Overview
A B2B platform had grown into a tightly coupled backend where changes in one module regularly broke unrelated workflows.
Challenge
API behavior varied by endpoint and historical quirks had become de facto contracts for multiple clients. A full rewrite would have broken active delivery commitments.
Approach
We defined domain boundaries first, then moved high-risk flows behind compatibility layers. Versioned contracts and contract tests were introduced before migrating clients so behavior changes were explicit.
Architecture
Core concerns were split into domain modules with clear ownership. Schema validation, contract checks in CI, and migration toggles reduced drift while old and new paths ran in parallel.
Outcome
The team could ship features without re-testing the whole platform each cycle. Client integrations became more predictable because contract changes were versioned and reviewable.
Lessons
Refactors fail when teams postpone migration mechanics. Sequencing and contract discipline have to be first-class work from day one.