Distributed Configuration. T.C Software + T.C. Hardware = One OS in a multiprocessor system (DOS) Communication : shared memory T.C. Software + L.C. Hardware = Homogeneous OS in network (DOS) Communication : message passing
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T.C. software + L.C. hardware ->T.C software + T.C harwarewhy?communication by shared data(control mechanisms: semaphores , monitors)is easiercommunication by message(control mechanisms : algorithms:reliable group communication)
-The application is modelled as a set of services that are provided by servers and a set of clients that use these services.
-Clients know of servers but servers need not know of clients.
-Clients and servers are logical processes
Thin:Used when legacy systems are migrated to client server architectures. The legacy system acts as a server with a graphical interface implemented on a client.
Fat:suitable for new C/S systems where the capabilities of the client system are known in advance.
More complex than a thin client model.
Fat + Thin: A 3-tier C/S architecture
No distinction between clients and servers.
Each distributable entity is an object that provides services to other objects and receives services from other objects.
Object communication: a middleware:ORB.
More complex to design than C/S systems.
allows to delay design decisions: where and how services should be provided.
a very open system architecture: new resources to be added to it.
The system is flexible and scaleable.
reconfigure the system dynamically with objects migrating across the network as required.