关于OCP的小知识点，实例受限启动，实例启动的知识点很多，也是OCP学习的重点，需要多看官方文档和多做测试。
Restricting Access to an Instance at Startup 
--实例启动限制

You can start an instance, and optionally mount and open a database, in restricted mode so that the instance is available only to administrative personnel (not general database users).
Use this mode of instance startup when you must accomplish one of the following tasks:
--实例启动限制模式适用的场景，一定是我们不希望其他连接访问数据，对数据库数据进行改动，数据备份，loader，临时阻止已经升级迁移会常常使用到。

•Perform an export or import of data
•Perform a data load (with SQL*Loader)
•Temporarily prevent typical users from using data
•Perform certain migration or upgrade operations

Typically, all users with the CREATE SESSION system privilege can connect to an open database. Opening a database in restricted mode allows database access only to users with both the CREATE SESSION and RESTRICTED SESSION system privilege. Only database administrators should have the RESTRICTED SESSION system privilege. Further, when the instance is in restricted mode, a database administrator cannot access the instance remotely through an Oracle Net listener, but can only access the instance locally from the system that the instance is running on. 

The following command starts an instance (and mounts and opens the database) in restricted mode:

小实验：
[root@11g-ocp ~]# su - oracle
<11g-ocp:orcl:/home/oracle>$sqlplus / as sysdba

SQL*Plus: Release 11.2.0.4.0 Production on Wed Dec 12 18:12:11 2018

Copyright (c) 1982, 2013, Oracle.  All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, Automatic Storage Management, OLAP and Real Application Testing options

SQL> startup restrict force 
ORACLE instance started.
--限制模式启动

Total System Global Area 1870647296 bytes
Fixed Size                  2254304 bytes
Variable Size             503319072 bytes
Database Buffers         1358954496 bytes
Redo Buffers                6119424 bytes
Database mounted.
Database opened.
SQL> drop user roidba;
--删除测试用户

User dropped.

SQL> create user roidba identified by roidba;
--创建测试用户

User created.

SQL> grant connect,resource to roidba;
--授权访问

Grant succeeded.

SQL> conn roidba/roidba;
ERROR:
ORA-01035: ORACLE only available to users with RESTRICTED SESSION privilege
--登陆报错，缺少系统权限restricted session 

Warning: You are no longer connected to ORACLE.
SQL> conn / as sysdba
Connected.
SQL> grant restricted session to roidba;

Grant succeeded.
--授予系统权限restricted session 

SQL> conn roidba/roidba
Connected.
--使用roidba连接数据库，成功

SQL> conn / as sysdba
Connected.
SQL> revoke restricted session from roidba;
--收回权限

Revoke succeeded.

SQL> alter system disable restricted session;
--取消限制

System altered.

SQL> conn roidba/roidba;
--收回权限和取消限制抵消，可以继续登陆
Connected.
SQL> 

这些都是OCP的小知识点，大家有时间多做测试。一定要多动手，多看文档，多思考。

另外有需要云服务器可以了解下创新互联cdcxhl.cn，海内外云服务器15元起步，三天无理由+7*72小时售后在线，公司持有idc许可证，提供“云服务器、裸金属服务器、高防服务器、香港服务器、美国服务器、虚拟主机、免备案服务器”等云主机租用服务以及企业上云的综合解决方案，具有“安全稳定、简单易用、服务可用性高、性价比高”等特点与优势，专为企业上云打造定制，能够满足用户丰富、多元化的应用场景需求。

创新互联公司服务紧随时代发展步伐，进行技术革新和技术进步，经过十多年的发展和积累，已经汇集了一批资深网站策划师、设计师、专业的网站实施团队以及高素质售后服务人员，并且完全形成了一套成熟的业务流程，能够完全依照客户要求对网站进行成都网站设计、做网站、建设、维护、更新和改版，实现客户网站对外宣传展示的首要目的，并为客户企业品牌互联网化提供全面的解决方案。
网页名称：Oraclestartupinrestrictedmode-创新互联
网站网址：http://pwwzsj.com/article/iphso.html