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https://issues.apache.org/jira/browse/HELIX-654?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16017024#comment-16017024
 ] 

ASF GitHub Bot commented on HELIX-654:
--------------------------------------

Github user jiajunwang commented on a diff in the pull request:

    https://github.com/apache/helix/pull/88#discussion_r117420922
  
    --- Diff: helix-core/src/main/java/org/apache/helix/task/JobRebalancer.java 
---
    @@ -455,6 +454,44 @@ private ResourceAssignment 
computeResourceMapping(String jobResource,
         return ra;
       }
     
    +  /**
    +   * If assignment is different from previous assignment, drop the old 
running task if it's no
    +   * longer assigned to the same instance, but not removing it from 
excludeSet because the same task
    +   * should not be assigned to the new instance right way.
    +   */
    +  private void dropRebalancedRunningTasks(Map<String, SortedSet<Integer>> 
newAssignment,
    +      Map<String, SortedSet<Integer>> oldAssignment, Map<Integer, 
PartitionAssignment> paMap,
    +      JobContext jobContext) {
    +    for (String instance : oldAssignment.keySet()) {
    +      for (Integer pId : oldAssignment.get(instance)) {
    +        if (jobContext.getPartitionState(pId) == TaskPartitionState.RUNNING
    +            && !newAssignment.get(instance).contains(pId)) {
    +          paMap.put(pId, new PartitionAssignment(instance, 
TaskPartitionState.DROPPED.name()));
    +          jobContext.setPartitionState(pId, TaskPartitionState.DROPPED);
    --- End diff --
    
    I think even the task is not re-scheduled, the information will be removed 
after cancel, right?
    So there shouldn't be the case that the context is RUNNING but the task is 
dropped. Unless something wrong happens during cancellation. In which case, 
showing RUNNING is correct.


> Rebalance running task
> ----------------------
>
>                 Key: HELIX-654
>                 URL: https://issues.apache.org/jira/browse/HELIX-654
>             Project: Apache Helix
>          Issue Type: New Feature
>          Components: helix-core
>            Reporter: Weihan Kong
>
> h3. Feature summary
> Helix Task Framework empowers user to run tasks on instances managed by 
> Helix. There're 2 type of tasks: generic task and fixed target task. For 
> fixed target task, the task always follows the targeted partition and is 
> rebalanced if the partition is rebalanced. For generic task, Helix provides 
> user the choice to rebalance the running task or not, when the topology of 
> the cluster changes.
> For most users, it's better to disabled this feature(as default) since 
> there's no need to re-run the task every time new node is added. For users 
> with long-running tasks, enabling this feature can be very useful so that 
> when new node is added, the load of the tasks are better balanced among the 
> cluster.
> h3. Defined system behavior
> h4. When a node fails,
> h6. Feature disabled:
> * Running tasks on that failed node will be rebalanced to a live node, since 
> the task no longer exists and failed with the node.
> h6. Feature enabled:
> * Same.
> h4. When a new node is added,
> h6. Feature disabled:
> * Running tasks will continue to run on the current instance.
> * If a running task fails after a while, it might be rebalanced and run on 
> other instances, according to the new rebalance assignment under the new 
> cluster topology.
> h6. Feature enabled:
> * Running task might be cancelled and rebalanced immediately, according to 
> the new rebalance assignment under the new cluster topology.
> h3. Configuration
> A job level config field(RebalanceRunningTask) in JobConfig to enable/disable 
> this feature. By default it's false.



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