Unit Test a Sample Kafka Consumer and returned messages for a topic

[Geane Norm]

Hi, I am extremely new to Kafka and I have just written a Kafka Consumer following the example here: https://www.packtpub.com/books/content/writing-consumers
Kafka is running on a EC2 instance and I would like to test that my consumer is actually returning the messages from a topic as created by the Producer. I have been trying to understand how to Unit Test the Consumer app and came across this: https://ransilberman.wordpress.com/2013/07/19/how-to-unit-test-kafka/ that says that the Kafka API comes with an embedded Kafka server that can easily be used for unit tests. However, I am not sure I need to go down that road in order to test the outputted messages from a particular topic. Can you help? Is there an easy way to test the returned topic messages?

Thank you so much,

I.

[EDIT]

I am expecting my message in a JSON format, do I need a serializer to read them?

[Junilu Lacar]

I would try to stop thinking about the Kafka infrastructure altogether. The point of unit testing is to see behavior in isolation. If you're going to unit test, then isolate your class under test (CUT). Use the test to say something like this: "Ok, I have this CUT, and if it were actually hooked up to Kafka, this is how it would be called..."

In code, that would look like this:

public class MyConsumerTest {
@Test
public void should_do_whatever_when_some_stimulus() {
MyConsumer testSubject = new MyConsumer();
... // do other things to set the test context
SomeResult actual = testSubject.methodToTest(...);
assertThat(actual, is(expected));
}

Whatever code would go where line 6 is, that should not include actually setting up Kafka, etc. That code would just do things to make your testSubject "think" that it's getting real input from the real Kafka.

It's kind of the same thing I taught my son. If you have a program that needs the user to enter something, don't test by asking the user to enter something. Instead, write your test so that it pretends that it's using something that the user entered. That way, you take the User out of the test equation.

public class FooTest {
@Test
public void should_accept_positive_number() {
String input = "200";
Foo testSubject = new Foo();
assertTrue(testSubject.accepts(input));
}
@Test
public void should_reject_negative_number() {
String input = "-10";
Foo testSubject = new Foo();
assertFalse(testSubject.accepts(input));
}
}