Github user pnowojski commented on a diff in the pull request:

    https://github.com/apache/flink/pull/4533#discussion_r145980535
  
    --- Diff: 
flink-runtime/src/test/java/org/apache/flink/runtime/io/network/netty/PartitionRequestClientHandlerTest.java
 ---
    @@ -208,6 +211,53 @@ public void testCancelBeforeActive() throws Exception {
                client.cancelRequestFor(inputChannel.getInputChannelId());
        }
     
    +   /**
    +    * Verifies that {@link RemoteInputChannel} is enqueued in the 
pipeline, and
    +    * {@link AddCredit} message is sent to the producer.
    +    */
    +   @Test
    +   public void testNotifyCreditAvailable() throws Exception {
    +           final CreditBasedClientHandler handler = new 
CreditBasedClientHandler();
    +           final EmbeddedChannel channel = new EmbeddedChannel(handler);
    +
    +           final RemoteInputChannel inputChannel = 
mock(RemoteInputChannel.class);
    --- End diff --
    
    I know that mocking was extensively used so far, but we had quite bad 
experience with it.
    
    I don't agree that it is not interacting, because it is for example 
checking `inputChannel.isReleased()`, which you need to mock one test below. 
Now imagine that during some refactor/implementing new feature someone modifies 
handlers code, by adding some additional interaction/sanity check on 
`inputChannel`. With mocks it will lead with thousands of changes in all of the 
mocked unit tests.


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