A distributed Apache HBase installation depends on a running ZooKeeper cluster. All
participating nodes and clients need to be able to access the running ZooKeeper ensemble. Apache
HBase by default manages a ZooKeeper "cluster" for you. It will start and stop the ZooKeeper
ensemble as part of the HBase start/stop process. You can also manage the ZooKeeper ensemble
independent of HBase and just point HBase at the cluster it should use. To toggle HBase
management of ZooKeeper, use the HBASE_MANAGES_ZK variable in
conf/hbase-env.sh. This variable, which defaults to
true, tells HBase whether to start/stop the ZooKeeper ensemble servers as
part of HBase start/stop.
When HBase manages the ZooKeeper ensemble, you can specify ZooKeeper configuration using its
native zoo.cfg file, or, the easier option is to just specify ZooKeeper
options directly in conf/hbase-site.xml. A ZooKeeper configuration option
can be set as a property in the HBase hbase-site.xml XML configuration file
by prefacing the ZooKeeper option name with hbase.zookeeper.property. For
example, the clientPort setting in ZooKeeper can be changed by setting the
hbase.zookeeper.property.clientPort property. For all default values used
by HBase, including ZooKeeper configuration, see HBase Default Configuration. Look for the
hbase.zookeeper.property prefix. For the full list of ZooKeeper configurations, see ZooKeeper's
zoo.cfg. HBase does not ship with a zoo.cfg so
you will need to browse the conf directory in an appropriate ZooKeeper
download.
You must at least list the ensemble servers in hbase-site.xml using the
hbase.zookeeper.quorum property. This property defaults to a single
ensemble member at localhost which is not suitable for a fully distributed
HBase. (It binds to the local machine only and remote clients will not be able to connect).
You can run a ZooKeeper ensemble that comprises 1 node only but in production it is recommended that you run a ZooKeeper ensemble of 3, 5 or 7 machines; the more members an ensemble has, the more tolerant the ensemble is of host failures. Also, run an odd number of machines. 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.
Give each ZooKeeper server around 1GB of RAM, and if possible, its own dedicated disk (A dedicated disk is the best thing you can do to ensure a performant ZooKeeper ensemble). For very heavily loaded clusters, run ZooKeeper servers on separate machines from RegionServers (DataNodes and TaskTrackers).
For example, to have HBase manage a ZooKeeper quorum on nodes
rs{1,2,3,4,5}.example.com, bound to port 2222 (the default is 2181)
ensure HBASE_MANAGE_ZK is commented out or set to true in
conf/hbase-env.sh and then edit conf/hbase-site.xml
and set hbase.zookeeper.property.clientPort and
hbase.zookeeper.quorum. You should also set
hbase.zookeeper.property.dataDir to other than the default as the default
has ZooKeeper persist data under /tmp which is often cleared on system
restart. In the example below we have ZooKeeper persist to
/user/local/zookeeper.
<configuration>
...
<property>
<name>hbase.zookeeper.property.clientPort</name>
<value>2222</value>
<description>Property from ZooKeeper's config zoo.cfg.
The port at which the clients will connect.
</description>
</property>
<property>
<name>hbase.zookeeper.quorum</name>
<value>rs1.example.com,rs2.example.com,rs3.example.com,rs4.example.com,rs5.example.com</value>
<description>Comma separated list of servers in the ZooKeeper Quorum.
For example, "host1.mydomain.com,host2.mydomain.com,host3.mydomain.com".
By default this is set to localhost for local and pseudo-distributed modes
of operation. For a fully-distributed setup, this should be set to a full
list of ZooKeeper quorum servers. If HBASE_MANAGES_ZK is set in hbase-env.sh
this is the list of servers which we will start/stop ZooKeeper on.
</description>
</property>
<property>
<name>hbase.zookeeper.property.dataDir</name>
<value>/usr/local/zookeeper</value>
<description>Property from ZooKeeper's config zoo.cfg.
The directory where the snapshot is stored.
</description>
</property>
...
</configuration>The newer version, the better. For example, some folks have been bitten by ZOOKEEPER-1277. If
running zookeeper 3.5+, you can ask hbase to make use of the new multi operation by enabling hbase.zookeeper.useMulti" in your hbase-site.xml.
Be sure to set up the data dir cleaner described under Zookeeper Maintenance else you could have 'interesting' problems a couple of months in; i.e. zookeeper could start dropping sessions if it has to run through a directory of hundreds of thousands of logs which is wont to do around leader reelection time -- a process rare but run on occasion whether because a machine is dropped or happens to hiccup.
To point HBase at an existing ZooKeeper cluster, one that is not managed by HBase, set
HBASE_MANAGES_ZK in conf/hbase-env.sh to
false
... # Tell HBase whether it should manage its own instance of Zookeeper or not. export HBASE_MANAGES_ZK=false
Next set ensemble locations and client port, if non-standard, in
hbase-site.xml, or add a suitably configured
zoo.cfg to HBase's CLASSPATH. HBase will prefer
the configuration found in zoo.cfg over any settings in
hbase-site.xml.
When HBase manages ZooKeeper, it will start/stop the ZooKeeper servers as a part of the regular start/stop scripts. If you would like to run ZooKeeper yourself, independent of HBase start/stop, you would do the following
${HBASE_HOME}/bin/hbase-daemons.sh {start,stop} zookeeper
Note that you can use HBase in this manner to spin up a ZooKeeper cluster, unrelated to
HBase. Just make sure to set HBASE_MANAGES_ZK to false
if you want it to stay up across HBase restarts so that when HBase shuts down, it doesn't take
ZooKeeper down with it.
For more information about running a distinct ZooKeeper cluster, see the ZooKeeper Getting Started Guide. Additionally, see the ZooKeeper Wiki or the ZooKeeper documentation for more information on ZooKeeper sizing.