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From "Lin Changrui (Jira)" <j...@apache.org>
Subject [jira] [Created] (ZOOKEEPER-4306) CloseSessionTxn contains too many ephemal nodes cause cluster crash
Date Fri, 28 May 2021 03:10:00 GMT
Lin Changrui created ZOOKEEPER-4306:

             Summary: CloseSessionTxn contains too many ephemal nodes cause cluster crash
                 Key: ZOOKEEPER-4306
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-4306
             Project: ZooKeeper
          Issue Type: Bug
    Affects Versions: 3.6.2
            Reporter: Lin Changrui
         Attachments: cs.jpg, f.jpg, l1.png, l2.jpg, r.jpg

We took a test about how many ephemal nodes can client create under one parent node with defalut
configuration. The test caused cluster crash at last, exception stack trace like this.






It seems that leader sent a too large txn packet to followers. When follower try to deserialize
the txn, it found the txn length out of its buffer size(default 1MB+1MB, jute.maxbuffer +
jute.maxbuffer.extrasize). That causes followers crashed, and then, leader found there was
no sufficient followers synced, so leader shutdown later. When leader shutdown, it called
zkDb.fastForwardDataBase() , and leader found the txn read from txnlog out of its buffer size,
so it crashed too.

After the servers crashed, they try to restart the quorum. But they would not success because
the last txn is too large. We lose the log at that moment, but the stack trace is same as
this one.



*Root Cause*

We use org.apache.zookeeper.server.LogFormatter(-Djute.maxbuffer=74827780) visualize this
log and found this. !cs.jpg|width=1400,height=581! So closeSessionTxn contains all ephemal
nodes with absolute path. We know we will get a large getChildren respose if we create too
many children nodes under one parent node, that is limited by jute.maxbuffer of client. If
we create plenty of ephemal nodes under different parent nodes with one session, it may not
cause out of buffer of client, but when the session close without delete these node first,
it probably cause cluster crash.

Is it a bug or just a unspecified feature?If it just so, how should we judge the upper limit
of creating nodes? 


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