resilient-nestjs-patterns

Use the Service Orchestration pattern: group methods into clusters by concern, extract each cluster into its own injectable child service, and convert the original service into a thin orchestrator that delegates. Apply this when the file exceeds 500 lines or has more than 6 constructor dependencies.

Wrap external calls in a withRetry utility that doubles the delay each attempt, starting from a base delay like 1000ms with a maximum of 3 attempts. Only retry transient errors such as timeouts and HTTP 408, 429, 500, 502, 503, and 504 status codes.

No. Use the 3-tier layering pattern: controllers handle only HTTP concerns, services contain business rules, and a dedicated DbService owns all Prisma queries and transactions. This keeps business logic testable and prevents transaction management from scattering across controllers.

Use events for consequences like sending emails, invalidating caches, or writing audit logs that must not block the response. Use direct calls when the result is needed for the operation to complete, such as validating a payment before creating a subscription.

It is a static factory method pair on a module: forRoot wires real providers using environment configuration, while forTesting registers mock implementations of external clients. This lets integration tests import the same module with controlled dependencies via Test.createTestingModule.

Avoid splitting when the service is under 200 lines, all methods share the same dependencies, or tests are already easy to write. Premature decomposition of a small service adds file overhead without improving maintainability.