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https://issues.apache.org/jira/browse/IGNITE-12133?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16945269#comment-16945269
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Moti Nisenson-Ken commented on IGNITE-12133:
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I'm not familiar enough with Ignite internals - from a message ordering
perspective, since the coordinator is the one initiating this, and the
coordinator doesn't change without a change to the major topology version,
shouldn't it be simple enough to establish an ordering? The change to the next
major topology version should be the last message sent under the previous
topology version. Nodes can then hold on to messages received out of order
(possibly up to some maximum size) for handling in order. I'd assume that when
the coordinator goes down, the new coordinator will need to notify under the
previous topology version.
I agree that the ability to mutate messages is lost; this requires sending the
updates back to the coordinator which then has additional work to do merging
them.
Regarding [~ivan.glukos] note - top level node failures are not an issue. The
issue is around consecutive node failures at the bottom most layer. If you look
in my diagram above 1 -> 3 and 1 -> 2. Thus, failure of 3 doesn't prevent 4
from getting a message, since 2 -> 4 (3 is not a near neighbour of 2 because
it's max level is higher than 2's). Again, adding comms in both directions
(where we have 1 -> 5 and 1 -> 6 as well), makes this even more robust.
> 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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