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Date: Tue, 22 Dec 2015 01:15:46 +0000 (UTC)
From: "ASF GitHub Bot (JIRA)" <jira@apache.org>
To: dev@apex.incubator.apache.org
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Subject: [jira] [Commented] (APEXCORE-60) Iterative processing support
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    [ https://issues.apache.org/jira/browse/APEXCORE-60?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15067387#comment-15067387 ] 

ASF GitHub Bot commented on APEXCORE-60:
----------------------------------------

Github user davidyan74 commented on a diff in the pull request:

    https://github.com/apache/incubator-apex-core/pull/185#discussion_r48212355
  
    --- Diff: engine/src/main/java/com/datatorrent/stram/plan/logical/LogicalPlan.java ---
    @@ -1495,6 +1509,11 @@ private void validateThreadLocal(OperatorMeta om) {
           return;
         }
     
    +    if (om.getOperator() instanceof Operator.DelayOperator) {
    +      String msg = String.format("Locality %s invalid for delay operator %s", Locality.THREAD_LOCAL, om);
    +      throw new ValidationException(msg);
    --- End diff --
    
    This is because if there is a THREAD_LOCAL for the loop, it would end up being an infinite recursive call within the thread.  However, it's technically possible to have THREAD_LOCAL for part of the cycle.  I have reopened this JIRA: https://issues.apache.org/jira/browse/APEXCORE-262.  We will tackle this in the future.


> Iterative processing support
> ----------------------------
>
>                 Key: APEXCORE-60
>                 URL: https://issues.apache.org/jira/browse/APEXCORE-60
>             Project: Apache Apex Core
>          Issue Type: New Feature
>            Reporter: David Yan
>            Assignee: David Yan
>
> We would like to support iterative processing by introducing cycles in the graph (known as DAG now, but no longer if we support iterative processing).
> Initial idea is as follow:
> {noformat}
>      |----|
>      v    |
> A -> B -> C -> D
> ^         |
> |---------|
> {noformat} 
> C has two separate backward streams to A and B.  The input ports of A and B that C connects to will have a special attribute on how many window IDs ahead the incoming windows should be treated as, and A and B will be responsible for the initial data for such input ports.
> Another idea is to have C advance the window ID on its output ports and have C generate the initial data on its output ports to A and B.


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