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A next reason for building a distributed system is that some applications are inherently distributed. A supermarket chain might have many stores, each of which gets goods delivered locally (possibly from local farms), makes local sales, and makes local decisions about which vegetables are so old or rotten that they must be thrown out. It therefore makes sense to keep track of inventory at each store on a local computer rather than centrally at corporate headquarters. After all, most queries and updates will be done locally. Nevertheless, from time to time, top management may want to find out how many rutabagas it currently owns. One way to accomplish this goal is to make the complete system look like a single computer to the application programs, but implement decentrally, with one computer per store as we have described. This would then be a commercial distributed system.. To understand how RPC works, it is important first to fully understand how a conventional (i.e., single machine) procedure call works. Consider a call like. It is easy to see that the algorithm guarantees mutual exclusion: the coordinator only lets one process at a time into each critical region. It is also fair, since requests are granted in the order in which they are received. No process ever waits forever (no starvation). The scheme is easy to implement, too, and requires only three messages per use of a critical region (request, grant, release). It can also be used for more general resource allocation rather than just managing critical regions.. The algorithm also runs into trouble if a process crashes, but recovery is easier than in the other cases. If we require a process receiving the token to acknowledge receipt, a dead process will be detected when its neighbor tries to give it the token and fails. At that point the dead process can be removed from the group, and the token holder can throw the token over the head of the dead process to the next member down the line, or the one after that, if necessary. Of course, doing so requires that everyone maintains the current ring configuration.. However, mean response time is not everything. There are also arguments in favor of distributed computing, such as cost. If a single 1000-MIPS CPU is much more expensive than 100 10-MIPS CPUs, the price/performance ratio of the latter may be much better. It may not even be possible to build such a large machine at any price. Reliability and fault tolerance are also factors.. Algorithm 1 picks a machine at random and just sends the new process there. If the receiving machine itself is overloaded, it, too, picks a random machine and sends the process off. This process is repeated until either somebody is willing to take it, or a hop counter is exceeded, in which case no more forwarding is permitted.. 4.5.7. Agreement in Faulty Systems. A third preemptive dynamic algorithm first computes for each task the amount of time it has to spare, called thelaxity(slack). For a task that must finish in 200 msec but has another 150 msec to run, the laxity is 50 msec. This algorithm, calledleast laxity,chooses the task with the least laxity, that is, the one with the least breathing room.. An interesting variation on voting isvoting with ghosts(Van Renesse and Tanenbaum, 1988). In most applications, reads are much more common than writes, soNris typically a small number andNwis nearlyN.This choice means that if a few servers are down, it may be impossible to obtain a write quorum.. An optimization of this ownership location protocol is shown in Fig. 6-28(b). Here the page manager forwards the request directly to the owner, which then replies directly back toP, saving one message.. Amoeba has an extremely simple memory model. A process can have any number of segments it wants to have, and they can be located wherever it wants in the process’ virtual address space. Segments are not swapped or paged, so a process must be entirely memory resident to run. Furthermore, although the hardware MMU is used, each segment is stored contiguously in memory.. 1. get_request — indicates a server’s willingness to listen on a port.. In addition to the calls listed in Fig. 8-8, a few other calls also exist. These calls are concerned primarily with getting and setting attributes, protection modes, and various kinds of statistical information.. 10.3.3. Binding a Client to a Server. 16. What is the function of the CDS clerk? Would it have been possible to have designed DCE without the CDS clerks? What would the consequences have been?.