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From Jordan Zimmerman <jor...@jordanzimmerman.com>
Subject Re: Leader election
Date Thu, 06 Dec 2018 12:50:29 GMT
It is not possible to achieve the level of consistency you're after in an eventually consistent
system such as ZooKeeper. There will always be an edge case where two ZooKeeper clients will
believe they are leaders (though for a short period of time). In terms of how it affects Apache
Curator, we have this Tech Note on the subject: https://cwiki.apache.org/confluence/display/CURATOR/TN10
<https://cwiki.apache.org/confluence/display/CURATOR/TN10> (the description is true
for any ZooKeeper client, not just Curator clients). If you do still intend to use a ZooKeeper
lock/leader I suggest you try Apache Curator as writing these "recipes" is not trivial and
have many gotchas that aren't obvious. 

-Jordan

http://curator.apache.org <http://curator.apache.org/>


> On Dec 5, 2018, at 6:20 PM, Michael Borokhovich <michaelbor@gmail.com> wrote:
> 
> Hello,
> 
> We have a service that runs on 3 hosts for high availability. However, at
> any given time, exactly one instance must be active. So, we are thinking to
> use Leader election using Zookeeper.
> To this goal, on each service host we also start a ZK server, so we have a
> 3-nodes ZK cluster and each service instance is a client to its dedicated
> ZK server.
> Then, we implement a leader election on top of Zookeeper using a basic
> recipe:
> https://zookeeper.apache.org/doc/r3.1.2/recipes.html#sc_leaderElection.
> 
> I have the following questions doubts regarding the approach:
> 
> 1. It seems like we can run into inconsistency issues when network
> partition occurs. Zookeeper documentation says that the inconsistency
> period may last “tens of seconds”. Am I understanding correctly that during
> this time we may have 0 or 2 leaders?
> 2. Is it possible to reduce this inconsistency time (let's say to 3
> seconds) by tweaking tickTime and syncLimit parameters?
> 3. Is there a way to guarantee exactly one leader all the time? Should we
> implement a more complex leader election algorithm than the one suggested
> in the recipe (using ephemeral_sequential nodes)?
> 
> Thanks,
> Michael.


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