Incremental Monolith Migration Without Breaking Production
Lessons from decomposing a 24+ module transaction platform while preserving API contracts and Camunda orchestration.
Large monoliths rarely fail because teams lack ambition � they fail because migration plans ignore production contracts. Here is a pragmatic pattern that worked on a transaction-processing platform with 24+ modules.
Start with external contracts, not internal modules
Before carving services, map every external API consumer and shared context path. Your migration succeeds when those contracts stay stable while internals change.
Use a reverse proxy as a compatibility layer
Traefik (or similar) let us route traffic to newly extracted services while keeping the same URLs. Consumers did not need to know whether a module lived in the monolith or a new ECS service.
Keep orchestration intact
Camunda BPMN processes spanned multiple modules. We migrated services without breaking process definitions by maintaining orchestration compatibility at each cutover step.
Ship incrementally
One service at a time. Validate. Roll forward. The goal is not a big-bang rewrite � it is a sequence of reversible steps with measurable risk reduction.
What I would do differently
Invest even earlier in workflow test automation. BDD-style Camunda tests became our safety net � building that framework in parallel with the first extractions would have accelerated confidence.