Moti Nisenson-Ken created IGNITE-12133:
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Summary: 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
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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