Idempotency: a practical guide

Teams tend to reach for this after something has already gone wrong. This guide covers what idempotency actually involves, where it usually goes wrong, and how to tell whether yours is in reasonable shape.

Architecture is the set of decisions that are expensive to reverse, which is the only reason they deserve the name. Nothing below assumes a large team or a large budget — most of it is a decision somebody has to make and then write down.

What is actually at stake

Networks retry, so assume every request can arrive twice. None of that requires a large budget, only a decision and someone to own it. It rarely shows up as a line item, which is exactly why it slips.

For most businesses the question is not whether this matters but how much of it is worth doing right now. That depends on what you are trying to achieve in the next few months, not on best practice in the abstract. Getting it slightly wrong is survivable. Ignoring it entirely is not.

How we handle it

An idempotency key turns a hard problem into a simple one. That sounds obvious written down. It is still the thing most often skipped. If it only works because one person remembers to do something, it does not work yet.

The right architecture for a team of three is the wrong one for a team of thirty, and vice versa. The version that works in practice is usually less elaborate than the version described in the guides.

Payment and order endpoints are where this matters most. That sounds obvious written down. It is still the thing most often skipped. It is worth deciding this deliberately rather than inheriting whatever the last person set up.

A working checklist

If you want a quick read on where you stand, work through this. Anything you cannot answer confidently is where to start.

• Networks retry, so assume every request can arrive twice
• An idempotency key turns a hard problem into a simple one
• Payment and order endpoints are where this matters most
• Someone is named as the owner, not just assumed to be
• There is a date in the calendar to review it again
• The decision and the reasoning behind it are written down somewhere findable
• You could explain the current setup to a new hire in five minutes

Where it usually goes wrong

The most common failure is not doing this badly. It is doing it once, during a launch, and never revisiting it. Circumstances move, the setup does not, and the gap widens quietly until something breaks or somebody notices the numbers.

• It was configured during a launch and has not been touched since
• Different people in the business believe different things are true about it
• There is no way to tell whether the last change helped or hurt
• The only person who understands it has left, or is about to

Most systems fail at the seams rather than inside any one component. The cost of getting this wrong is rarely visible on the day it happens.

How we approach it

On our projects this gets handled during the build rather than added afterwards, because retrofitting it costs several times more than including it. We write down what was decided and why, so the next person to touch it is not guessing.

If you are working with someone else, the questions worth asking are simple: who owns this, how will we know it is working, and what happens when it needs to change?

What to do next

Pick the single item from the checklist above that would cause the most trouble if it turned out to be wrong. Fix that one, confirm it worked, then move on. None of this needs a rewrite. Most of it is a morning's work once someone decides to do it.