What would you check first in a microservice: functionality or contracts?

Contracts.

Contract testing (API specifications) ensures that interaction between microservices is possible. The functionality of a specific microservice is important, but without valid contracts, integration will inevitably not work.

An approximate sequence of actions:

1. Contract testing: Verify compliance of implementation with the contract (OpenAPI, gRPC Protobuf, etc.). Ensure that requests and responses match the specifications.
2. Unit tests: Check the internal logic of the microservice in isolation.
3. Integration tests: Verify the interaction of the microservice with its dependencies (database, other services).
4. Functional tests (API): Ensure that the microservice performs its business functions by sending real requests through the API.

Prioritizing contracts early in development and testing helps identify and fix integration errors, which can be costly to fix at later stages.