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前言：本篇主要梳理了HBase的基础知识，不涉及环境的搭建及原理的深入探讨，算是阶段性学习的总结。文中观点大部分源于HBase官方文档，并且为了不曲解其原意，不做多余的中文翻译。更多详情请参照HBase官方文档：

一、什么是HBase？

HBase(Hadoop Database, 以下简称HBase)是Apache Hadoop项目的子项目，是一个开源的、分布式的、面向列的非关系型数据库。它参考了Google的BigTable建模，在HDFS上提供了类似于BigTable的能力。

附注：准确来说，应该是面向列族的。

二、HBase的应用场景？

1.概述

• Make sure you have enough data. 若数据达数亿乃至数十亿行，则可以考虑HBase。
• Make sure you can live without all the extra features that an RDBMS provides(e.g., typed columns, secondary indexes, transactions, advanced query languages, etc.)
• Make sure you have enough hardware. HDFS doesn't do well with anything less than 5 DataNodes, plus a NameNode.

2.应用实例

• 存储云服务数据，如用户的联系人、短信、便签等数据。
• 存储用户的点击流，通过分析点击流，可精准化用户运营。

三、HBase的特性

• Strongly consistent reads/writes - 强一致性的读写
• Automatic sharding - 自动分片
• Automatic RegionServer failover - 自动RegionServer故障转移
• Hadoop/HDFS Integration
• MapReduce
• Java Client API/Thrift/REST API
• Block Cache and Bloom Filters
• Operational Management

四、HBase与Hadoop/HDFS的区别？

• Hadoop/HDFS：HDFS是一个适用于大文件存储的分布式文件系统，不支持在文件中快速查找记录。
• HBase：HBase基于HDFS，利用了HDFS的大文件存储优势，同时支持在大表中快速查找和更新记录。

五、HBase的数据模型

1.命名空间(Namespace)

• A namespace is a logical grouping of tables.
• There are two predefined special namespaces.

hbase - system namespace, used to contain HBase internal tables.

default - tables with no explicit specified namespace will automatically fall into this namespace.

2.表(Table)

• An HBase table consists of multiple rows.

3.行(Row)

• A row in HBase consists of a row key and one or more columns with values associated with them.
• Rows are sorted alphabetically by the row key as they are stored.

4.列(Column)

• A column in HBase consists of  a column family and a column qualifier, which are delimited by a ':' character.

5.列族(Column Family)

• Column families physically colocate a set of columns and their values, often for performance reasons.
• Column families must be declared up front at schema definition time.
• The column family prefix must be composed of printable characters.
• HBase currently does not do well with anything above two or three column families.

6.列描述符(Column Qualifier)

• A column qualifier is added to a column family to provide the index for a given piece of data.
• Column qualifiers are mutable and may differ greatly between rows.

7.单元格(Cell)

• A cell is a combination of row, column family, and column qualifier, and contains a value and a timestamp, which represents the value's version.
• A {row, column, version} tuple exactly specifies a cell in HBase. 

8.时间戳(Timestamp)

• A timestamp is written alongside each value, and is the identifier for a given version of a value.
• By default, the timestamp represents the time on the RegionServer when the data was written, but you can specify a different timestamp value when you put data into the cell.
• In physical view, timestamp are stored in descending order.

六、HBase的逻辑视图（表格视图&多维Map视图）

表描述：

• 表名：webtable
• 行：2行，RowKey为com.cnn.www、com.example.www
• 列族：3个，contents、anchor、people
• 对于RowKey=com.cnn.www，列族anchor包含两个列（anchor:cssnsi.com、anchor:my.look.ca），列族contents包含一个列（contents:html）
• 对于RowKey=com.cnn.www，具有5个数据版本；对于RowKey=com.example.www，具有一个数据版本。

七、HBase的物理视图（列族存储）

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