A data source is simply the source of the data. It can be a file, a particular database on a DBMS, or even a live data feed. For example, a data source might be an Oracle DBMS running on an OS/2 operating system, accessed by Novell NetWare, an IBM DB2 DBMS accessed through a gateway; a collection of Xbase files in a server directory, or a local Microsoft Access database file.
The purpose of a data source is to gather all of the technical information needed to access the data (i.e. the driver name, network address, network software, and so on) into a single place and make it available to the user, without exposing him to the complexity of this information. The user should be able to look at a list that includes Payroll, Inventory, and Personnel, choose Payroll from the list, and have the application connect to the payroll data, all without knowing where the payroll data resides or how the application got to it.
The term data source should not be confused with similar terms. In this manual, DBMS or database refers to a database program or engine. A further distinction is made between desktop databases, designed to run on personal computers and often lacking in full SQL and transaction support, and server databases, designed to run in a client/server situation and characterized by a stand-alone database engine and rich SQL and transaction support. Database also refers to a particular collection of data, such as a collection of Xbase files in a directory or a database on SQL Server. It is generally equivalent to the term catalog, used elsewhere in this manual, or the term qualifier in earlier versions of ODBC.
There are two types of data sources: machine data sources and file data sources. Although both contain similar information about the source of the data, they differ in the way this information is stored. Because of these differences, they are used in somewhat different manners.
Machine data sources are stored on the system with a user-defined name. Associated with the data source name is all of the information the Driver Manager and driver need to connect to the data source. For an Xbase data source, this might be the name of the Xbase driver, the full path of the directory containing the Xbase files, and in addition, some options that tell the driver how to use those files, such as single-user mode or read-only. For an Oracle data source, this might be the name of the Oracle driver, the server where the Oracle DBMS resides, the SQL*Net connection string that identifies the SQL*Net driver to use, and the system ID of the database on the server.
File data sources are stored in a file and allow connection information to be used repeatedly by a single user or shared among several users. When a file data source is used, the Driver Manager makes the connection to the data source using the information in a .dsn file. This file can be manipulated like any other file. A file data source does not have a data source name, as does a machine data source, and is not registered to any one user or machine.
A file data source streamlines the connection process, because the .dsn file contains the connection string that would otherwise have to be built for a call to the SQLDriverConnect function. Another advantage of the .dsn file is that it can be copied to any machine. Therefore, identical data sources can be used by many machines as long as they have the appropriate driver installed. A file data source can also be shared by applications. A shareable file data source can be placed on a network and used simultaneously by multiple applications.
A .dsn file can also be unshareable. An unshareable .dsn file resides on a single machine and points to a machine data source. Unshareable file data sources exist mainly to allow the easy conversion of machine data sources to file data sources so that an application can be designed to work solely with file data sources. When the Driver Manager receives the information in an unshareable file data source, it connects as necessary to the machine data source to which the .dsn file points to.