What to ask your agency about eventual consistency

This is cheap to get right at the start and expensive to retrofit. If you are briefing an agency or a freelancer on eventual consistency, these questions are worth asking early.

Architecture is the set of decisions that are expensive to reverse, which is the only reason they deserve the name. It is worth deciding this deliberately rather than inheriting whatever the last person set up.

Questions worth asking

• Who will actually do this work, and have they done it before?
• How will we know afterwards whether it worked?
• What happens if it needs changing in a year?
• What are you assuming that we have not confirmed?

What a good answer sounds like

Users notice inconsistency long before your monitoring does. The reasoning matters more than the rule, because the rule has exceptions. If two people in the business would answer this differently, that gap is the actual problem.

Explain the delay in the interface rather than hiding it. In practice this is a scheduling problem more than a technical one. Assume whoever inherits this will have half your context and none of your patience.

How to tell if yours is fine

The right architecture for a team of three is the wrong one for a team of thirty, and vice versa. Three things worth confirming about eventual consistency before you move on:

• Someone can say what the current setup is without going to look
• Users notice inconsistency long before your monitoring does — and you know whether that is true here
• There is a way to tell whether the last change to this helped

Most of the value here comes from doing the first two things, not all of them.