What is contract testing and when should you use it?

Contract testing is a technique for testing an integration point by checking each application in isolation to ensure the messages it sends or receives conform to a shared understanding documented in a contract.

Contract testing is immediately applicable anywhere where you have two services that need to communicate — such as an API client and a web front-end.

It really shines in an environment with many services (as is common in a microservice architecture). Having well-formed contract tests makes it easy for developers to avoid version hell.

In microservices, the consumer (client) defines what it expects from the provider (API server). The contract is tested independently on both sides.

Consumer (Frontend) ──→ defines expected API shape ──→ Contract
Provider (API Server) ──→ verifies it fulfils the contract ──→ Pass/Fail

@ExtendWith(PactConsumerTestExt.class)
public class UserServiceConsumerTest {

    @Pact(consumer = "FrontendApp", provider = "UserAPI")
    public RequestResponsePact createPact(PactDslWithProvider builder) {
        return builder
            .given("user 1 exists")
            .uponReceiving("get user by id")
                .path("/api/users/1")
                .method("GET")
            .willRespondWith()
                .status(200)
                .body(new PactDslJsonBody()
                    .integerType("id", 1)
                    .stringType("name", "John"))
            .toPact();
    }
}

Contract testing is a killer app for microservice development and deployment.