Architecting For Scale by Lee Atchison

Author:Lee Atchison
Language: eng
Format: epub, pdf
Publisher: O'Reilly Media, Inc.
Published: 2016-07-24T16:00:00+00:00

What Is “Two Mistakes High”?

If you’ve ever flown radio control (R/C) airplanes before, you might have heard the expression “keep your plane two mistakes high.”

When you learn to fly R/C planes, especially when you began learning how to do acrobatics, you learn this quickly. You see, mistakes equate to altitude. You make a mistake, you lose altitude. You lose too much altitude, and well, badness happens. Keeping your plane “two mistakes high” means keeping it high enough that you have enough altitude to recover from two, independent mistakes.

Why two mistakes? Simple. You always want to be operating your plane high enough so that you can recover if (when) you make a mistake. Now, suppose that you make a mistake and lose a bunch of altitude. During your recovery from that mistake, you also want to be high enough that you can recover from a mistake. Think about it: during your recovery process, you are typically stressed and perhaps in an awkward situation doing potentially abnormal things—just the type of situation that can cause you to make another mistake. If you aren’t high enough, you can crash.

Put another way, if you normally fly two mistakes high, you can always have a backup plan for recovering from a mistake, even if you are currently recovering from a mistake.

This same philosophy is important to understand when building highly available, high-scale applications.

How do we “keep two mistakes high” in an application? For starters, when we identify the failure scenarios that we anticipate our application facing, we walk through the ramifications of those scenarios and our recovery plan for them. We make sure the recovery plan itself does not have mistakes or other shortcomings built into it—in short, we check that the recovery plan is able to work. If we find that it doesn’t work, then it’s not a recovery plan.

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