[ZOOKEEPER-4306] CloseSessionTxn contains too many ephemal nodes cause cluster crash

• Type: Bug
• Status: Open
• Priority: Critical
• Resolution: Unresolved
• Affects Version/s: 3.6.2
• Labels: pull-request-available
• Assignee: Unassigned
• Reporter: Lin Changrui
• Created: 28/May/21 03:09
• Updated: 07/Nov/24 06:50

Description

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:

leader:

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. 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? 

 

Issue Links

• is caused by: ZOOKEEPER-3145 Potential watch missing issue due to stale pzxid when replaying CloseSession txn with fuzzy snapshot
• is related to: ZOOKEEPER-4874 Avoid using fuzzy snapshots.
• links to: GitHub Pull Request #1716
• links to: GitHub Pull Request #2201