Implementation Model versus Mental Model

The implementation model of the file system runs contrary to the mental model almost all users bring to it. Most users picture electronic files like printed documents in the real world, and they imbue them with the behavioral characteristics of those real objects. In the simplest terms, users visualize two salient facts about all documents: First, there is only one document; and second, it belongs to them. The file system's implementation model violates both these rules: There are always two copies of the document, and they both belong to the program.

Every data file, every document, and every program, while in use by the computer, exists in two places at once: on disk and in main memory. The user, however, imagines his document as a book on a shelf. Let's say it is a journal. Occasionally, it comes down off the shelf to have something added to it. There is only one journal, and it either resides on the shelf or it resides in the user's hands. On the computer, the disk drive is the shelf, and main memory is the place where editing takes place, equivalent to the user's hands. But in the computer world, the journal doesn't come off the shelf. Instead a copy is made, and that copy is what resides in computer memory. As the user makes changes to the document, he is actually making changes to the copy in memory, while the original remains untouched on disk. When the user is done and closes the document, the program is faced with a decision: whether to replace the original on disk with the changed copy from memory, or to discard the altered copy. From the programmer's point of view, equally concerned with all possibilities, this choice could go either way. From the software's implementation model point of view, the choice is the same either way. However, from the user's point of view, there is no decision to be made at all. He made his changes, and now he is just putting the document away. If this were happening with a paper journal in the physical world, the user would have pulled it off the shelf, penciled in some additions, and then replaced it on the shelf. It's as if the shelf suddenly were to speak up, asking him if he really wants to keep those changes!