How do you test APIs that use asynchronous communication?

1 What is asynchronous communication?

Asynchronous communication is a mode of interaction where the sender and the receiver do not need to be available or synchronized at the same time. For example, when you send an email, you do not expect an immediate reply from the recipient, and you can continue with other activities while waiting. Similarly, when an API sends a request to another API, it does not have to wait for a response before proceeding with other tasks. Instead, it can handle the response later, when it is available, using a callback function, an event listener, or a message queue.

2 Why test asynchronous APIs?

Testing asynchronous APIs is essential to verify the functionality, reliability, and performance of your application or system. It can ensure that the API can handle multiple concurrent requests without errors or delays, send and receive the correct data in the expected format and structure, and handle different scenarios such as timeouts, retries, failures, and exceptions. Additionally, it can guarantee that the API integrates well with other APIs or services that use asynchronous communication.

3 How to test asynchronous APIs?

Testing asynchronous APIs requires different methods and techniques, depending on the type and complexity of the communication. Polling, subscribing, and mocking are some of the common methods. Polling involves sending repeated requests to the API until a response is received or a condition is met, and is useful for APIs that use long-polling or server-sent events. Subscribing involves subscribing to a channel or a topic where the API publishes or broadcasts messages or events, and is suitable for APIs that use websockets or message brokers. Mocking involves creating fake or simulated responses or messages that mimic the behavior of the API or the service, and is advantageous for APIs that depend on external or third-party APIs or services that are not available or reliable.