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Over the years, the main computing paradigm has oscillated between centralized and decentralized
computing. The first computers, such as the ENIAC, were, in fact, personal computers, albeit large
ones, because only one person could use one at once. Then came time-sharing systems, in which
many remote users at simple terminals shared a big central computer. Next came the PC era, in which
the user had their own personal computers again. While the decentralized PC model has advantages,
it also has some severe disadvantages that are only beginning to be taken seriously. Probably the
biggest problem is that each PC has a large hard disk and complex software that must be maintained.
For example, when a new release of the operating system comes out, a great deal of work has to be
done to perform the update on each machine separately. At most corporations, the labor costs of doing
this kind of software maintenace dwarf the actual hardware and software costs. For home users, the
labor is technically free, but few people are capable of doing it correctly and fewer still enjoy doing it.
With a centralized system, only one or a few machines have to be updated and those machines have
a staff of experts to do the work. A related issue is that users should make regular backups of their
gigabyte file systems, but few of them do. When disaster strikes, a great deal of moaning and wringing
of hands tends to follow. With a centralized system backups can be made every night by automated
tape robots.
Another advantage is the resource sharing is easier with centralized systems. A system with 256 remote
users, each with 256 MB of RAM will have most of that RAM idle most of the time. With a centralized
system with 64 GB of RAM, it never happens that some user temporarily needs a lot of RAM but can not
get it because it is on someone else‘s PC. The same argument holds for disk space and other resources.
Finally, we are starting to see a shift from PC-centric computing to Web-centric computing. One area
where this shift is very far along is e-mail.
It is probably a fair conclusion to say that most users want high-perfromance interactive computing,
but do not really wnat to administer a computer. This has led researchers to reexamine timesharing
using dumb terminals (now politely called thin clients) that meet modern terminal expections.
Modern Operating System --- Thin Clients
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原文地址：http://www.cnblogs.com/miaoyong/p/4886764.html
