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Embedded Systems November 2000 Vol13_12

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BRAN SELIC Distributed Software Design: Challenges and Solutions In contrast to centralized systems, distributed software systems add a new layer of complexity to the already difficult problem of software design. In spite of that and for a variety of reasons, mol'e and more modern-day software systems are distributed. n some cases, such as telecommunications systems, distribution is inherent in the problem domain and cannot be avoided. In other cases, we rely on physical distribution to achieve properties that cannot be effectively realized with centralized systems. For instance, we may want to use multiple physically distributed processors to achieve a high degree of availability. Thus, if one processor experiences a hard fail- ure, another one can take over its functionali ty. Another reason for distribution is performance. By splitting the processing load across multiple processors, it is generally possible to achieve higher throughput and faster response than with just a single processor. We define a distributed software system (see Figure 1) as a system with two or more independent fJrocessing sites that communicate with each other over a medium whose transmission delays may exceed the time between successive state changes. Note that this definition is broad enough to encompass logically distributed systems. These are software systems that are concentrated on a single processing node but, for one reason or another, I exhibit the above properties. An example of such a system is one that spans multiple heavyweight processes running on the same processor and which interact by exchanging asynchronous messages. Since each process is an independent unit of fai lure, it fits our definition. (For this reason, our term distributed site should not be taken necessarily as a synonym for the term "pro- cessing node.") While logically distributed systems cannot achieve the full bene- fits of physically distributed systems, the problems that they present to develop- ers are practically the same. Embedded Systems Programming NOVEMBER 2ooo 127

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