Writing contract tests for an asynchronous API

The objective of contract testing for asynchronous APIs is to ensure backwards-compatibility between the provider and the consumer—just as it is with synchronous APIs. Testing asynchronous communication is naturally flaky, as there is the unreliability of networks plus the unreliable latency of asynchronous messaging. All too often these tests fail because messages are slow to arrive and the tests timeout. To solve this problem, we isolate the tests from the messaging system; we record the send message request on one end, then relay the message and assert the contract on the receiving side.

How to do it...

1. Create the upstream and downstream projects from the following...