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https://issues.apache.org/jira/browse/ZOOKEEPER-1046?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13049607#comment-13049607
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Benjamin Reed commented on ZOOKEEPER-1046:
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it turns out that the fix is really easy and small and easy to make backwards
compatible except for one killer problem: closeSession. The closeSession may do
a massive amount of deletes, so potentially we would need to go through and
figure out everything to delete and send a cversion for each.
i've been looking and i think we can simplify things. cversion is only used to
pick suffixes for sequential nodes, so we really only need to increment on
creates. if we did this, the fix would really become trivial and would also
make the numbering of creates sequential.
of course this is the only way we use it, others may use it for something else.
it would be an api semantic break.
> Creating a new sequential node results in a ZNODEEXISTS error
> -------------------------------------------------------------
>
> Key: ZOOKEEPER-1046
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1046
> Project: ZooKeeper
> Issue Type: Bug
> Components: server
> Affects Versions: 3.3.2, 3.3.3
> Environment: A 3 node-cluster running Debian squeeze.
> Reporter: Jeremy Stribling
> Assignee: Vishal K
> Priority: Blocker
> Labels: sequence
> Fix For: 3.4.0
>
> Attachments: ZOOKEEPER-1046-for333, ZOOKEEPER-1046.patch,
> ZOOKEEPER-1046.patch, ZOOKEEPER-1046.patch1, ZOOKEEPER-1046.tgz
>
>
> On several occasions, I've seen a create() with the sequential flag set fail
> with a ZNODEEXISTS error, and I don't think that should ever be possible. In
> past runs, I've been able to closely inspect the state of the system with the
> command line client, and saw that the parent znode's cversion is smaller than
> the sequential number of existing children znode under that parent. In one
> example:
> {noformat}
> [zk:<ip:port>(CONNECTED) 3] stat /zkrsm
> cZxid = 0x5
> ctime = Mon Jan 17 18:28:19 PST 2011
> mZxid = 0x5
> mtime = Mon Jan 17 18:28:19 PST 2011
> pZxid = 0x1d819
> cversion = 120710
> dataVersion = 0
> aclVersion = 0
> ephemeralOwner = 0x0
> dataLength = 0
> numChildren = 2955
> {noformat}
> However, the znode /zkrsm/000000000000002d_record0000120804 existed on disk.
> In a recent run, I was able to capture the Zookeeper logs, and I will attach
> them to this JIRA. The logs are named as nodeX.<zxid_prefixes>.log, and each
> new log represents an application process restart.
> Here's the scenario:
> # There's a cluster with nodes 1,2,3 using zxid 0x3.
> # All three nodes restart, forming a cluster of zxid 0x4.
> # Node 3 restarts, leading to a cluster of 0x5.
> At this point, it seems like node 1 is the leader of the 0x5 epoch. In its
> log (node1.0x4-0x5.log) you can see the first (of many) instances of the
> following message:
> {noformat}
> 2011-04-11 21:16:12,607 16649 [ProcessThread:-1] INFO
> org.apache.zookeeper.server.PrepRequestProcessor - Got user-level
> KeeperException when processing sessionid:0x512f466bd44e0002 type:create
> cxid:0x4da376ab zxid:0xfffffffffffffffe txntype:unknown reqpath:n/a Error
> Path:/zkrsm/00000000000000b2_record0001761440 Error:KeeperErrorCode =
> NodeExists for /zkrsm/00000000000000b2_record0001761440
> {noformat}
> This then repeats forever as my application isn't expecting to ever get this
> error message on a sequential node create, and just continually retries. The
> message even transfers over to node3.0x5-0x6.log once the 0x6 epoch comes
> into play.
> I don't see anything terribly fishy in the transition between the epochs; the
> correct snapshots seem to be getting transferred, etc. Unfortunately I don't
> have a ZK snapshot/log that exhibits the problem when starting with a fresh
> system.
> Some oddities you might notice in these logs:
> * Between epochs 0x3 and 0x4, the zookeeper IDs of the nodes changed due to a
> bug in our application code. (They are assigned randomly, but are supposed
> to be consistent across restarts.)
> * We manage node membership dynamically, and our application restarts the
> ZooKeeperServer classes whenever a new node wants to join (without restarting
> the entire application process). This is why you'll see messages like the
> following in node1.0x4-0x5.log before a new election begins:
> {noformat}
> 2011-04-11 21:16:00,762 4804 [QuorumPeer:/0.0.0.0:2888] INFO
> org.apache.zookeeper.server.quorum.Learner - shutdown called
> {noformat}
> * There is in fact one of these dynamic membership changes in
> node1.0x4-0x5.log, just before the 0x4 epoch is formed. I'm not sure how
> this would be related though, as no transactions are done during this period.
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