DEPARTMENT OF COMPUTER SCIENCE
COMP 414:DISTRIBUTED ASSIGNMENT 1, DUE DATE 31/01/2014
(a) With examples describe Access, Location and Migration transparency in a distributed system. (4mks) Access Transparency: Clients should be unaware of the distribution of the files. The files could be present on a totally different set of servers which are physically distant apart and a single set of operations should be provided to access these remote as well as the local files. Applications written for the local file should be able to be executed even for the remote files. The examples illustrating this property are the File system in Network File System (NFS), SQL queries, and Navigation of the web.
Location Transparency: Clients should see a uniform file name space. Files or groups of files may be relocated without changing their pathnames. A location transparent name contains no information about the named object’s physical location. This property is important to support the movement of the resources and the availability of services. The location and access transparencies together are sometimes referred as Network transparency. The examples are File system in NFS and the pages of the web.
Migration Transparency: This transparency allows the user to be unaware of the movement of information or processes within a system without affecting the operations of the users and the applications that are running. This mechanism allows for the load balancing of any particular client, which might be overloaded. The systems that implement this transparency are NFS and Web pages.
(b) With examples describe what Middleware is.(2 mks)
Middleware is systems software that resides between the applications and the underlying operating systems, network protocol stacks, and hardware. Its primary role is to; 1. Functionally bridge the gap between application programs and the lower-level hardware and software infrastructure in order to coordinate how parts of applications are connected and how they interoperate and 2. Enable and simplify the integration of components developed by multiple technology suppliers.
The diagram below shows an example of a middleware system
(c) With examples describe the client server model? (2 mks)
is a distributed application structure that partitions tasks or workloads between the providers of a resource or service, called servers, and service requesters, called clients. Often clients and servers communicate over a computer network on separate hardware, but both client and server may reside in the same system. Examples of computer applications that use the client–server model are Email, network printing, and the World Wide Web.
(d)What is redundancy and why is it used in a distributed system.(2 mks) Redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the form of a backup or fail-safe. Redundancy is used as a method for achieving fault tolerance in distributed systems. It guards the primary system from failure by acting as a back up system. Redundant components can include both hardware elements of a system -- such as disk drives, peripherals, servers, switches, routers -- and software elements -- such as operating systems, applications and databases.
(e) What are Client and Server Stubs and how are they used in remoteprocedure calls?
(2 mks) Remote Procedure Call achieves its transparency in an analogous way. When read is actually a remote procedure e.g., one that will run on the file server's machine, a different version of read, called a client stub, is put into the library. Like the original one, it, too, is called using the calling sequence of. Also like the original one, it too, does a call to the local operating system. Only unlike the original one, it does not ask the operating system to give it data. Instead, it packs the parameters...
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