现场报网公司数据库连不上，先检查了下数据库processes=1500,session=2200.我觉得很大啊，这个数据库没有几个人用。

查看v$session中的session最多是哪个machine发起的，发现是省公司的数据库发起的session，找开发梳理了下业务，省公司同步dblink操作网公司表，且是通过weblogic的连接池。

哦，有点明白了，是dblink引起的，weblogic连接池是一直存在的，所以在网公司端session是不释放的，如果省公司把应用都停掉，那在网公司端的session都会释放。要验证想法，做个试验：

目标：数据库A上建dblink,修改数据库B上的表。

环境准备：

1.在数据库A上建dblink

create public database link TO_B

connect to TEST_DB identified by  TEST_DB

using '(DESCRIPTION =(ADDRESS_LIST =(ADDRESS = (PROTOCOL = TCP)(HOST = 10.10.15.150)(PORT = 1521)))(CONNECT_DATA =(SID = orcl)))';

2.在数据库B上建表

create table TEST(  ID NUMBER );

insert into test values(1);

开始测试：

1.在数据库B上select count(1) from v$session;

2.在数据库A上update test@TO_B set id=1;

commit;

3.在数据库B上select count(1) from v$session; 可以看到涨了一个

对比测试：

1.在数据库B上select count(1) from v$session;

2.在数据库A上update test@TO_B set id=1;

commit;

alter session close database link TO_B;

3.在数据库B上select count(1) from v$session; 可以看到没有变化

总结： 出现这种问题，归根结底是操作dblink不规范，用了之后没有关闭。

附录为官方文档：

If you access a database link in a session, then the link remains open until you close the session. A link is open in the sense that a process is active on each of the remote databases accessed through the link. This situation has the following consequences:

• If 20 users open sessions and access the same public link in a local database, then 20 database link connections are open.

• If 20 users open sessions and each user accesses a private link, then 20 database link connections are open.

• If one user starts a session and accesses 20 different links, then 20 database link connections are open.

After you close a session, the links that were active in the session are automatically closed. You may have occasion to close the link manually. For example, close links when:

• The network connection established by a link is used infrequently in an application.

• The user session must be terminated.

To close a link, issue the following statement, where linkname refers to the name of the link:

ALTER SESSION CLOSE DATABASE LINK linkname;

Note that this statement only closes the links that are active in your current session. 

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