Query Interface

Custom API

HBase does not provide native support for SQL. Unlike RDBMS, HBase has four primary operations: Get, Put, Scan and Delete. It also has some DDL operations, e.g., Create. HBase provides a shell which users can fire queries from. Users can specify table name, column names and apply filters in their query. HBase also offers Java Client API and Thrift/REST API. Some third-party drivers are also available for other programming languages.

Storage Architecture

Disk-oriented

HBase leverages HDFS as the backend storage. Currently HDFS is disk-oriented.

Data Model

Column Family / Wide-Column

An HBase table consists of rows and columns. Rows are referenced by row keys which are raw byte arrays and are sorted by row key. The sort is byte-ordered. Each row contains columns. A column's content is also an uninterpreted array of bytes. All columns belong to a column family.

Views

Not Supported

HBase does not provide views. But users can write MapReduce programs to approximate views. Apache Phoenix, a SQL interface for HBase, provides support for views.

Query Compilation

Not Supported

Isolation Levels

Read Uncommitted Read Committed

HBase only provide "read committed" isolation level. Users can downgrade the isolation level to "read uncommitted" by modifying the source code.

Storage Model

Decomposition Storage Model (Columnar)

HBase is schema-less column-oriented datastore.

Query Execution

Tuple-at-a-Time Model

Checkpoints

Non-Blocking

HBase Snapshots allows the users to make checkpoints of a table with little impact on RegionServers. Creating snapshots does not block reads and writes, but for each table only one snapshot can be created at a time.

Stored Procedures

Not Supported

Stored procedures are not directly supported in HBase. But users can use coprocessors to resemble store procedures.

Logging

Logical Logging

HBase's write-ahead-log is named HLog.

System Architecture

Shared-Nothing

HBase is organized as a cluster of HBase nodes that complies with the master-slave architecture. There are two types of nodes: a master node, and one or more slave nodes called RegionServers. RegionServers serve data for reads and writes. An HBase Region is a subset of an HBase table that has a continuous range of sorted rowkeys. Region assignment and DDL operations are handled by the HBase Master. HBase is built on top of Hadoop. The Hadoop DataNodes store the data that RegionServers are managing. The HDFS Zookeeper maintains the server status in the cluster.

Indexes

B+Tree

For each table, HBase only provide an B+Tree like index on row keys. HBase does not natively support secondary indexes. Users can use filters for querying on non-rowkey columns. There are some techniques to create another table which can be used as a secondary index.

Joins

Not Supported

HBase does not support join operations. Users can implement joins in their application code.

Concurrency Control

Multi-version Concurrency Control (MVCC)

HBase guarantees ACID semantics per-row. HBase uses a form of Multiversion Concurrency Control (MVCC) to avoid row locks for read operations. Write operations still need to acquire row locks.