Persistence, page 2

Imagine a program such as a word processor that was unable to save information from one session to the next. With such a program, you would have to type in a document from scratch every time you needed a printout - roughly akin to the way offices used typewriters just a few decades ago. Much of the value of computers comes from the ability of applications to store information - to persist their data. There are many technologies available for persisting data, and not all of them are appropriate for every situation.

There are three major reasons for persisting data in an application and one or all may be appropriate for any given program. They are as follows:

1. Application configuration information:
   Many applications save state information from one session to the next. It may remember user settings such as whether a backup should be generated for each file, or the positions of windows so that the application will restore itself to the same state that it was in when it was last closed. The application might save general configuration information as well as user specific configuration settings (for applications that support multiple users).
2. Documents:
   Many applications work on documents which are opened at the beginning of a session and saved or closed when the session is over. Typical documents are word processing documents, spreadsheets, pictures and so forth. Documents can often contain many types of objects. The key thing to remember about documents is that they generally correspond to a single disk file. This makes a document easy to manage as an individual unit. For example: it can be copied to a diskette, or sent via Email.
3. Records:
   Applications that need to keep track of multiple records which are similar to each other will typically use some sort of database storage mechanism. Databases support searching and filtering and are designed to be easily shared by multiple applications simultaneously.

Each of these reasons for storing data influences the choice of technology that can best solve the particular storage problem. But the choice of technology is not always obvious. Let's take a look at the major storage technologies in use under Windows and how appropriate each is for the problems listed above.

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