HBase笔记之namespace
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
内置的namespace
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
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
查看namespace信息
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
修改namespace
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'}
查看已创建的所有namespace
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
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namespace
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下的表
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"]
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