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From "Ivan Rakov (Jira)" <j...@apache.org>
Subject [jira] [Commented] (IGNITE-12133) O(log n) partition exchange
Date Tue, 03 Sep 2019 17:04:00 GMT

    [ https://issues.apache.org/jira/browse/IGNITE-12133?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16921575#comment-16921575

Ivan Rakov commented on IGNITE-12133:

Folks, what if we'll try to apply skip-list approach not to discovery messages routing, but
to GridDhtPartitionsFullMessage delivery to all server nodes?
This may make onDone() phase of PME faster, especially when full message is heavy and cluster
topology is large. On the other hand, handling of failed nodes during PME will be dramatically
complicated: if one of top-level nodes will fail, its "children" should somehow request full
map from coordinator or another server node (currently, we have extra failover logic only
for coordinator leave case).

> O(log n) partition exchange
> ---------------------------
>                 Key: IGNITE-12133
>                 URL: https://issues.apache.org/jira/browse/IGNITE-12133
>             Project: Ignite
>          Issue Type: Improvement
>            Reporter: Moti Nisenson-Ken
>            Priority: Major
> Currently, partition exchange leverages a ring. This means that communications is O\(n)
in number of nodes. It also means that if non-coordinator nodes hang it can take much longer
to successfully resolve the topology.
> Instead, why not use something like a skip-list where the coordinator is first. The coordinator
can notify the first node at each level of the skip-list. Each node then notifies all of its
"near-neighbours" in the skip-list, where node B is a near-neighbour of node-A, if max-level(nodeB)
<= max-level(nodeA), and nodeB is the first node at its level when traversing from nodeA
in the direction of nodeB, skipping over nodes C which have max-level(C) > max-level(A). 
> 1
> 1 .  .  .3
> 1        3 . .  . 5
> 1 . 2 . 3 . 4 . 5 . 6
> In the above 1 would notify 2 and 3, 3 would notify 4 and 5, 2 -> 4, and 4 -> 6,
and 5 -> 6.
> One can achieve better redundancy by having each node traverse in both directions, and
having the coordinator also notify the last node in the list at each level. This way in the
above example if 2 and 3 were both down, 4 would still get notified from 5 and 6 (in the backwards
> The idea is that each individual node has O(log n) nodes to notify - so the overall time
is reduced. Additionally, we can deal well with at least 1 node failure - if one includes
the option of processing backwards, 2 consecutive node failures can be handled as well. By
taking this kind of an approach, then the coordinator can basically treat any nodes it didn't
receive a message from as not-connected, and update the topology as well (disconnecting any
nodes that it didn't get a notification from). While there are some edge cases here (e.g.
2 disconnected nodes, then 1 connected node, then 2 disconnected nodes - the connected node
would be wrongly ejected from the topology), these would generally be too rare to need explicit
handling for.

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