That??™s because the mainframe doesn??™t have to be changed, and the software in the PCs
isn??™t constrained by limitations of the mainframe. Of course, the PC software usually needs to
adapt to an asynchronous programming model, but that??™s usually worth the effort, especially
because modern windowing systems are all message-based anyway.
MOM servers and libraries usually provide many services based around the message
passing. Many of these can be important, and a real MOM is not usable for a real application
if it doesn??™t feature most of these. Persistent messages are central. This means that one application
can register interest in a topic, and the MOM then saves away all messages for this
application, even if that application isn??™t online at that moment. This is close to how e-mail
works, so it shouldn??™t come as a surprise. A MOM can also guarantee that a message will never
get lost. If it cannot be delivered, there are usually many ways of describing how the sender or
the MOM should handle this. Further, many MOM systems include advanced support for rollback
of different degrees, which means that you can sometimes integrate your MOM system
with databases using two-phase commit, and get transactional safety all over the system.
Of course, that??™s a major feature that most applications don??™t often need.
MOM services don??™t necessarily need to be located in one server. In fact, they??™re at their
most useful when you have hundreds of MOM servers, all routing messages among each other
to different applications and services that need them.
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