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From "Marco P. (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ZOOKEEPER-2748) Four-letter command to voluntarily drop client connections
Date Thu, 06 Apr 2017 22:04:41 GMT
Marco P. created ZOOKEEPER-2748:

             Summary: 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

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

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.

-- -- --

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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