Service boundaries: a practical guide

Most teams know this matters. Fewer have decided who owns it. This guide covers what service boundaries 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.

What it costs to ignore

The wrong boundary is more expensive than no boundary. The reasoning matters more than the rule, because the rule has exceptions. It rarely shows up as a line item, which is exactly why it slips.

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. Where this goes wrong is almost never a lack of knowledge.

How to approach it

If two services always deploy together they are one service. 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.

Architecture is the set of decisions that are expensive to reverse, which is the only reason they deserve the name. The version that works in practice is usually less elaborate than the version described in the guides.

Data ownership is the clearest line to draw. None of that requires a large budget, only a decision and someone to own it. Budget a little time for it every quarter and it never becomes a project of its own.

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.

• The wrong boundary is more expensive than no boundary
• If two services always deploy together they are one service
• Data ownership is the clearest line to draw
• 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

Where it usually goes wrong

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

The right architecture for a team of three is the wrong one for a team of thirty, and vice versa. This is the sort of thing that compounds, quietly, in both directions.

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?

Turning this into a decision

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 you are not sure where your systems currently stand on this, it takes us about an hour to find out.