A service boundary is useful when it reduces the number of things a team must understand at once. Splitting a codebase is easy. Creating a boundary that preserves that reduction as the business changes is harder.
Boundaries are promises
A durable boundary states who owns a decision, which data is authoritative, and what other parts of the system may assume. The API is only the visible part of that agreement.
When two services write the same business fact, or when one service needs to understand another service’s internal state to do its work, the boxes are separate but the system is not.
Follow business invariants
The best starting point is often an invariant rather than a noun. Ask which rules must be evaluated together and which facts must change atomically. Code that protects the same invariant usually benefits from one owner, even if it serves several user journeys.
Good boundaries also make change legible. A team should be able to explain the effect of a new rule without tracing a chain of synchronous calls through the entire platform. That clarity becomes especially important when designing for partial failure.
Prefer explicit coupling
No distributed design removes coupling. It chooses its form. Versioned contracts, owned events, and documented consistency expectations make coupling visible enough to manage.
Architecture survives growth when its boundaries help people reason about change. If a decomposition increases deployment independence but destroys that clarity, it has traded one constraint for a more expensive one.