[jira] [Commented] (ZOOKEEPER-4306) CloseSessionTxn contains too many ephemal nodes cause cluster crash

["Damien Diederen (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-4306?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17360734#comment-17360734 ] 

Damien Diederen commented on ZOOKEEPER-4306:
--------------------------------------------

Hi [~Changrui Lin],

I am currently looking into fixing this.

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

I would definitely put this in the "bug" category \:)

It seems that we would want ephemeral node creation to start failing when the session gets "too big" to fit in a transaction.  [~eolivelli], [~lvfangmin]: would you agree?

Here are a few related tickets which include ideas for minimizing {{jute.maxbuffer}} annoyances:

# ZOOKEEPER-1162: Suggests \(among others) controlling node size during child creation (similar to what I am proposing above);
# ZOOKEEPER-1644: Suggests compressing some of the data, which would allow for a larger {{CloseSessionTxn}} (related to your comment about "absolute paths").

Note that it is currently possible to work around this issue by setting this undocumented flag:

{noformat}
closeSessionTxn.enabled = false
{noformat}

(The flag was introduced as part of ZOOKEEPER-3145. Of course, disabling it "unfixes" the "potential watch missing issue." Still, probably better than suffering crashing ensembles.)


> 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
>            Priority: Critical
>         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.
> follower:
> !f.jpg!
> leader:
> !l1.png!
> !l2.jpg!
> 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.
> !r.jpg|width=1468,height=598!
>  
> *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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