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From "Andre Medeiros (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-5397) [book] zookeeper quorum mistake
Date Tue, 14 Feb 2012 12:36:59 GMT
[book] zookeeper quorum mistake
-------------------------------

                 Key: HBASE-5397
                 URL: https://issues.apache.org/jira/browse/HBASE-5397
             Project: HBase
          Issue Type: Bug
            Reporter: Andre Medeiros
            Priority: Minor


In Chapter 2, section 2.5 "ZooKeeper" under "How many ZooKeepers should I run?" there is the
sentence
*There can be no quorum if the number of members is an even number.*

This is not true. In ZooKeeper, an even number of peers is supported, but it is normally not
used because an even sized ensemble requires, proportionally, more peers to form a quorum
than an odd sized ensemble requires. For example, an ensemble with 4 peers requires 3 to form
a quorum, while an ensemble with 5 also requires 3 to form a quorum. Thus, an ensemble of
5 allows 2 peers to fail, and thus is more fault tolerant than the ensemble of 4, which allows
only 1 down peer.

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