yes, i think you have summarized the problem nicely jeremy.
i'm curious about your reasoning for running servers in standalone mode
and then merging. can you explain that a bit more?
thanx
ben
On 11/01/2010 04:51 PM, Jeremy Stribling wrote:
I think this is caused by stupid behavior on our application's part, and
the error message just confused me. Here's what I think is happening.
1) 3 servers are up and accepting data, creating sequential znodes under
/zkrsm.
2) 1 server dies, the other 2 continue creating sequential znodes.
3) The 1st server restarts, but instead of joining the other 2 servers,
it starts an instance by itself, knowing only about the znodes created
before it died. [This is a bug in our application -- it is supposed to
join the other 2 servers in their cluster.]
4) Another server (#2) dies and restarts, joining the cluster of server
#1. It knows about more sequential znodes under /zkrsm than server #1.
5) At this point, trying to create a new znode in the #1-#2 cluster
might be problematic, because servers #1 and #2 know about different
sets of znode. If #1 allocates what it thinks is a new sequential
number for a new znode, it could be one already used by server #2, and
hence a "node exists" code might be returned.
So, in summary, our application is almost certainly using Zookeeper
wrong. Sorry to waste time on the list, but maybe this thread can help
someone in the future.
(If this explanation sounds totally off-base though, let me know. I'm
not 100% certain this is what's happening, but it definitely seems likely.)
Thanks,
Jeremy
On 11/01/2010 02:56 PM, Jeremy Stribling wrote:
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
