answers07 - COMP212 May 2007 exam: answers Question 1 A....

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Unformatted text preview: COMP212 May 2007 exam: answers Question 1 A. Give four examples of resources to be shared in a distributed system. 2 marks Possible examples are: hardware resources (disk space, printers, memory, CPU power, etc) and data/software resources (distributed objects, web- pages, databases, etc.). B. Explain what is meant by (distribution) transparency. 4 marks Distribution transparency is the phenomenon by which distribution as- pects in a system are hidden from users and applications. Examples in- clude access transparency, location transparency, migration transparency, relocation transparency, replication transparency, concurrency transparency, failure transparency, and persistence transparency. C. What is the role of middleware in a distributed system? 4 marks Middleware is an additional layer of software between applications and the network operating system, offering a higher level of abstraction. It aims at enhancing distributed transparency that is missing in network operating systems and to hide heterogeneity of the underlying platforms from applications. D. Name two middleware communication services. 2 marks RMI (or Java RMI), RPC, Message-oriented communication, streams E. In Java RMI, all methods of a remote object must be declared able to throw instances of a particular subclass of the Exception class. What is this subclass? 3 marks RemoteException F. Explain the role of the data level in a three-tired client/server architecture. 3 marks The data level in the client-server model contains the programs that main- tain the actual data on which the application operate. In its simplest form, the data level consists of a file system, but it is more common to use a fill-size database. In the client-server model, the data level is typically implemented at the server side. 1 G. Consider a network consisting of 5 computers, A (coordinator), B , C , D , and E . At 14 : 40 the coordinator decides to synchronise the clock of all computers in the network. At that moment, the clock of every computer in the network shows the following. Computer Clock A 14:40 B 14:37 C 14:35 D 14:28 E 14:25 Apply the Berkley clock synchronisation algorithm to this situation, show the stages of computation, and write what will be the outcome of the syn- chronisation. The time needed for computation and for network commu- nication is negligible. 5 marks At 14:40 the time daemon tells the other machines its time and asks for theirs. They respond with how far ahead or behind the time. The reply from processes is depicted in the following table: Computer Time drift A 00:00 B-00:03 C-00:05 D-00:12 E-00:15 The time daemon computes the average time (14:33) and tells each ma- chine how to adjust its clock, Computer Needed adjustment A-00:07 B-00:04 C-00:02 D +00:05 E +00:08 H. Communication in Jini is based on which of the following: a) message queueing, b) Java RMI, or c) remote procedure calls? 2 mark Java RMI 2 Question 2 A. Classify the following protocols: IP, TCP, HTTP according to the ISO...
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answers07 - COMP212 May 2007 exam: answers Question 1 A....

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