Eventual consistency: a practical guide

There is no clever trick in this one, just a handful of decisions worth making deliberately. This guide covers what eventual consistency actually involves, where it usually goes wrong, and how to tell whether yours is in reasonable shape.

The right architecture for a team of three is the wrong one for a team of thirty, and vice versa. Nothing below assumes a large team or a large budget — most of it is a decision somebody has to make and then write down.

The reason this keeps coming up

Users notice inconsistency long before your monitoring does. There is a version of this that is over-engineered, and it is worth avoiding. Write the reasoning down alongside the decision, because the reasoning is what changes first.

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.

What good looks like

Decide which reads must be immediate and which can lag. The cost of getting this wrong is rarely visible on the day it happens. It rarely shows up as a line item, which is exactly why it slips.

Most systems fail at the seams rather than inside any one component. The version that works in practice is usually less elaborate than the version described in the guides.

Explain the delay in the interface rather than hiding it. Where this goes wrong is almost never a lack of knowledge. Write the reasoning down alongside the decision, because the reasoning is what changes first.

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.

• Users notice inconsistency long before your monitoring does
• Decide which reads must be immediate and which can lag
• Explain the delay in the interface rather than hiding it
• 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

Common failure modes

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

Architecture is the set of decisions that are expensive to reverse, which is the only reason they deserve the name. 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.