一、什么是namespace

在RDBMS中有database的概念，用来对table进行分组，那么在HBase中当表比较多的时候如何对表分组呢，就是namespace，可以简单的把namespace理解为RDBMS中的database。

二、namespace相关操作

HBase有两个内置的namespace：

hbase(main):009:0> list_namespace
NAMESPACE
default
hbase
2 row(s)
Took 0.0683 seconds

hbase：用来存放系统相关的一些元数据等，相当于mysql中的mysql数据库：

hbase(main):025:0> list_namespace_tables "hbase""
TABLE
meta
namespace
2 row(s)
Took 0.0174 seconds
=> ["meta", "namespace"]

default：创建表时未指定命名空间的话默认挂在default下。

创建namespace：

hbase(main):010:0> create_namespace "test"
Took 0.2781 seconds

在创建namespace的时候还可以添加一些说明信息：

hbase(main):018:0> create_namespace "test002", {"author"=>"CC11001100", "create_time"=>"2018-11-4 17:51:53"}
Took 0.2262 seconds

上面创建namespace时添加了一些附加信息，如何查看这些附加信息呢：

hbase(main):019:0> describe_namespace "test002""
DESCRIPTION
{NAME => 'test002', author => 'CC11001100', create_time => '2018-11-4 17:51:53'}
Took 0.0042 seconds
=> 1

查看附加信息发现作者写错了，需要修改：

hbase(main):014:0> alter_namespace "test002", {METHOD=>"set", "author"=>"ChenEr"}
Took 0.2458 seconds
hbase(main):015:0> describe_namespace "test002"
DESCRIPTION
{NAME => 'test002', author => 'ChenEr', create_time => '2018-11-4 17:51:53'}
Took 0.0056 seconds
=> 1

改完发现还漏了一项，需要追加自己的联系方式：

hbase(main):016:0> alter_namespace "test002", {METHOD=>"set", "email"=>"CC11001100@qq.com"}
Took 0.2313 seconds
hbase(main):017:0> describe_namespace "test002"
DESCRIPTION
{NAME => 'test002', author => 'ChenEr', create_time => '2018-11-4 17:51:53', email => 'CC11001100@qq.com'}
Took 0.0038 seconds
=> 1

想了下放上去自己的联系方式搞不好会被不断骚扰，还是去掉此项属性：

hbase(main):010:0> alter_namespace "test002", {METHOD=>"unset", NAME=>"email"}
Took 0.2419 seconds
hbase(main):011:0> describe_namespace "test002"
DESCRIPTION
{NAME => 'test002', author => 'ChenEr', create_time => '2018-11-4 17:51:53'}
Took 0.0056 seconds
=> 1

总结：

添加或修改属性：

alter_namespace 'ns1', {METHOD => 'set', 'PROPERTY_NAME' => 'PROPERTY_VALUE'}

删除属性：

alter_namespace 'ns1', {METHOD => 'unset', NAME=>'PROPERTY_NAME'}

查看当前已经创建的namespace，相当于RDBMS中的show databases：

hbase(main):019:0> list_namespace
NAMESPACE
default
hbase
test
test002
4 row(s)
Took 0.0122 seconds

删除namespace：

hbase(main):019:0> list_namespace
NAMESPACE
default
hbase
test
test002
4 row(s)
Took 0.0122 seconds
hbase(main):020:0> drop_namespace "test002"
Took 0.2321 seconds
hbase(main):021:0> list_namespace
NAMESPACE
default
hbase
test
3 row(s)
Took 0.0111 seconds

注意要删除的namespace必须是空的，其下没有表，否则会删除失败：

hbase(main):051:0> create_namespace "test_drop_namespace"
Took 0.2376 seconds
hbase(main):052:0> create "test_drop_namespace:t1", "cf1"
Created table test_drop_namespace:t1
Took 2.2312 seconds
=> Hbase::Table - test_drop_namespace:t1
hbase(main):053:0> drop_namespace "test_drop_namespace"

ERROR: org.apache.hadoop.hbase.constraint.ConstraintException: Only empty namespaces can be removed. Namespace test_drop_namespace has 1 tables
at org.apache.hadoop.hbase.master.procedure.DeleteNamespaceProcedure.prepareDelete(DeleteNamespaceProcedure.java:217)
at org.apache.hadoop.hbase.master.procedure.DeleteNamespaceProcedure.executeFromState(DeleteNamespaceProcedure.java:78)
at org.apache.hadoop.hbase.master.procedure.DeleteNamespaceProcedure.executeFromState(DeleteNamespaceProcedure.java:45)
at org.apache.hadoop.hbase.procedure2.StateMachineProcedure.execute(StateMachineProcedure.java:189)
at org.apache.hadoop.hbase.procedure2.Procedure.doExecute(Procedure.java:850)
at org.apache.hadoop.hbase.procedure2.ProcedureExecutor.execProcedure(ProcedureExecutor.java:1473)
at org.apache.hadoop.hbase.procedure2.ProcedureExecutor.executeProcedure(ProcedureExecutor.java:1241)
at org.apache.hadoop.hbase.procedure2.ProcedureExecutor.access$800(ProcedureExecutor.java:75)
at org.apache.hadoop.hbase.procedure2.ProcedureExecutor$WorkerThread.run(ProcedureExecutor.java:1761)

For usage try 'help "drop_namespace"'

Took 0.1217 seconds
hbase(main):054:0> disable "test_drop_namespace:t1"
Took 0.4357 seconds
hbase(main):055:0> drop "test_drop_namespace:t1"
Took 0.2351 seconds
hbase(main):056:0> drop_namespace "test_drop_namespace"
Took 0.2172 seconds

创建表时指定表所在的namespace，使用namespace-name:tale-name的格式指定：

hbase(main):022:0> create "test:user", "userInfo"
Created table test:user
Took 2.3992 seconds
=> Hbase::Table - test:user

查看namespace下都有哪些表：

hbase(main):024:0> list_namespace_tables "test"
TABLE
user
1 row(s)
Took 0.0252 seconds
=> ["user"]