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https://issues.apache.org/jira/browse/YARN-6596?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16299704#comment-16299704
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Konstantinos Karanasos commented on YARN-6596:
----------------------------------------------

bq. Figured allocation tags to nodes mapping is managed by 
AllocationTagsManager which is done in YARN-7522.
Yep, I was just writing the same :) Also, YARN-6858 will do the same for node 
attributes.
Note that constraints can be defined at three levels: at the cluster level, at 
the application level, and at the scheduling request level.
Probably if we had constraints only at the scheduling request level, we could 
do without Placement Constraint Manager. Although even in that case, it would 
not be easy to do it if we want to place multiple scheduling request at a time.
But most of our use cases are at the application level and at the cluster 
level, so that you can more easily express constraints across scheduling 
requests. Also think that a service will typically define its constraints at 
the application level when it gets first submitted.
For all those cases, we need a Placement Constraint Manager to hold the 
constraints across scheduling requests.

> Introduce Placement Constraint Manager module
> ---------------------------------------------
>
>                 Key: YARN-6596
>                 URL: https://issues.apache.org/jira/browse/YARN-6596
>             Project: Hadoop YARN
>          Issue Type: Sub-task
>            Reporter: Konstantinos Karanasos
>            Assignee: Konstantinos Karanasos
>         Attachments: YARN-6596-YARN-6592.001.patch
>
>
> This RM module will be responsible for storing placement constraints, 
> allocation tags, and node attributes.
> It will be used when determining the placement of SchedulingRequests with 
> constraints.



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