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From Andor Molnar <an...@apache.org>
Subject Re: Some question about Observer
Date Wed, 02 Jan 2019 09:54:03 GMT
Hi Muzhi!

Unfortunately I’m not aware of such performance analysis in the ZooKeeper documentation,
but you could get an idea of the performance impact by considering the following:

From the wiki:
https://wiki.apache.org/hadoop/ZooKeeper/Observers <https://wiki.apache.org/hadoop/ZooKeeper/Observers>
- The Leader must keep track of all Observers. The cost is the same as for each Follower -
in fact in the current implementation FollowerHandlers are used to handle Observer connections.
- Observers must receive every proposal via a single INFORM message, adding N messages to
every proposal for N Observers.

The idea behind using observers is to improve read performance without affecting write performance
(observers don’t participate in voting). However, as the first statement indicates, the
leader must track of observers and using many observers can be a bottleneck on the leader.
In order to address this issue the following Jira has been merged recently to allow Followers
to host observers too:

https://issues.apache.org/jira/browse/ZOOKEEPER-3140 <https://issues.apache.org/jira/browse/ZOOKEEPER-3140>

The feature will be available in the 3.6.0 release.


> On 2018. Dec 27., at 17:13, Muzhi <2807282710@qq.com> wrote:
> Hello!
>  I'm a user of Zookeeper,I have some questions about Observer.In the Zookeeper Documentation
there's a figure,Zookeeper Throughtput as the Read-Write Ratio Varie,not include the Observer
and the documentation just talk about how to set Observer or example use case.So I'm confused
if we set Observer in cluster,how many Observer should be set can have a higher performance?
For example,if I have 3 servers set to Leader of Follower,How many Observer should I set that
cluster have a higher performance?Do you have a figure such as "Zookeeper Throughtput as the
Read-Write Ratio Varie" that include Observer?
>    Looking forward to your reply,thanks!

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