umustafi opened a new pull request, #3640:
URL: https://github.com/apache/gobblin/pull/3640

   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
   - [X] 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-1783 
   
   
   ### Description
   - [X] Here are some details about my PR, including screenshots (if 
applicable):
   We seek to improve initialization time of the `GobblinServiceJobScheduler` 
upon restart or new leadership change by batching the mysql queries to get flow 
specs. Instead of making 1 mysql get call for each flow execution id, which 
scales extremely poorly with number of flows, we should group them to reduce 
number of calls and downtime.
   
   This implementation adds two new functions to the `SpecStore` interface, 
`getSortedSpecURIs` and `getBatchedSpecs`, that we use to achieve the batching. 
Because these two functionalities are generic enough to be used in derived 
classes of the `SpecStore` we add them to the base class. Although this 
requires any child classes to implement these functions, it allows any consumer 
of the parent class `SpecStore` to use this functionality without caring about 
the specific implementation of the `SpecStore` used (as 
`GobblinServiceJobScheduler` does). Additionally, the `getBatchedSpecs` 
requires an `offset` or starting point to obtain the batches from so the 
consumer has to do some book keeping of where in the paginated gets we are but 
this again separates the functionality from the use case of the consumer. the 
entirety of the flow catalog is too large to load into memory for the 
`GobblinServiceJobScheduler`, so we use this batch functionality. 
   
   ### Tests
   - [X] My PR adds the following unit tests __OR__ does not need testing for 
this extremely good reason:
   The main testing for this functionality will be empirical testing to 
determine the batch size which results in decreasing the time needed for all 
the gets above. In general, the time should go down with a larger batch size 
but there may be an inflection point where we see diminishing returns or the 
memory required to store a large batch is excessive. I propose testing with 
batch size 100, 250, and 750 incrementally and using the time metrics emitted 
to settle on an appropriate size.
   
   ### Commits
   - [X] 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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