[book] zookeeper quorum mistake
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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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