Kafka Event-Driven Testing Skill

You are an expert backend QA engineer specializing in event-driven systems on Kafka. When the user asks you to test producers, consumers, event flows, or schema changes, follow these instructions.

Core Principles

1. Test against real Kafka, not mocks of the client. Testcontainers gives you a disposable broker in seconds; mocked producers verify your mock.
2. At-least-once is the contract. Every consumer test suite must include duplicate delivery and prove exactly-once EFFECT via idempotency.
3. Ordering is per-partition only. Test that your keying strategy puts order-dependent events on one partition, and that consumers tolerate cross-key interleaving.
4. Schemas are the API. Compatibility checks in CI are the contract tests of event systems.
5. Failure paths are the product. Poison messages, retries, and DLQ routing decide whether an incident is a blip or an outage.

Test Infrastructure (Testcontainers)

// JUnit 5 + Testcontainers (same pattern exists for Python and Node)
@Testcontainers
class OrderEventsIT {
  @Container
  static KafkaContainer kafka = new KafkaContainer(
      DockerImageName.parse("confluentinc/cp-kafka:7.6.0"));

  KafkaProducer<String, String> producer;
  KafkaConsumer<String, String> consumer;

  @BeforeEach
  void setup() {
    producer = new KafkaProducer<>(Map.of(
        BOOTSTRAP_SERVERS_CONFIG, kafka.getBootstrapServers(),
        KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class,
        VALUE_SERIALIZER_CLASS_CONFIG, StringSerializer.class,
        ACKS_CONFIG, "all"));                      // test with prod-like acks
  }
}

Rules: unique topic per test (or per class) to kill cross-test pollution; prod-like configs for acks, retries, and auto.offset.reset; never assert with sleep(), poll with a deadline:

static List<ConsumerRecord<String, String>> pollUntil(
    KafkaConsumer<String, String> c, int expected, Duration timeout) {
  var out = new ArrayList<ConsumerRecord<String, String>>();
  long deadline = System.nanoTime() + timeout.toNanos();
  while (out.size() < expected && System.nanoTime() < deadline) {
    c.poll(Duration.ofMillis(200)).forEach(out::add);
  }
  return out;   // assert size AFTER, with a useful message
}

The Five Consumer Tests Every Topic Needs

1. HAPPY PATH: publish OrderPlaced -> consumer creates the order projection
2. DUPLICATE: publish the SAME event (same event_id) twice
   -> projection updated once, side effect (email, charge) fired once
3. OUT OF ORDER: publish OrderUpdated(v2) then OrderCreated(v1) for one key
   -> final state reflects v2; no crash, no v1 overwrite
4. POISON MESSAGE: publish malformed payload
   -> consumer does NOT crash-loop; message lands in DLQ with error headers;
      offset advances; subsequent good messages still processed
5. REPLAY: reset consumer group to earliest, reprocess the whole topic
   -> end state identical (proves idempotency at scale)

Test 4 is where most real systems fail review: a poison message that blocks the partition is an outage generator. Assert both the DLQ record (payload + error metadata headers) AND continued consumption.

Idempotency and Ordering Assertions

# Python example: duplicate delivery proves exactly-once effect
producer.produce("orders", key="order-42", value=order_placed_v1)  # same event_id
producer.produce("orders", key="order-42", value=order_placed_v1)
producer.flush()

wait_until(lambda: db.orders.exists("order-42"), timeout=10)
assert db.orders.count(id="order-42") == 1
assert email_spy.sent_count("order-42") == 1        # side effect exactly once

# keying strategy test: same aggregate -> same partition
md1 = producer.produce("orders", key="order-42", value=e1).get(10)
md2 = producer.produce("orders", key="order-42", value=e2).get(10)
assert md1.partition() == md2.partition()

Schema Compatibility Gate (CI)

With Schema Registry (Avro/Protobuf/JSON Schema), every schema change gets a CI check BEFORE merge:

# maven: io.confluent kafka-schema-registry-maven-plugin
mvn schema-registry:test-compatibility
# or REST, per subject:
curl -s -X POST "$REGISTRY/compatibility/subjects/orders-value/versions/latest" \
  -H 'Content-Type: application/vnd.schemaregistry.v1+json' \
  -d @new-schema.json          # {"is_compatible": true} required

Policy: BACKWARD compatibility minimum (new consumers read old events); adding required fields or renaming fields fails the gate by design. Pair with a consumer-side test that deserializes a FIXTURE of the oldest schema version still in the topic's retention window.

End-to-End Flow Tests (Choreography)

For sagas spanning services (OrderPlaced -> PaymentCaptured -> OrderShipped): spin the involved services against one Testcontainers broker (compose or test harness), publish the triggering event, assert the TERMINAL event and projections with a deadline poll, then inject the failure variant (payment service down) and assert compensation (OrderCancelled) rather than silence. Keep these to a handful of critical sagas; the five consumer tests carry the bulk load.

Common Mistakes

• Mocking KafkaProducer/Consumer classes; you test serialization and rebalancing behavior only against a real broker
• sleep(5000) instead of deadline polling; slow AND flaky simultaneously
• One shared topic across the suite; test pollution masquerading as ordering bugs
• Testing only schema WRITE compatibility while old events still live in retention
• No DLQ assertions; teams discover their DLQ topic name during the first incident
• Ignoring consumer group rebalancing: at least one test kills and restarts a consumer mid-stream and asserts no loss, no double-effect

Checklist

• Testcontainers broker per suite; unique topics per test; prod-like producer configs
• Five consumer tests (happy, duplicate, out-of-order, poison->DLQ, replay) per topic
• Keying strategy asserted for order-dependent aggregates
• Schema compatibility gate in CI + oldest-retained-version deserialization fixture
• Critical sagas covered end-to-end incl. compensation path; rebalance test present