Backward Compatibility & Contract Testing

// Consumer side — define contract in a consumer test
@ExtendWith(PactConsumerTestExt.class)
@PactTestFor(providerName = "inventory-service")
public class InventoryClientContractTest {

    @Pact(consumer = "order-service")
    public RequestResponsePact stockLookupPact(PactDslWithProvider builder) {
        return builder
            .given("product 101 exists with stock 50")
            .uponReceiving("a request for product stock")
                .path("/stock/101")
                .method("GET")
            .willRespondWith()
                .status(200)
                .body(new PactDslJsonBody()
                    .integerType("productId", 101)    // type-only matching (flexible)
                    .integerType("quantity", 50)
                    .stringType("unit", "units"))
            .toPact();
    }

    @Test
    @PactTestFor(pactMethod = "stockLookupPact")
    public void testStockLookup(MockServer mockServer) {
        InventoryClient client = new InventoryClient(mockServer.getUrl());
        StockResponse response = client.getStock(101L);
        assertThat(response.getQuantity()).isGreaterThanOrEqualTo(0);
        // Pact generates a contract JSON file — publish to Pact Broker
    }
}

// Provider side — verify contracts from Pact Broker
@Provider("inventory-service")
@PactBroker(
    url = "https://pact-broker.example.com",
    authentication = @PactBrokerAuth(token = "${PACT_BROKER_TOKEN}")
)
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
public class InventoryProviderContractTest {

    @LocalServerPort
    private int port;

    @BeforeEach
    void setUp(PactVerificationContext context) {
        context.setTarget(new HttpTestTarget("localhost", port));
    }

    // Set up test state matching "given()" in consumer contract
    @State("product 101 exists with stock 50")
    public void setupProduct101() {
        // Insert test data into database or mock the repository
        inventoryRepository.save(new Product(101L, "Widget", 50, "units"));
    }

    @TestTemplate
    @ExtendWith(PactVerificationInvocationContextProvider.class)
    void pactVerificationTestTemplate(PactVerificationContext context) {
        context.verifyInteraction();
    }
}

// Publish results to Pact Broker (in CI)
// pact.verifier.publishResults=true
// pact.provider.version=${GIT_COMMIT}
// pact.provider.branch=${GIT_BRANCH}

# Confluent Schema Registry compatibility modes
# BACKWARD (default): new schema can read messages from previous schema
#   → consumers can be upgraded before producers
#   → safe to: add optional fields, remove fields
#   → NOT safe to: remove required fields, change field types

# FORWARD: old schema can read messages from new schema
#   → producers can be upgraded before consumers
#   → safe to: add fields with defaults

# FULL: both BACKWARD + FORWARD
#   → safe additions only: new optional fields with defaults

# Set compatibility for a subject
curl -X PUT http://schema-registry:8081/config/orders-value \
  -H "Content-Type: application/json" \
  -d '{"compatibility": "FULL"}'

# Avro schema evolution example (BACKWARD compatible change)
# V1: {"type":"record","name":"Order","fields":[
#   {"name":"id","type":"int"},
#   {"name":"total","type":"double"}
# ]}

# V2: add optional field with default (BACKWARD compatible)
# {"name":"customerId","type":["null","string"],"default":null}

# NOT compatible (would break BACKWARD): removing "total" field