ZihanLi58 opened a new pull request, #3635:
URL: https://github.com/apache/gobblin/pull/3635

   Dear Gobblin maintainers,
   
   Please accept this PR. I understand that it will not be reviewed until I 
have checked off all the steps below!
   
   
   ### JIRA
   - [ ] My PR addresses the following [Gobblin 
JIRA](https://issues.apache.org/jira/browse/GOBBLIN/) issues and references 
them in the PR title. For example, "[GOBBLIN-XXX] My Gobblin PR"
       - https://issues.apache.org/jira/browse/GOBBLIN-1778
   
   
   ### Description
   - [ ] Here are some details about my PR, including screenshots (if 
applicable):
   Now dag managers have the assumption that it is the only process that can 
update mysql table and the in-memory state is always in sync with mysql. But we 
do notice that during the leader transforms period, it's possible that two dag 
manager can run concurrently and update the mysql db at the same time. 
   
   To address that, we need either add a lock to make sure only one dag manager 
is working at one time, or we need to have a housekeeping thread to 
periodically sync the in-memory state with the mysql table. After discussion, 
we choose to go with the later approach and we do have the assumption that GaaS 
submit jobs without specifying job.id, so jobs with same flow execution id will 
not share the staging dir and can be executed concurrently. 
   
   Besides that, during adding test, I figure out there will be NPE if we try 
to set the dag manager as inactive, add a small fix for that as well. 
   
   
   ### Tests
   - [ ] My PR adds the following unit tests __OR__ does not need testing for 
this extremely good reason:
   Just add a single thread for housekeeping so will not affect other 
functions. Also add unit test to make sure we close the thread when de-active 
DagManager to avoid memory leak 
   
   ### Commits
   - [ ] My commits all reference JIRA issues in their subject lines, and I 
have squashed multiple commits if they address the same issue. In addition, my 
commits follow the guidelines from "[How to write a good git commit 
message](http://chris.beams.io/posts/git-commit/)":
       1. Subject is separated from body by a blank line
       2. Subject is limited to 50 characters
       3. Subject does not end with a period
       4. Subject uses the imperative mood ("add", not "adding")
       5. Body wraps at 72 characters
       6. Body explains "what" and "why", not "how"
   
   


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