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https://issues.apache.org/jira/browse/ZOOKEEPER-2748?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15963227#comment-15963227
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Alexander Shraer commented on ZOOKEEPER-2748:
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Hi Marco,
this looks like a very useful feature. I added a couple of comments on GitHub.
I agree with Michael regarding an admin interface for this. But instead of
introducing new plumbing for this as an admin command, perhaps its worth
considering adding this as a reconfiguration mode ? This should be much easier.
I'm not sure if as part of this JIRA or in a separate JIRA. Then, in the
future, the leader could perhaps use the same command to do the load
rebalancing automatically.
Alex
> Four-letter command to voluntarily drop client connections
> ----------------------------------------------------------
>
> Key: ZOOKEEPER-2748
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2748
> Project: ZooKeeper
> Issue Type: New Feature
> Components: server
> Reporter: Marco P.
> Assignee: Marco P.
> Priority: Minor
>
> In certain circumstances, it would be useful to be able to move clients from
> one server to another.
> One example: a quorum that consists of 3 servers (A,B,C) with 1000 active
> client session, where 900 clients are connected to server A, and the
> remaining 100 are split over B and C (see example below for an example of how
> this can happen).
> A will do a lot more work than B, C.
> Overall throughput will benefit by having the clients more evenly divided.
> In case of A failure, all its client will create an avalanche by migrating en
> masse to a different server.
> There are other possible use cases for a mechanism to move clients:
> - Migrate away all clients before a server restart
> - Migrate away part of clients in response to runtime metrics (CPU/Memory
> usage, ...)
> - Shuffle clients after adding more server capacity (i.e. adding Observer
> nodes)
> The simplest form of rebalancing which does not require major changes of
> protocol or client code consists of requesting a server to voluntarily drop
> some number of connections.
> Clients should be able to transparently move to a different server.
> Patch introducing 4-letter commands to shed clients:
> https://github.com/apache/zookeeper/pull/215
> -- -- --
> How client imbalance happens in the first place, an example.
> Imagine servers A, B, C and 1000 clients connected.
> Initially clients are spread evenly (i.e. 333 clients per server).
> A: 333 (restarts: 0)
> B: 333 (restarts: 0)
> C: 334 (restarts: 0)
> Now restart servers a few times, always in A, B, C order (e.g. to pick up a
> software upgrades or configuration changes).
> Restart A:
> A: 0 (restarts: 1)
> B: 499 (restarts: 0)
> C: 500 (restarts: 0)
> Restart B:
> A: 250 (restarts: 1)
> B: 0 (restarts: 1)
> C: 750 (restarts: 0)
> Restart C:
> A: 625 (restarts: 1)
> B: 375 (restarts: 1)
> C: 0 (restarts: 1)
> The imbalance is pretty bad already. C is idle while A has a lot of work.
> A second round of restarts makes the situation even worse:
> Restart A:
> A: 0 (restarts: 2)
> B: 688 (restarts: 1)
> C: 313 (restarts: 1)
> Restart B:
> A: 344 (restarts: 2)
> B: 657 (restarts: 1)
> C: 0 (restarts: 1)
> Restart C:
> A: 673 (restarts: 2)
> B: 328 (restarts: 1)
> C: 0 (restarts: 1)
> Large cluster (5, 7, 9 servers) make the imbalance even more evident.
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