How is an integration contract generally fixed? How do you describe the interaction?

An integration contract is usually documented in the form of a formal document or specification that describes the format and protocol of interaction between systems. This can be:

• OpenAPI/Swagger for REST API — describing endpoints, methods, data structures, response codes.
• WSDL for SOAP services.
• JSON Schema or XML Schema for message format descriptions.
• A document outlining business rules and interaction scenarios.

Interaction is described through:

• Data contracts (message structures, mandatory fields, data types).
• Transmission protocols (HTTP, AMQP, gRPC, etc.).
• Error and exception handling agreements.
• API versioning agreements.

Example: For REST API, an OpenAPI specification is created, serving as a single source of truth for developers on both sides.