HBase is a distributed column-oriented database built on top of HDFS. HBase is the Hadoop application to use when you require real-time read/write random-access to very large datasets.
Although there are countless strategies and implementations for database storage and retrieval, most solutions—especially those of the relational variety—are not built with very large scale and distribution in mind. Many vendors offer replication and partitioning solutions to grow the database beyond the confines of a single node but these add-ons are generally an afterthought and are complicated to install and maintain. They also come at some severe compromise to the RDBMS feature set. Joins, complex queries, triggers, views, and foreign-key constraints become prohibitively expensive to run on a scaled RDBMS or do not work at all.
HBase comes at the scaling problem from the opposite direction. It is built from the ground-up to scale linearly just by adding nodes. HBase is not relational and does not support SQL, but given the proper problem space it is able to do what an RDBMS cannot: ably host very large, sparsely populated tables on clusters made from commodity hardware.
The canonical HBase use case is the webtable, a table of crawled web pages and their attributes (such as language and MIME type) keyed by the web page URL. The webtable is large with row counts that run into the billions. Batch analytic and parsing MapReduce jobs are continuously ...