Distribute

HBase is a distributed key-value database that supports automatic load balancing. With the balance switch (balance_switch) enabled, the HMaster process automatically selects regions according to a specified policy and assigns them to RegionServers with lower load. The official distribution currently supports two region-selection policies: DefaultLoadBalancer and StochasticLoadBalancer, both described in detail below. Because all HBase data (including HLog, meta, HStoreFile, and so on) is written to HDFS, region moves are very lightweight....

In an HBase master-slave replication cluster, as shown on the left in the figure below, Region-Server-X and Region-Server-Y are two RegionServers in the master cluster. Under normal conditions, writes to Region-A append logs to Hlog-X on Region-Server-X, and Region-Server-X asynchronously applies those HLog entries in batches to the slave cluster. If Region-Server-X then crashes, Region-A is taken over by Region-Server-Y. Subsequent writes to Region-A append logs to Hlog-Y, while Region-Server-Y starts a new thread to replay Hlog-X....