Distributed computing system models.pdf

Definition is one that looks to its users like an

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Q: What is A Distributed Operating System (DOS)? Definition: “ Is one that looks to its users like an ordina ry centralized OS but runs on multiple , independent CPUs.” (Tanenbaum and Van Renesse [1985]) The OS commonly used for DCS are broadly classified into two namely “network OS” and “distributed OS”. Three features that differentiate them are: a. System image in a network OS, users view the DCS as a collection of distinct machines connected by a communication subsystem i.e. users are aware that multiple computers are being used, but DOS hides the existence of multiple computers and provides a single-system image to its users as a “virtual uniprocessor.” b. Autonomy for NOS, each computer has its own local OS and functions independent of others except when they must intercommunicate whereby they must use a mutually agreed on communication protocol. For a DOS, there is a single system-wide OS and each computer runs a part of this global OS. Processes and resources are managed globally and there is a single set of system calls (globally valid) available on all computers of the DCS. c. Fault tolerance capability this is usually high for a DOS than a NOS e.g. a 10% loss in NOS affects 10% users but in DOS only 10% loss in performance is experienced, Issues in Designing a DOS A DOS is more difficult to design than a centralized OS (COS) for several reasons e.g. in COS, it is assumed that the OS has access to complete and accurate information about the environment in which it is functioning but DOS must be designed with the assumption that complete information will never be available i.e. resources are physically separated, there are no common clocks among the multiple processors, delivery of messages is delayed and messages could even be lost. The following therefore are some of the key design issues: 1. Transparency: One main aim of DOS is to display the existence of multiple computers as invisible (transparent) and provide a single system image to users (virtual uniprocessor). The eight forms of transparency include:
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Distributed computing system models: 9 | P a g e B y P a u l K a t h a l e a. Access transparency users should not need or be able to recognize whether a resource, both h/w and s/w is remote or local i.e. the user interface should not distinguish between remote and local resources. b. Location transparency which has two aspects, name transparency , i.e. the name of a resource s/w and h/w should not reveal any hint as to the physical location of the resource i.e. name should be independent of the physical connectivity/topology or the current location of the resource , and must be unique system wide. User mobility , i.e. no matter which machine a user is logged onto, he/she should be able to access a resource with the same name.
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