Was it necessary to test interactions with message brokers (e.g., Kafka)?

Yes, testing interaction points with message brokers such as Kafka is necessary. It helps ensure that messages are correctly sent and received, and that the system properly handles potential failures and delays.

For testing, you can use integration tests that verify interaction with a real or test broker, as well as mock the message broker for unit tests to isolate application logic.

An example of an integration test in Java using Kafka Testcontainers:

@Test
public void testSendMessage() {
    // Setting up the Kafka test container
    // Sending a message
    // Checking that the message is received and processed
}

Thus, testing interaction points with the broker helps improve the system's reliability and robustness.