A short guide to queues and message brokers

It is rarely the thing that gets a project approved, and often the thing that decides how it goes. Everything we would tell a client about queues and message brokers in the time it takes to drink a coffee.

Architecture is the set of decisions that are expensive to reverse, which is the only reason they deserve the name. Anything you cannot measure here, you are deciding by taste, which is fine as long as everyone knows it.

What is actually at stake

Queues absorb spikes that would otherwise take a system down. It is worth being explicit about, because assumptions differ quietly. If it only works because one person remembers to do something, it does not work yet.

How to approach it

Every queue needs a dead letter destination and someone watching it. The teams that handle this well are rarely the ones with the biggest budgets. Doing this properly once is usually cheaper than doing it approximately three times.

What to watch for

Ordering guarantees are weaker than most teams assume. Where this goes wrong is almost never a lack of knowledge. Budget a little time for it every quarter and it never becomes a project of its own.

In practice

Most systems fail at the seams rather than inside any one component. Three things worth confirming about queues and message brokers before you move on:

• Someone can say what the current setup is without going to look
• Ordering guarantees are weaker than most teams assume — and you know whether that is true here
• There is a way to tell whether the last change to this helped

The point is not perfection, it is knowing which of these you have consciously chosen to skip.