Information Technology Reference
In-Depth Information
you update the file often. Serverinitiated updates require effort,
processing time, and network communications, but they can keep
multiple copies of a file synchronized. Of course, the importance of
keeping copies up to date may depend greatly on the specific file
and circumstances. Serverinitiated updates are not implemented in
many applications, because the gain does not seem worth the effort
for just a few users on occasional files. However, some applications
may require that everyone have completely current information, and
serverinitiated update might be explored in these cases.
The most complex environment for the cachecoherence prob
lem arises when several users might want to access and change all or
part of a remote file. The problem, called the
concurrent update
problem
, is that one user might try to make a change in one way,
while a second user tries to do something contradictory. For exam
ple, suppose that both Steve and Ella are using remote access to
view and edit Karen's draft of their paper. If Steve and Ella both
have plans to edit the introduction, how will the network know
what changes to make to the original file? One way the network
could handle this situation is to take the
ostrich approach
and just
ignore the problem (just as ostriches are reported to stick their
heads in the sand). Is this irresponsible? Maybe, but it can work
fairly well if multiple users agree to take turns editing the file, or if
their work schedules dictate that only one will be changing the file
at a time. In this scenario, several people have the potential to mod
ify the same file, but only one actually tries to change the file during
any interval of time. To work, all parties would need to be in good
communication; for example, Steve would tell Ella when he's work
ing on the file and vice versa, so that two people are not working on
the paper at the same time. Effectively, there is no need for a com
plicated solution in software to a potential problem that has been
resolved in another way.
Alternatively, the network could enforce a policy that only one
person can change the file at a time. In this mode, any person ask
ing for access to a file must indicate whether the access will involve
reading only or possibly editing. Those interested in reading the file
are allowed access at any time. However, if someone asks to edit the
file, then the software checks whether someone else already has edit
ing privileges. If not, then the request can be granted, but if so, the
second person is denied access. We say the first person activates a
lock
on the file, and this lock prevents others from changing the
file's contents. Thus, if Ella were making changes on the paper,
