Monoliths and microservices: a practical guide

It comes up on almost every project, usually later than it should. This guide covers what monoliths and microservices actually involves, where it usually goes wrong, and how to tell whether yours is in reasonable shape.

Most systems fail at the seams rather than inside any one component. Nothing below assumes a large team or a large budget — most of it is a decision somebody has to make and then write down.

Why it matters

A well-structured monolith beats a badly split set of services. The teams that handle this well are rarely the ones with the biggest budgets. Assume whoever inherits this will have half your context and none of your patience.

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. That sounds obvious written down. It is still the thing most often skipped.

Where to start

Split along boundaries the business already has, not along technical layers. Small and consistent beats large and occasional here. Budget a little time for it every quarter and it never becomes a project of its own.

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.

Distributed systems trade a coding problem for an operations problem. Small and consistent beats large and occasional here. 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.

• A well-structured monolith beats a badly split set of services
• Split along boundaries the business already has, not along technical layers
• Distributed systems trade a coding problem for an operations problem
• 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

Warning signs

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 teams that handle this well are rarely the ones with the biggest budgets.

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?

Where to go from here

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. If any of that sounds like a description of your current setup, it is fixable.