How do services communicate with each other in a microservices architecture? Who writes the contracts?

In a microservices architecture, services typically communicate through well-defined APIs, most often REST or gRPC. Data exchange uses JSON, Protobuf, or other serialization formats. Contracts are interface specifications that describe which requests and responses the service supports.

Contracts are usually written by developers responsible for the API, often in collaboration with the team that will consume the service. It is important that the contract is documented and agreed upon to avoid misunderstandings and errors during integration.

For example, a gRPC contract is described in .proto files, which then generate code for clients and servers. For REST APIs, OpenAPI (Swagger) can be used to describe contracts.