What is contract testing and how does it apply to Go microservices?

Contract testing verifies that services honour their agreed API contracts without requiring a full integration test environment. In a microservices system, consumer-driven contract testing (Pact) lets service consumers define the API shape they expect, and providers verify they match.

// Pact consumer test (user-service-client)
// Defines what the consumer expects from the user service
func TestUserServiceContract_GetUser(t *testing.T) {
    // Define the expected interaction
    pact := dsl.Pact{
        Consumer: "order-service",
        Provider: "user-service",
    }
    defer pact.Teardown()

    pact.
        AddInteraction().
        Given("user 1 exists").
        UponReceiving("a request for user 1").
        WithRequest(dsl.Request{
            Method:  "GET",
            Path:    "/users/1",
            Headers: dsl.MapMatcher{"Accept": "application/json"},
        }).
        WillRespondWith(dsl.Response{
            Status: 200,
            Body: dsl.Match(User{}), // matches structure, not values
        })

    if err := pact.Verify(func() error {
        client := NewUserClient(pact.Server.URL)
        user, err := client.GetUser(context.Background(), 1)
        if err != nil { return err }
        if user.ID != 1 { return fmt.Errorf("expected ID 1, got %d", user.ID) }
        return nil
    }); err != nil {
        t.Fatal(err)
    }
}

// Protobuf contracts are already self-documenting
// For gRPC: test that the proto file matches what clients use
// Use protoc-gen-validate for field-level validation in the schema
message CreateUserRequest {
    string email = 1 [(validate.rules).string.email = true];
    string name  = 2 [(validate.rules).string = {min_len: 1, max_len: 100}];
}

gRPC contract testing: Protobuf provides the contract itself. The pattern is different — ensure the generated Go code from the consumer's proto copy matches the provider's. Tools like buf breaking detect breaking changes in .proto files, acting as automated contract enforcement.

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