[GitHub] [druid] kfaraz commented on pull request #13074: Add test framework to simulate segment loading and balancing

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kfaraz commented on PR #13074:
URL: https://github.com/apache/druid/pull/13074#issuecomment-1248869153

   Thanks a lot for the review, @paul-rogers !
   I will be sure to include a README to help other developers write more of these tests.
   
   Your understanding of the changes is correct.
   
   > We verify results by looking at the state of the simulated nodes.
   
   We also verify the state of the coordinator itself and the emitted metrics, as the `DruidCoordinator` is the primary entity under test (I will clarify these in the README).
   
   > It seems that the tests don't cover the dynamic aspects: load, the threads which decided when to fire off the control tasks in the coordinator, latencies, etc.
   
   - Yes, we do not verify latency of an operation.
   - The behaviour to actually load a segment would always be mocked (as it happens on a historical).
   Here, we would only want to control when the load happens and whether it succeeds or fails.
   - The simulation maintains a handle to all the executors used inside the coordinator. It can thus choose
   to invoke pending tasks of a certain executor at a certain step to recreate race conditions. For example,
   a sequence of steps could be to: run coordinator, load one segment from queue, sync inventory, load
   remaining segments from queue and verify the final state.


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