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Date: Tue, 11 Apr 2017 16:57:41 +0000 (UTC)
From: "Marco P. (JIRA)" <jira@apache.org>
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Subject: [jira] [Updated] (ZOOKEEPER-2748) Admin command to voluntarily drop
 client connections
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archived-at: Tue, 11 Apr 2017 16:57:46 -0000


     [ https://issues.apache.org/jira/browse/ZOOKEEPER-2748?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Marco P. updated ZOOKEEPER-2748:
--------------------------------
    Summary: Admin command to voluntarily drop client connections  (was: Four-letter command to voluntarily drop client connections)

Re-titling: s/4-letter command/admin command/

> Admin 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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