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From "Rajini Sivaram (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ZOOKEEPER-3100) ZooKeeper client times out due to random choice of resolved addresses
Date Mon, 23 Jul 2018 14:14:00 GMT
Rajini Sivaram created ZOOKEEPER-3100:
-----------------------------------------

             Summary: ZooKeeper client times out due to random choice of resolved addresses
                 Key: ZOOKEEPER-3100
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3100
             Project: ZooKeeper
          Issue Type: Bug
          Components: java client
    Affects Versions: 3.4.13
            Reporter: Rajini Sivaram


The changes to ZooKeeper clients to re-resolve hosts made under ZOOKEEPER-2184 results in
delays when only a subset of the addresses that a host resolves to are actually reachable.
This can result in connection timeouts on the client.

For example, when running tests with a single ZooKeeper server accepting connections on 127.0.0.1
on a host that has both IPv4 and IPv6, we have seen connection timeouts in tests if client
connects using `localhost` rather than `127.0.0.1`. ZooKeeper client resolves `localhost`
to both the IPv4 and IPv6 addresses and chooses a random one. If IPv6 was chosen, a fixed
one second backoff is applied before retry since there is only one hostname specified. After
backoff, 'localhost' is resolved again and a random address chosen, which could also be the
unconnectable IPv6 address.

For the list of host names specified for connection, the clients do round-robin without backoffs
until connections to all hostnames are attempted. Can we also do the same for addresses that
each of the hosts resolves to, so that backoffs are only applied after connection to each
address is attempted once and every address is connected to once using round-robin rather
than random selection? This will avoid delays in cases where at least one address can be connected
to.

 



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