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https://issues.apache.org/jira/browse/ZOOKEEPER-3100?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16553039#comment-16553039
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Rajini Sivaram commented on ZOOKEEPER-3100:
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[~andorm] In the failing tests, ZooKeeper was being started on `127.0.0.1`
explicitly
([https://github.com/apache/kafka/blob/trunk/core/src/test/scala/unit/kafka/zk/EmbeddedZookeeper.scala).]
Clients were connecting using `localhost`. We are fixing this for now in the
tests by changing the clients to use `127.0.0.1`, but it used to work fine
before upgrading ZK clients.
> 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
> 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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