Information Technology Reference
In-Depth Information
perspective, editing the remote file would have the same appearance
in a window as editing a local file. Behind the scenes, however,
Ella's machine must make a constant series of requests to the server
so that the file can be viewed and updated. Some requests will leave
the file unchanged, and others will modify the file. It is important to
keep track of what work is done by the client computer and what is
done by the server. One way to accomplish this is to make edits and
additions directly onto the file server's copy of the file itself.
If you use a word processor (such as Microsoft Word, Open
Office, or ThinkFree Office) on the file server itself, the application
would tell the server about every keystroke the user makes, and
send updates to the user's machine whenever something visible
changes. Consequently, using a word processor on the file server
generates a great deal of traffic. Thus, although this approach oc
curs in some applications, most applications provide for some pro
cessing on the user's machine. For example, part of the computer
science paper may be sent from the file server to Ella's computer, the
client Ella may edit that section, and updates would be sent periodi
cally back to the file server. With this piecebypiece communica
tion, one can think of the local machine as providing another level
of cache for the file server, just as cache memory holds frequently
used data items or program parts, and just as main memory holds
recently used segments of virtual memory. During editing, the word
processing software (such as Word or Open Office) runs on the
client
machine, and asks the file server for at least enough of the file
to fill the editing window on the screen.
With remove file access, reading and displaying a file is reason
ably straightforward. The client computer requests a part of a file,
and that material is displayed on the screen. When the user moves
to another part of the file, the client computer requests the informa
tion in the next section of the file. If the user has only read or dis
played the old material—not changed it—there is no need to copy
material back to the file server, because the server already has the
original version of that material. Thus, when requesting new mater
ial, the client computer can simply throw away any previous part of
the file that might still be stored in the client computer.
The process of modifying a file, however, is somewhat more
complicated, because any changes in one portion of the file must be
reported back to the file server. After all, the file server maintains
the authoritative copy of the file, so the server's copy of the file must
