How to Contract Test Async Message Queues

Message pacts verify the payload a consumer handler can process, decoupled from any specific queue technology like Kafka or RabbitMQ.

The consumer test asserts its handler accepts a described message. The provider test proves the producer emits a message matching that description. The transport itself is not part of the contract.

Steps

• Use MessageConsumerPact to describe the message the handler expects.
• Assert your real handler processes the generated example.
• On the producer side, verify the message it builds satisfies the pact.

Message consumer test

const { MessageConsumerPact, synchronousBodyHandler } = require('@pact-foundation/pact');

const messagePact = new MessageConsumerPact({ consumer: 'notifier', provider: 'orders-events' });

await messagePact
  .expectsToReceive('an order shipped event')
  .withContent({ type: 'order.shipped', id: 42 })
  .verify(synchronousBodyHandler(handleOrderShipped));

Gotchas

• Message pacts describe the payload only, so wiring the actual topic and serializer still needs its own test.
• Keep the handler pure enough that it can run against the example message in isolation.

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