title: why is it bad to execute Flink job with parallelism = 1?
body: <p>I'm trying to understand what are the important features I need to take into consideration before submitting a Flink job.</p>
  
  <p>My question is what is the number of parallelism, is there an upper bound(physically)? and how can the parallelism impact the performance of my job?</p>
  
  <p>For example, I have a CEP Flink job that detects a pattern from unkeyed Stream, the number of parallelism will always be 1 unless I partition the datastream with KeyBy operator. </p>
  
  <p>Plz Correct me if I'm wrong :</p>
  
  <p>If I partition the data stream, then I will have a number of parallelism equals to the number of different keys. but the problem is that the pattern matching is being done independently for each key so I can't define a pattern that requires information from 2 partitions that have different keys.</p>