1.1. Region Servers and Secondary Mode

HBase is a key-value store, which allows fast table scanning and data retrieval at petabyte scale. Table keys are assigned to one or more Region Servers. In the following image, for example, Region Server 1 is responsible for responding to queries and scans for keys 10 through 40. If Region Server 1 crashes, keys 10-40 are unavailable.

HA provides a way to access keys 10-40 even if Region Server 1 is not available by assigning other Region Servers as backups. In the following image, Region Server 2 is the secondary region server for keys 10-20 and Region Server 3 is the secondary region server for keys 20-40. Region Server 2 is also the secondary region server for keys 80-100. Secondary Region Servers are not separate processes. Rather, they are Region Servers acting in secondary mode. When Region Server 2 services queries and scans for keys 10-20, it acts in secondary mode.

[Note]Note

Region Servers acting in secondary mode are also known as Secondary Region Servers. However, there is no second server process.

A Region Server in secondary mode can read but not write data. In addition, the data it returns may be stale, as described in the following section.


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