In a multitier architecture environment, an application server provides data for clients
and serves as an interface between clients and database servers. This architecture is
particularly important because of the prevalence of Internet use.
This architecture enables use of an application server to:
■ Validate the credentials of a client, such as a Web browser
■ Connect to a database server
■ Perform the requested operation

多层体系结构概述
1. 在一个采用多层体系结构的系统中，客户端与数据库服务器间还存在着为两者交换数据的应用
服务器
2. 应用服务器的作用
* 验证客户端身份
* 连接数据库服务器
* 执行用户提交的请求