In hbase book, It recommends the zookeeper.session.timeout is default 60s.  
but, the default value is configured for 18000 in Hbase-default.xml.

Should we modify the Hbase-default.xml or book ?


Hbase Book:
13.6.2.6. ZooKeeper SessionExpired events

If you wish to increase the session timeout, add the following to your 
hbase-site.xml to increase the timeout from the default of 60 seconds to 120 
seconds.
<property>
    <name>zookeeper.session.timeout</name>
    <value>1200000</value>
</property>
<property>
    <name>hbase.zookeeper.property.tickTime</name>
    <value>6000</value>
</property>

Be aware that setting a higher timeout means that the regions served by a 
failed RegionServer will take at least that amount of time to be transfered to 
another RegionServer. For a production system serving live requests, we would 
instead recommend setting it lower than 1 minute and over-provision your 
cluster in order the lower the memory load on each machines (hence having less 
garbage to collect per machine).



Hbase-default.xml:
  <property>
    <name>zookeeper.session.timeout</name>
    <value>180000</value>
    <description>ZooKeeper session timeout.
      HBase passes this to the zk quorum as suggested maximum time for a
      session.  See 
http://hadoop.apache.org/zookeeper/docs/current/zookeeperProgrammers.html#ch_zkSessions
      "The client sends a requested timeout, the server responds with the
      timeout that it can give the client. "
      In milliseconds.
    </description>
  </property>

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