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https://issues.apache.org/jira/browse/YARN-7783?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16333937#comment-16333937
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Arun Suresh commented on YARN-7783:
-----------------------------------
[~leftnoteasy], to alleviate your concerns, I have moved all the
add/remove/cleanup temp containers/tags methods to a another class, which is
used only by the algorithm. The EpehmeralAlloactionTagsManager is a delegating
TagsManager - that is instantiated once per run of the algorithm, and manages
all temporary tags which are created and removed during the algorithms run. It
will also cleanup the tags from the underlying ATM after it is done.
Please do take a look [~cheersyang] / [~kkaranasos] as well.
I agree with [~kkaranasos] that fixing / validating the constraints at register
time via (YARN-6621) should be our long term solution, but in any case, this
validation phase of the algorithm loop is also needed - and YARN-6621 is a far
more complex undertaking.
> Add validation step to ensure constraints are not violated due to order in
> which a request is processed
> -------------------------------------------------------------------------------------------------------
>
> Key: YARN-7783
> URL: https://issues.apache.org/jira/browse/YARN-7783
> Project: Hadoop YARN
> Issue Type: Sub-task
> Reporter: Arun Suresh
> Assignee: Arun Suresh
> Priority: Blocker
> Attachments: YARN-7783-YARN-6592.001.patch,
> YARN-7783-YARN-6592.002.patch
>
>
> When the algorithm has placed a container on a node, allocation tags are
> added to the node if the constraint is satisfied, But depending on the order
> in which the algorithm sees the request, it is possible that a constraint
> that happen to be valid during placement of an earlier-seen request, might
> not be valid after all subsequent requests have been placed.
> For eg:
> Assume nodes n1, n2, n3, n4 and n5
> Consider the 2 constraints:
> # *foo* -> anti-affinity with *foo*
> # *bar* -> anti-affinity with *foo*
> And 2 requests
> # req1: NumAllocations = 4, allocTags = [foo]
> # req2: NumAllocations = 1, allocTags = [bar]
> If *req1* is seen first, the algorithm can place the 4 containers in n1, n2,
> n3 and n4. And when it gets to *req2*, it will see that 4 nodes have the
> *foo* tag and will place it on n5. But if *req2* is seen first, then *bar*
> tag will be placed on any node, since no node will at that point have *foo*,
> and then when it gets to *req1*, since *foo* has no anti-affinity with *bar*,
> the algorithm can end up placing *foo* on a node with *bar* violating the
> second constraint.
> To prevent the above, we need a validation step: after the placements for a
> batch of requests are made, then for each req, we remove its tags from the
> node and try to see of constraints are still satisfied if the tag were to be
> added back on the node.
> When applied to the example above, after the algorithm has run through *req2*
> and then *req1*, we remove the *bar* tag from the node and try to add it back
> on the node. This time, constraint satisfaction will fail, since there is now
> a *foo* tag on the node and *bar* cannot be added. The algorithm will then
> retry placing *req2* on another node.
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