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https://issues.apache.org/jira/browse/ZOOKEEPER-851?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12926016#action_12926016
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Henry Robinson commented on ZOOKEEPER-851:
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I think what happens is that the leader happily lets the new follower connect,
but that it won't be part of any voting procedure. It shouldn't become leader
because no other nodes know about it to propose or support a vote for it.
To add a new node, you'll need to incrementally restart every node in your
cluster with the new config.
> ZK lets any node to become an observer
> --------------------------------------
>
> Key: ZOOKEEPER-851
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-851
> Project: Zookeeper
> Issue Type: Bug
> Components: quorum, server
> Affects Versions: 3.3.1
> Reporter: Vishal K
> Priority: Critical
> Fix For: 3.4.0
>
>
> I had a 3 node cluster running. The zoo.cfg on each contained 3 entries as
> show below:
> tickTime=2000
> dataDir=/var/zookeeper
> clientPort=2181
> initLimit=5
> syncLimit=2
> server.0=10.150.27.61:2888:3888
> server.1=10.150.27.62:2888:3888
> server.2=10.150.27.63:2888:3888
> I wanted to add another node to the cluster. In fourth node's zoo.cfg, I
> created another entry for that node and started zk server. The zoo.cfg on the
> first 3 nodes was left unchanged. The fourth node was able to join the
> cluster even though the 3 nodes had no idea about the fourth node.
> zoo.cfg on fourth node:
> tickTime=2000
> dataDir=/var/zookeeper
> clientPort=2181
> initLimit=5
> syncLimit=2
> server.0=10.150.27.61:2888:3888
> server.1=10.150.27.62:2888:3888
> server.2=10.150.27.63:2888:3888
> server.3=10.17.117.71:2888:3888
> It looks like 10.17.117.71 is becoming an observer in this case. I was
> expecting that the leader will reject 10.17.117.71.
> # telnet 10.17.117.71 2181
> Trying 10.17.117.71...
> Connected to 10.17.117.71.
> Escape character is '^]'.
> stat
> Zookeeper version: 3.3.0--1, built on 04/02/2010 22:40 GMT
> Clients:
> /10.17.117.71:37297[1](queued=0,recved=1,sent=0)
> Latency min/avg/max: 0/0/0
> Received: 3
> Sent: 2
> Outstanding: 0
> Zxid: 0x200000065
> Mode: follower
> Node count: 288
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