ADO enables your application to directly gain access to and modify data sources (sometimes called a two-tier system). For example, if your connection is to the data source that contains your data, then that is a direct connection in a two-tier system.
However, you may want to access data sources indirectly through an intermediary such as Microsoft Internet Information Services (IIS). This arrangement is sometimes called a three-tier system. IIS is a client/server system that provides an efficient way for a local, or client, application to invoke a remote, or server, program across the Internet or an intranet. The server program gains access to the data source and optionally processes the acquired data.
For example, your intranet Web page contains an application written in Microsoft Visual Basic, Scripting Edition (VBScript), which connects to IIS. IIS in turn connects to the actual data source, retrieves the data, processes it in some way, and then returns the processed information to your application.
In this example, your application never directly connected to the data source; IIS did. And IIS accessed the data by means of ADO.
Note The client/server application doesn't have to be based on the Internet or an intranet (that is, Web-based)—it could consist solely of compiled programs on a local area network. However, the typical case is a Web-based application.
Because some visual control, such as a grid, check box, or list, may use the returned information, the returned information must be easily used by a visual control.
You want a simple and efficient application-programming interface that supports three-tier systems, and returns information as easily as if it had been retrieved on a two-tier system. Remote Data Service (RDS) is this interface.
RDS defines a programming model—the sequence of activities necessary to gain access to and update a data source—to gain access to data through an intermediary, such as Internet Information Services (IIS). The programming model summarizes the entire functionality of RDS.