whatever the shortcomings of any particular investigation, a habit of investigating surprises has many attractive qualities. The main one is that we get to fix problems before those problems get worse, start bouncing off other problems, and cause fires. In other words: our system runs smoother. Consider what that’s worth.