Introducing HBase - HBase Essentials - page 12

Understanding HBase cluster
components
In fully distributed and pseudo-distributed modes, a HBase cluster has many
components such as HBase Master, ZooKeeper, RegionServers, HDFS DataNodes,
and so on, discussed as follows:
• HBase Master : HBase Master coordinates the HBase cluster and is responsible
for administrative operations. It is a lightweight process that does not require
too many hardware resources. A large cluster might have multiple HBase
Master components to avoid cases that have a single point of failure. In this
highly available cluster with multiple HBase Master components, only once
HBase Master is active and the rest of HBase Master servers get in sync with
the active server asynchronously. Selection of the next HBase Master in case
of failover is done with the help of the ZooKeeper ensemble.
• ZooKeeper : ZooKeeper is a centralized service for maintaining coniguration
information, naming, providing distributed synchronization, and providing
group services. Similar to HBase Master, ZooKeeper is again a lightweight
process. By default, a ZooKeeper process is started and stopped by HBase
but it can be managed separately as well. The HBASE_MANAGES_ZK variable
in conf/hbase-env.sh with the default value true signiies that HBase is
going to manage ZooKeeper. We can specify the ZooKeeper coniguration
in the native zoo.cfg ile or its values such as client, port, and so on directly
in conf/hbase-site.xml . It is advisable that you have an odd number of
ZooKeeper ensembles such as one/three/ive for more host failure tolerance.
The following is an example of hbase-site.xml with ZooKeeper settings:
<property>
<name>hbase.zookeeper.property.clientPort</name>
<value>2222</value>
</property>
<property>
<name>hbase.zookeeper.quorum</name>
<value>ZooKeeperhost1, ZooKeeperhost2, ZooKeeperhost3<value>
</property>
<property>
<name>hbase.zookeeper.property.dataDir</name>
<value>/opt/zookeeper<value>
</property>