HMaster is the implementation of the Master Server. The Master server is
responsible for monitoring all RegionServer instances in the cluster, and is the interface
for all metadata changes. In a distributed cluster, the Master typically runs on the Section 9.9.1, “NameNode”. J Mohamed Zahoor goes into some more detail on the Master
Architecture in this blog posting, HBase HMaster
Architecture .
If run in a multi-Master environment, all Masters compete to run the cluster. If the active Master loses its lease in ZooKeeper (or the Master shuts down), then then the remaining Masters jostle to take over the Master role.
A common dist-list question involves what happens to an HBase cluster when the Master
goes down. Because the HBase client talks directly to the RegionServers, the cluster can
still function in a "steady state." Additionally, per Section 9.2, “Catalog Tables”, hbase:meta exists as an HBase table and is not
resident in the Master. However, the Master controls critical functions such as
RegionServer failover and completing region splits. So while the cluster can still run for
a short time without the Master, the Master should be restarted as soon as possible.
The methods exposed by HMasterInterface are primarily metadata-oriented methods:
Table (createTable, modifyTable, removeTable, enable, disable)
ColumnFamily (addColumn, modifyColumn, removeColumn)
Region (move, assign, unassign)
For example, when the HBaseAdmin method disableTable is invoked, it is serviced by the Master server.
The Master runs several background threads:
Periodically, and when there are no regions in transition, a load balancer will run and move regions around to balance the cluster's load. See Section 2.6.3.1, “Balancer” for configuring this property.
See Section 9.7.2, “Region-RegionServer Assignment” for more information on region assignment.
Periodically checks and cleans up the hbase:meta table. See Section 9.2.2, “hbase:meta” for more information on META.