Excerpt from Optimization of Distributed Database Systems and Computer Networks

In previous works, most of the research focused on the problem of minimizing the operating cost of a distributed database. A great deal of attention has been devoted to the problem of Optimal distribution of the files over a network of computer systems.

One of the earliest studies of the file allocation problem was' done by Chu He developed a linear-programming model allocating files so that the allocation yields minimum overall operating costs subject to the following constraints: (i) the expected time to access each file is less than a given bound, (ii) the amount of storage needed at each computer does not exceed the available storage capacity. His model includes storage costs, queuing delays, and communication costs. But he assumed that the number of copies of each -file in the system is known. In a later paper, Chu(10) developed a procedure to determine in advance how many redundant copies of a file are required to achieve a desired level of reliability. Then, he inserted this number into the model, and the basic scheme remains unchanged.

Whitney(24) also formulated a similar model. He applied it to the design of a network topology and to the allocation of file Copies. A communication network optimization procedure is developed. He showed that, for certain communication cost functions, the tree top -ology is less expensive than any non-tree topology. In addition, he showed that the system delay is minimized when there are as few independent channels as possible.

Casey (4) developed a procedure for finding a minimal cost solution. Heuristic methods are used in this paper to find good solutions. The main difference between his paper and Chu's paper (10)is that the number of copies of files and their locations are treated as variables. He showed that the proportions of update traffic to query traffic generated by the users of a given file in the network could be used to determine an upper bound on the number of copies of the file present in the least cost network. He applied his algorithm to real data for the arpa network and has thus shown the process feasible for networks of moderate size. He indicated that when update -traffic-equals query traffic, it is efficient to store all files at a central node.

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