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https://issues.apache.org/jira/browse/ZOOKEEPER-1599?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13536436#comment-13536436
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Flavio Junqueira commented on ZOOKEEPER-1599:
---------------------------------------------

Pat, I'm not sure what you're trying to get out of this discussion at this 
point. Here is my understanding out of the comments I've seen here, including 
my own observations:

# The code we have for 3.3 and 3.4 enable rolling upgrades in the way you guys 
have been suggesting to people. There is this corner case that could cause some 
downtime, but I have no reason to suspect that it would be severe. 
# Making changes to try to fix the corner case is likely to open a can of 
worms, so I'd rather leave it alone and it sounds like others too.
# Moving forward, we won't have to depend on rolling upgrades any longer when 
ZOOKEEPER-107 gets in.

Is there a strong reason to keep discussing this?
                
> 3.3 server cannot join 3.4 quorum
> ---------------------------------
>
>                 Key: ZOOKEEPER-1599
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1599
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: quorum
>    Affects Versions: 3.3.6, 3.4.5
>            Reporter: Skye Wanderman-Milne
>            Assignee: Skye Wanderman-Milne
>            Priority: Blocker
>             Fix For: 3.4.6
>
>         Attachments: ZOOKEEPER-1599.patch
>
>
> When a 3.3 server attempts to join an existing quorum lead by a 3.4 server, 
> the 3.3 server is disconnected while trying to download the leader's 
> snapshot. The 3.3 server restarts and starts the process over again, but is 
> never able to join the quorum.
> 3.3 server log:
> {code}
> 2012-12-07 10:44:34,582 - INFO  
> [QuorumPeer:/0:0:0:0:0:0:0:0:2183:Learner@294] - Getting a snapshot from 
> leader
> 2012-12-07 10:44:34,582 - INFO  
> [QuorumPeer:/0:0:0:0:0:0:0:0:2183:Learner@325] - Setting leader epoch 12
> 2012-12-07 10:44:54,604 - WARN  
> [QuorumPeer:/0:0:0:0:0:0:0:0:2183:Follower@82] - Exception when following the 
> leader
> java.io.EOFException
>         at java.io.DataInputStream.readInt(DataInputStream.java:392)
>         at 
> org.apache.jute.BinaryInputArchive.readInt(BinaryInputArchive.java:63)
>         at 
> org.apache.zookeeper.server.quorum.QuorumPacket.deserialize(QuorumPacket.java:84)
>         at 
> org.apache.jute.BinaryInputArchive.readRecord(BinaryInputArchive.java:108)
>         at 
> org.apache.zookeeper.server.quorum.Learner.readPacket(Learner.java:148)
>         at 
> org.apache.zookeeper.server.quorum.Learner.syncWithLeader(Learner.java:332)
>         at 
> org.apache.zookeeper.server.quorum.Follower.followLeader(Follower.java:75)
>         at 
> org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:645)
> 2012-12-07 10:44:54,605 - INFO  
> [QuorumPeer:/0:0:0:0:0:0:0:0:2183:Follower@165] - shutdown called
> java.lang.Exception: shutdown Follower
>         at 
> org.apache.zookeeper.server.quorum.Follower.shutdown(Follower.java:165)
>         at 
> org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:649)
> {code}
> 3.4 leader log:
> {code}
> 2012-12-07 10:51:35,178 [myid:2] - INFO  
> [WorkerReceiver[myid=2]:FastLeaderElection$Messenger$WorkerReceiver@273] - 
> Backward compatibility mode, server id=3
> 2012-12-07 10:51:35,178 [myid:2] - INFO  
> [WorkerReceiver[myid=2]:FastLeaderElection@542] - Notification: 3 (n.leader), 
> 0x1100000000 (n.zxid), 0x2 (n.round), LOOKING (n.state), 3 (n.sid), 0x11 
> (n.peerEPoch), LEADING (my state)
> 2012-12-07 10:51:35,182 [myid:2] - INFO  
> [LearnerHandler-/127.0.0.1:37654:LearnerHandler@263] - Follower sid: 3 : info 
> : org.apache.zookeeper.server.quorum.QuorumPeer$QuorumServer@262f4873
> 2012-12-07 10:51:35,182 [myid:2] - INFO  
> [LearnerHandler-/127.0.0.1:37654:LearnerHandler@318] - Synchronizing with 
> Follower sid: 3 maxCommittedLog=0x0 minCommittedLog=0x0 
> peerLastZxid=0x1100000000
> 2012-12-07 10:51:35,182 [myid:2] - INFO  
> [LearnerHandler-/127.0.0.1:37654:LearnerHandler@395] - Sending SNAP
> 2012-12-07 10:51:35,183 [myid:2] - INFO  
> [LearnerHandler-/127.0.0.1:37654:LearnerHandler@419] - Sending snapshot last 
> zxid of peer is 0x1100000000  zxid of leader is 0x1200000000sent zxid of db 
> as 0x1200000000
> 2012-12-07 10:51:55,204 [myid:2] - ERROR 
> [LearnerHandler-/127.0.0.1:37654:LearnerHandler@562] - Unexpected exception 
> causing shutdown while sock still open
> java.net.SocketTimeoutException: Read timed out
>         at java.net.SocketInputStream.socketRead0(Native Method)
>         at java.net.SocketInputStream.read(SocketInputStream.java:150)
>         at java.net.SocketInputStream.read(SocketInputStream.java:121)
>         at java.io.BufferedInputStream.fill(BufferedInputStream.java:235)
>         at java.io.BufferedInputStream.read(BufferedInputStream.java:254)
>         at java.io.DataInputStream.readInt(DataInputStream.java:387)
>         at 
> org.apache.jute.BinaryInputArchive.readInt(BinaryInputArchive.java:63)
>         at 
> org.apache.zookeeper.server.quorum.QuorumPacket.deserialize(QuorumPacket.java:83)
>         at 
> org.apache.jute.BinaryInputArchive.readRecord(BinaryInputArchive.java:108)
>         at 
> org.apache.zookeeper.server.quorum.LearnerHandler.run(LearnerHandler.java:450)
> 2012-12-07 10:51:55,205 [myid:2] - WARN  
> [LearnerHandler-/127.0.0.1:37654:LearnerHandler@575] - ******* GOODBYE 
> /127.0.0.1:37654 ********
> {code}

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