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From "Rajini Sivaram (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-3100) ZooKeeper client times out due to random choice of resolved addresses
Date Fri, 27 Jul 2018 14:26:00 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-3100?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16559837#comment-16559837
] 

Rajini Sivaram commented on ZOOKEEPER-3100:
-------------------------------------------

[~andorm] In the failing Kafka test, ZooKeeper was not listening on the wilcard address, it
was listening specifically on 127.0.0.1 (([https://github.com/apache/kafka/blob/trunk/core/src/test/scala/unit/kafka/zk/EmbeddedZookeeper.scala)|https://github.com/apache/kafka/blob/trunk/core/src/test/scala/unit/kafka/zk/EmbeddedZookeeper.scala).].
Hence the connection to the IPv6 address was failing. I think in the example above, you were
running ZooKeeper on the wildcard address and hence it worked for both IPv4 and IPv6.

We have fixed the Kafka tests by changing clients to connect to `127.0.0.1` instead of `localhost`
and that is a reasonable workaround since the server is bound explicitly to that address.
But since this used to work before, perhaps it would be better to guarantee that all the possible
addresses are attempted before applying backoff as large as a second?

> 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
>            Assignee: Andor Molnar
>            Priority: Major
>
> 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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