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From "Jeremy Stribling (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-1046) Creating a new sequential node results in a ZNODEEXISTS error
Date Fri, 15 Apr 2011 00:56:06 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-1046?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13020139#comment-13020139
] 

Jeremy Stribling commented on ZOOKEEPER-1046:
---------------------------------------------

For the record, I have another example of this happening, this time without a lot complicated
cluster dynamics.  The scenario is:

# There is a 3-node cluster
# A sequential node is created with seq. num. X
# The leader dies very shortly after that (within a couple of transactions)
# A new leader is elected (who has seen all of the transactions committed by the previous
leader)
# The new leader immediately hits the problem where it returns ZNODEEXISTS when a seq. node
is returned

I have logs from before and after the problem, as well as Zookeeper snapshots that contain
the problem, but unfortunately the transaction log containing the actual problematic transaction
was rotated away.  If anyone wants to see what I do have, let me know and I'll upload them
somewhere.

I will also try to write a simple test that creates lots of sequential nodes, kills the leader,
and sees if the problem occurs.


> 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
>              Labels: sequence
>             Fix For: 3.4.0
>
>         Attachments: 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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