Information Technology Reference
In-Depth Information
(or even Karen's mother) could download the draft of the group pa
per from this FTP site either from oncampus or from offcampus,
supplying only an email address as a password. Note that anony
mous FTP does not allow general access to a file server, and you
may have to make a conscious effort to move files to the special FTP
directories. Other files you might have, or even updates to your
files, will not be available to others unless you copy them to the spe
cial FTP area as well.
Sharing with Remote File Access: Turning to remote file access,
if you have authorized others to be able to read your original file,
they may be able to view it directly with an editor, such as Word or
OpenOffice, directly on the network. There are, however, a few lo
gistical problems with having many authorized people able to read
a file. A remote file service can send many copies of a file to various
folks, and all can view the same material without interference.
Complications arise, however, if the file changes. For example,
suppose many people can read the file, and you are the only one
who makes a change. Each viewer of the file has a separate cache
copy on their local machine. When you make a change, their copy
is no longer up to date, creating what is called a
cache coherence
problem; all copies of the file are no longer identical. Resolution of
this problem may take one of two basic approaches. The first ap
proach, called a
client-initiated update
, is reasonably passive. The
file server simply waits until a user asks for an updated version of
the file, supplying new copies only on demand. Thus, when you al
tered the file, all cache copies for the various people around the net
work became out of date, and they stay out of date until those folks
ask for revised versions. Of course, if those people know you are
editing, they may ask for updated versions frequently so they can re
fer to the newest information. On the other hand, if they do not
know you are updating, or if they simply do not want to be both
ered, then the materials in front of them could be outdated for an
extended length of time.
A second approach to resolving a cachecoherence problem,
called a
server-initiated update
, requires a file server to keep track
of all clients currently viewing a file. Whenever you change a file,
the server notifies all clients of the update, so that the cache at all
client computers can be revised. As you might expect, this approach
requires some administrative overhead for the server to keep track
of current clients and can generate network traffic—particularly if
