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From "Pavel Kovalenko (Jira)" <j...@apache.org>
Subject [jira] [Commented] (IGNITE-12133) O(log n) partition exchange
Date Wed, 04 Sep 2019 11:24:00 GMT

    [ https://issues.apache.org/jira/browse/IGNITE-12133?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16922393#comment-16922393
] 

Pavel Kovalenko commented on IGNITE-12133:
------------------------------------------

[~ivan.glukos] I guess the general solution of such problem will be in introducing Gossip
protocol like in Cassandra. In this case, we shouldn't have a pre-determined skip-list topology
for the nodes. The probabilistic nature of Gossip also gives us ~ log(N) rounds for the dissemination
of such messages.

> 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
direction).
>  
> 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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