Re: Getting a "node exists" code on a sequence create

Mon, 01 Nov 2010 14:57:45 -0700 

Yes, every znode in /zkrsm was created with the sequence flag.
We bring up a cluster of three nodes, though we do it in a slightly odd manner to support dynamism: each node starts up as a single-node instance knowing only itself, and then each node is contacted by a coordinator that kills the ZooKeeperServer object and starts a new QuorumPeer object using the full list of three servers. I know this is weird; perhaps this has something to do with it.
Other than the weird setup behavior, we are just writing a few sequential records into the system (which all seems to work fine), killing one of the nodes (one that has been elected leader via the standard recommended ZK leader election algorithm), restarting it, and then trying to create more sequential znodes. I'm guessing this is pretty well-tested behavior, so there must be something weird or wrong about the way I have stuff setup.
I'm happy to provide whatever logs or snapshots might help someone track this down. Thanks, 

Jeremy

On 11/01/2010 02:42 PM, Benjamin Reed wrote:

how were you able to reproduce it?

all the znodes in /zkrsm were created with the sequence flag. right?

ben

On 11/01/2010 02:28 PM, Jeremy Stribling wrote:

We were able to reproduce it.  A "stat" on all three servers looks
identical:

[zk:<ip:port>(CONNECTED) 0] stat /zkrsm
cZxid = 9
ctime = Mon Nov 01 13:01:57 PDT 2010
mZxid = 9
mtime = Mon Nov 01 13:01:57 PDT 2010
pZxid = 12884902218
cversion = 177
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0
dataLength = 0
numChildren = 177

Creating a sequential node through the command line also fails:

[zk:<ip:port>(CONNECTED) 1] create -s /zkrsm/0000000000000000_record
testdata
Node already exists: /zkrsm/0000000000000000_record

One potentially interesting thing is that numChildren above is 177,
though I have sequence numbers on that record prefix up to 214 or so.
There seem to be some gaps though -- I thin "ls /zkrsm" only shows about
177.  Not sure if that's relevant or not.

Thanks,

Jeremy

On 11/01/2010 12:06 PM, Jeremy Stribling wrote:

Thanks for the reply.  It happened every time we called create, not
just once.  More than that, we tried restarting each of the nodes in
the system (one-by-one), including the new master, and the problem
continued.

Unfortunately we cleaned everything up, and it's not in that state
anymore.  We haven't yet tried to reproduce, but I will try and report
back if I can get any "cversion" info.

Jeremy

On 11/01/2010 11:33 AM, Patrick Hunt wrote:

Hi Jeremy, this sounds like a bug to me, I don't think you should be
getting the nodeexists when the sequence flag is set.

Looking at the code briefly we use the parent's "cversion"
(incremented each time the child list is changed, added/removed).

Did you see this error each time you called create, or just once? If
you look at the cversion in the Stat of the znode "/zkrsm" on each of
the servers what does it show? You can use the java CLI to connect to
each of your servers and access this information. It would be
interesting to see if the data was out of sync only for a short period
of time, or forever. Is this repeatable?

Ben/Flavio do you see anything here?

Patrick

On Thu, Oct 28, 2010 at 6:06 PM, Jeremy Stribling<st...@nicira.com>
wrote:

HI everyone,

Is there any situation in which creating a new ZK node with the
SEQUENCE
flag should result in a "node exists" error? I'm seeing this happening 
after a failure of a ZK node that appeared to have been the master;
when the
new master takes over, my app is unable to create a new SEQUENCE
node under
an existing parent node.  I'm using Zookeeper 3.2.2.

Here's a representative log snippet:

--------------------------
3050756 [ProcessThread:-1] TRACE
org.apache.zookeeper.server.PrepRequestProcessor  -
:Psessionid:0x12bf518350f0001 type:create cxid:0x4cca0691
zxid:0xfffffffffffffffe txntype:unknown /zkrsm/0000000000000000_record 
3050756 [ProcessThread:-1] WARN
org.apache.zookeeper.server.PrepRequestProcessor - Got exception when 
processing sessionid:0x12bf518350f0001 type:create cxid:0x4cca0691
zxid:0xfffffffffffffffe txntype:unknown n/a
org.apache.zookeeper.KeeperException$NodeExistsException:
KeeperErrorCode =
NodeExists
         at
org.apache.zookeeper.server.PrepRequestProcessor.pRequest(PrepRequestProcessor.java:245) 

         at
org.apache.zookeeper.server.PrepRequestProcessor.run(PrepRequestProcessor.java:114) 

3050756 [ProcessThread:-1] DEBUG
org.apache.zookeeper.server.quorum.CommitProcessor  - Processing
request::
sessionid:0x12bf518350f0001 type:create cxid:0x4cca0691
zxid:0x50000027e
txntype:-1 n/a
3050756 [ProcessThread:-1] DEBUG
org.apache.zookeeper.server.quorum.Leader
- Proposing:: sessionid:0x12bf518350f0001 type:create cxid:0x4cca0691 
zxid:0x50000027e txntype:-1 n/a
3050756 [SyncThread:0] TRACE
org.apache.zookeeper.server.quorum.Leader  -
Ack zxid: 0x50000027e
3050757 [SyncThread:0] TRACE
org.apache.zookeeper.server.quorum.Leader  -
outstanding proposal: 0x50000027e
3050757 [SyncThread:0] TRACE
org.apache.zookeeper.server.quorum.Leader  -
outstanding proposals all
3050757 [SyncThread:0] DEBUG
org.apache.zookeeper.server.quorum.Leader  -
Count for zxid: 0x50000027e is 1
3050757 [FollowerHandler-/172.16.0.28:48776] TRACE
org.apache.zookeeper.server.quorum.Leader  - Ack zxid: 0x50000027e
3050757 [FollowerHandler-/172.16.0.28:48776] TRACE
org.apache.zookeeper.server.quorum.Leader  - outstanding proposal:
0x50000027e
3050757 [FollowerHandler-/172.16.0.28:48776] TRACE
org.apache.zookeeper.server.quorum.Leader - outstanding proposals all 
3050757 [FollowerHandler-/172.16.0.28:48776] DEBUG
org.apache.zookeeper.server.quorum.Leader  - Count for zxid:
0x50000027e is
2
3050757 [FollowerHandler-/172.16.0.28:48776] DEBUG
org.apache.zookeeper.server.quorum.CommitProcessor  - Committing
request::
sessionid:0x12bf518350f0001 type:create cxid:0x4cca0691
zxid:0x50000027e
txntype:-1 n/a
3050757 [CommitProcessor:0] DEBUG
org.apache.zookeeper.server.FinalRequestProcessor  - Processing
request::
sessionid:0x12bf518350f0001 type:create cxid:0x4cca0691
zxid:0x50000027e
txntype:-1 n/a
3050757 [CommitProcessor:0] TRACE
org.apache.zookeeper.server.FinalRequestProcessor  -
:Esessionid:0x12bf518350f0001 type:create cxid:0x4cca0691
zxid:0x50000027e
txntype:-1 n/a
3050757 [FollowerHandler-/172.16.0.28:41062] TRACE
org.apache.zookeeper.server.quorum.Leader  - Ack zxid: 0x50000027e
3050757 [FollowerHandler-/172.16.0.28:41062] TRACE
org.apache.zookeeper.server.quorum.Leader - outstanding proposals all 
3050757 [FollowerHandler-/172.16.0.28:41062] DEBUG
org.apache.zookeeper.server.quorum.Leader  - outstanding is 0
--------------------------

I'm still a n00b at understanding ZK log messages, so maybe there's
something obvious going on.  I looked in the JIRA and did my best to
search
the mailing list archives, but couldn't find anything related to
this.  Any
ideas?  Thanks very much,

Jeremy

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