Neha Narkhede created ZOOKEEPER-1740:
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Summary: Zookeeper 3.3.4 loses ephemeral nodes under stress
Key: ZOOKEEPER-1740
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1740
Project: ZooKeeper
Issue Type: Bug
Components: server
Affects Versions: 3.3.4
Reporter: Neha Narkhede
Priority: Critical
The current behavior of zookeeper for ephemeral nodes is that session
expiration and ephemeral node deletion is not an atomic operation.
The side-effect of the above zookeeper behavior in Kafka, for certain corner
cases, is that ephemeral nodes can be lost even if the session is not expired.
The sequence of events that can lead to lossy ephemeral nodes is as follows -
1. The session expires on the client, it assumes the ephemeral nodes are
deleted, so it establishes a new session with zookeeper and tries to re-create
the ephemeral nodes.
2. However, when it tries to re-create the ephemeral node,zookeeper throws back
a NodeExists error code. Now this is legitimate during a session disconnect
event (since zkclient automatically retries the
operation and raises a NodeExists error). Also by design, Kafka server doesn't
have multiple zookeeper clients create the same ephemeral node, so Kafka server
assumes the NodeExists is normal.
3. However, after a few seconds zookeeper deletes that ephemeral node. So from
the client's perspective, even though the client has a new valid session, its
ephemeral node is gone.
This behavior is triggered due to very long fsync operations on the zookeeper
leader. When the leader wakes up from such a long fsync operation, it has
several sessions to expire. And the time between the session expiration and the
ephemeral node deletion is magnified. Between these 2 operations, a zookeeper
client can issue a ephemeral node creation operation, that could've appeared to
have succeeded, but the leader later deletes the ephemeral node leading to
permanent ephemeral node loss from the client's perspective.
Thread from zookeeper mailing list:
http://zookeeper.markmail.org/search/?q=Zookeeper+3.3.4#query:Zookeeper%203.3.4%20date%3A201307%20+page:1+mid:zma242a2qgp6gxvx+state:results
The way to reproduce this behavior is as follows -
1. Bring up a zookeeper 3.3.4 cluster and create several sessions with
ephemeral ndoes on it using zkclient. Make sure the session expiration callback
is implemented and it re-registers the ephemeral node.
2. Run the following script on the zookeeper leader -
while true
do
kill -STOP $1
sleep 8
kill -CONT $1
sleep 60
done
3. Run another script to check for existence of ephemeral nodes.
This script shows that zookeeper loses the ephemeral nodes and the clients
still have a valid session.
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