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

    https://github.com/apache/flink/pull/1098#discussion_r39253553
  
    --- Diff: 
flink-staging/flink-streaming/flink-streaming-scala/src/test/java/org/apache/flink/streaming/scala/api/CsvOutputFormatITCase.java
 ---
    @@ -36,6 +37,10 @@ protected void preSubmit() throws Exception {
        @Override
        protected void testProgram() throws Exception {
                OutputFormatTestPrograms.wordCountToCsv(WordCountData.TEXT, 
resultPath);
    +           OutputFormatTestPrograms.wordCountToCsv(WordCountData.TEXT, 
resultPath, 1);
    --- End diff --
    
    It is expected that the test fails if you set the overwrite mode to 
`NO_OVERWRITE` because you start multiple programs that all write to the same 
location.
    
    I debugged the issue and found that the `ForkableFlinkMiniCluster` sets the 
overwrite mode to OVERWRITE. That explains the behavior. However, this also 
means that you cannot test the correctness of your methods by setting the 
overwrite mode to OVERWRITE because this is the default behavior.
    
    I would change the `CsvOuputFormatITCase` to extend 
`StreamingMultipleProgramsTestBase` and run each program in a dedicated test 
method. 


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