title: When does flink initiates RocksDBKeyedStateBackend directory delete?
body: <p>My Flink jobs runs for few days without any issues, but after somedays it kills the tm and restarts the entire job.
  in the log I found this,
  org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend  - Deleting existing instance base directory /tmp/flink-io-4b455efa-bcde-4ef2-aed3-c66ca9d8933e/job_152b986e7e5a6f411780849f13ce4bc8_op_KeyedProcessOperator_a1c286a47e97622aa92a8f6cd4115854__1_4__uuid_4b53ff24-e240-48d6-b438-3ab2d05cbdb8</p>
  <p>after it deleted the statestore file, it throws this error, because i am fetching data from statestore there.</p>
  <pre><code>java.lang.NullPointerException
      at c.c.w.d.s.b.aggregator.StateProcessFunction.addEvent(StateProcessFunction.java:81)
      at c.cs.w.d.s.b.a.StateProcessFunction.processElement(StateProcessFunction.java:113)
      at c.c.w.d.s.b.a.StateProcessFunction.processElement(ContactStateProcessFunction.java:26)
      at org.apache.flink.streaming.api.operators.KeyedProcessOperator.processElement(KeyedProcessOperator.java:85)
      at org.apache.flink.streaming.runtime.io.StreamOneInputProcessor.processElement(StreamOneInputProcessor.java:164)
      at org.apache.flink.streaming.runtime.io.StreamOneInputProcessor.processInput(StreamOneInputProcessor.java:143)
      at org.apache.flink.streaming.runtime.tasks.StreamTask.processInput(StreamTask.java:279)
      at org.apache.flink.streaming.runtime.tasks.StreamTask.run(StreamTask.java:301)
      at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:406)
      at org.apache.flink.runtime.taskmanager.Task.doRun(Task.java:705)
      at org.apache.flink.runtime.taskmanager.Task.run(Task.java:530)
      at java.lang.Thread.run(Thread.java:748)
  </code></pre>
  <p>question is when and why flink backend statestore file delete initiated ? is there something which is not printed in the log ?</p>