Three myths about eventual consistency

Most teams know this matters. Fewer have decided who owns it. A few things about eventual consistency that get repeated more often than they get checked.

Architecture is the set of decisions that are expensive to reverse, which is the only reason they deserve the name. It is the sort of thing that looks like polish right up until it costs you an enquiry.

“It only matters for big sites”

Users notice inconsistency long before your monitoring does. There is a version of this that is over-engineered, and it is worth avoiding. Doing this properly once is usually cheaper than doing it approximately three times.

“We can deal with it after launch”

Sometimes true, usually expensive. It is worth being explicit about, because assumptions differ quietly.

“Our platform handles it”

Explain the delay in the interface rather than hiding it. The teams that handle this well are rarely the ones with the biggest budgets. It rarely shows up as a line item, which is exactly why it slips.

What this looks like day to day

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
• Decide which reads must be immediate and which can lag — and you know whether that is true here
• There is a way to tell whether the last change to this helped

If you want a second opinion on how yours is set up, ask.