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From "Fangmin Lv (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-3099) ZooKeeper cluster is unavailable for session_timeout time when the leader shutdown in a three-node environment.  
Date Fri, 24 Aug 2018 06:55:00 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-3099?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16591225#comment-16591225
] 

Fangmin Lv commented on ZOOKEEPER-3099:
---------------------------------------

[~jiangjiafu] please change the title to be explicitly about the network ACL not shutdown
on leader. Because in shutdown case, the follower should detect the connection being reset
immediately after it, so it won't wait sync timeout before starting a new leader election.

In this case, the quorum is unavailable for syncLimit * tickTIme (+ leader election/activation
time), not the session timeout.

In ZK, improper syncLimit, tickTIme, session timeout could cause undesired behavior, for example,
if syncLimit * tickTime is larger than session timeout, then the client will expire due
to one server is slow due to full GC or network issue. You need to make the session timeout
relatively larger than the syncLimit * tickTime, so the global sessions won't expire due to
network partition/ACL like the scenario you mentioned here.

> ZooKeeper cluster is unavailable for session_timeout time when the leader shutdown in
a three-node environment.   
> ------------------------------------------------------------------------------------------------------------------
>
>                 Key: ZOOKEEPER-3099
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3099
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: c client, java client
>    Affects Versions: 3.4.11, 3.5.4, 3.4.12, 3.4.13
>            Reporter: Jiafu Jiang
>            Priority: Major
>
>  
> The default readTimeout timeout of ZooKeeper client is 2/3 * session_time, the default
connectTimeout is session_time/hostProvider.size(). If the ZooKeeper cluster has 3 nodes,
then connectTimeout is 1/3 * session_time.
>  
> Supports we have three ZooKeeper servers: zk1, zk2, zk3 deployed. And zk3 is now the
leader. Client c1 is now connected to zk2(follower). Then we shutdown the network of zk3(leader),
the same time, client c1 begin to write some data to ZooKeeper. After a (syncLimit * tick)
timeout, zk2 will disconnect with leader and begin a new election, and zk2 becomes the leader.
>  
> The write operation will not succeed due to the leader is shutdown. It will take at most
readTimeout time for c1 to discover the failure, and client c1 will try to choose another
ZooKeeper server. Unfortunately, c1 may choose zk3, which is unreachable now, then c1 will
spend connectTimeout to find out that zk3 is unused. Notice that readTimeout + connectTimeout
= sesstion_timeout in my case(three-node cluster).
>  
> Therefore, in this case, the ZooKeeper cluster is unavailable for session timeout time
when only one ZooKeeper server is shutdown.
>  
> I have some suggestions:
>  # The HostProvider used by ZooKeeper can be specified by an argument.
>  # readTimeout can also be specified in any way.
>  
>  
>  



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