连接的双重作用：
1、通信信道的概念：指令和数据的传输；
2、运行的上下文；
针对每一个线程的连接生成了一个工作上下文。
每一个连接都有自己的上下文；
上下文作为一个数据结构，有自己的创建、实用、销毁的流程；
需要保证数据的一致性；
为数据库的操作创建了上下文空间；相对于线程对于函数提供栈空间的支持；
A Database connection is a facility in computer science that allows client software to talk to database server software, whether on the same machine or not. A connection is required to send commands and receive answers, usually in the form of a result set.
Connections are a key concept in data-centric programming. Since some DBMS engines require considerable time to connect connection pooling was invented to improve performance. No command can be performed against a database without an "open and available" connection to it.
https://en.wikipedia.org/wiki/Database_connection。