There is only one active HBase master at any given time, but there can be any number of backup masters. The failover is automated and coordinated via ZooKeeper. Regionservers and clients use ZooKeeper to determine who is the current active master. You can run with as many as you want.
On larger clusters, I usually recommend around 5 master nodes. One possible configuration is to have all 5 with ZK (running on their own dedicated spindles), one for the NameNode, one for the SecondaryNameNode, and the other three can be HMasters (one will be the normal HMaster, the other backups).
> -----Original Message-----
> From: Joseph Coleman [mailto:[EMAIL PROTECTED]]
> Sent: Monday, February 07, 2011 11:05 AM
> To: [EMAIL PROTECTED]
> Subject: Master node Question
> I am in the process of building out a clustered environment that is going to
> start very small but grow extremely fast. I am just trying to figure out the
> 1. How many master nodes can I have?
> 2. If the answers is only 1 what happens if the master fails or dies. Does a
> slave take over and if so is the slave just referenced in the master files?
> Thanks for your help I am just trying to understand this before I order my
> equipment. The build out will be a couple master nodes for redundancy, 10
> data nodes running HDFS AND HBASE and a 2 to 3 node zookeeper cluster.
> We anticipate that out cluster will grow to 80 nodes before years end which is
> why I am thinking of the setup they way I am if anyone has feedback for a
> different setup please let me know. Also if there is a doc that covers that
> would be beneficial.
> Thanks for everyones help I do appreciate it.