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From "Hang Qi (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HELIX-596) Message throttling of controller behavior unexpectedly, throttled messages still take the constraint quota
Date Mon, 18 May 2015 06:05:00 GMT
Hang Qi created HELIX-596:
-----------------------------

             Summary: Message throttling of controller behavior unexpectedly, throttled messages
still take the constraint quota
                 Key: HELIX-596
                 URL: https://issues.apache.org/jira/browse/HELIX-596
             Project: Apache Helix
          Issue Type: Bug
          Components: helix-core
    Affects Versions: 0.6.4
            Reporter: Hang Qi
             Fix For: master


We found a very strange behavior on message throttling of controller when there is multiple
constraints. Here is our setup ( we are using helix-0.6.4, only one resource )
  - constraint 1: per node constraint, we only allow 3 state transitions happens on one node
concurrently.
  - constraint 2: per partition constraint, we define the state transition priorities in the
state model, and only allow one state transition happens on one single partition concurrently.

We are using MasterSlave state model, suppose we have two nodes A, B, each has 8 partitions
(p0-p7) respectively, and initially both A and B are shutdown, and now we start them at the
same time (say A is slightly earlier than B).

The expected behavior might be
  - p0, p1, p2 on A starts from Offline -> Slave; p3, p4, p5 on B starts from Offline ->
Slave
But the real result is:
  - p0, p1, p2 on A starts from Offline -> Slave, nothing happens on B
  - until p0, p1, p2 all transited to Master state, p3, p4, p5 on A starts from Offline ->
Slave; p0, p1, p2 on B starts from Offline -> Slave

As step Offline -> Slave might take long time, this behavior result in very long time to
bring up these two nodes (long down time result in long catch up time as well), though ideally
we should not let both nodes down at the same time.

Looked at the controller code, I like the stage and pipeline based implementation, it is well
design, very easy to understand and to reason about.

The logic of MessageThrottleStage#throttle, 

  - it goes through each messages selected by MessageSelectionStage, 
  - for each message, it goes through all selected matched constraints, and decrease the quota
of each constraints
     - if any constraint's quota is less than 0, this message will be marked as throttled.
 
I think there is something wrong here, the message will take the quota of constraints even
it is not going to be sent out (throttled). That explains our case, 
  - all the messages have been generated by the beginning, (p0, A, Offline->Slave), ...
(p7, A, Offline->Slave), (p0, B, Offline->Slave), ..., (p7, B, Offline->Slave)
  - in the messageThrottleStage#throttle
    - (p0, A, Offline->Slave), (p1, A, Offline->Slave), (p2, A, Offline->Slave) are
good, and constraint 1 on A reaches 0, constraint 2 on p0, p1, p2 reaches 0 as well
    - (p3, A, Offline->Slave), ... (p7, A, Offline->Slave) throttled by constraint 1
on A, also takes the quota of constraint 2 on those partitions as well.
    - (p0, B, Offline->Slave), ... (p7, B, Offline->Slave) throttled by constraint 2
    - thus only (p0, A, Offline->Slave), (p1, A, Oflline->Slave), (p2, A, Offline->Slave)
has been sent out by controller.

Does that make sense, or is there anything else you can think of to result in this unexpected
behavior? And is there any work around for it? One thing comes into my mind is update constraint
2 to be only one state transition is allowed of single partition on certain state transitions.

Thanks very much.



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