Who forms the contract during integration — backend-backend or frontend-backend interaction?

The contract during integration is an agreement on the format and behavior of interacting services or components. Its formation depends on the type of integration:

• Backend-Backend: the contract is usually formed jointly by the teams responsible for both services. Often, this is an API specification (e.g., OpenAPI/Swagger), describing endpoints, request and response formats, and error codes.

• Frontend-Backend: the contract is most often formed by the backend team providing the API and the frontend team consuming it. It is important to agree on data structures, methods, parameters, and API behavior.

In both cases, the process includes:

1. Discussing requirements and capabilities.
2. Documenting the API and data formats.
3. Versioning contracts for backward compatibility.
4. Testing compliance of the implementation with the contract.

Thus, the contract is the result of joint work of the teams, ensuring coordinated interaction of systems.