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That is the theory. The practice is that to function at all, current distributed systems count on a number of specific servers being up. As a result, some of them have an availability more closely related to the Boolean and of the components than to the Boolean OR. In a widely-quoted remark, Leslie Lamport once defined a distributed system as “one on which I cannot get any work done because some machine I have never heard of has crashed.” While this remark was (presumably) made somewhat tongue-in-cheek, there is clearly room for improvement here.. Distributed systems consist of autonomous CPUs that work together to make the complete system look like a single computer. They have a number of potential selling points, including good price/performance ratios, the ability to match distributed applications well, potentially high reliability, and incremental growth as the workload grows. They also have some disadvantages, such as more complex software, potential communication bottlenecks, and weak security. Nevertheless, there is considerable interest worldwide in building and installing them.. 4. Connectionless data traffic.. In short, the possibility of server crashes radically changes the nature of RPC and clearly distinguishes single-processor systems from distributed systems. In the former case, a server crash also implies a client crash, so recovery is neither possible nor necessary. In the latter it is both possible and necessary to take some action.Client Crashes. Although in this book we will study only operating system (i.e., process) groups, it is worth mentioning that other groups are also commonly encountered in computer systems. For example, on the USENET computer network, there are hundreds of news groups, each about a specific subject. When a person sends a message to a particular news group, all members of the group receive it, even if there are tens of thousands of them. These higher-level groups usually have looser rules about who is a member, what the exact semantics of message delivery are, and so on, than do operating system groups. In most cases, this looseness is not a problem.. 21. When a computation runs for a long time, it is sometimes wise to make checkpoints periodically, that is, to save the state of the process on disk in case it crashes. In that way, the process can be restarted from the check point instead of from the beginning. Try to devise a way of checkpointing a computation that consists of multiple processes running in parallel.. Primary-backup fault tolerance has two major advantages over active replication. First, it is simpler during normal operation since messages go to just one server (the primary) and not to a whole group. The problems associated with ordering these messages also disappear. Second, in practice it requires fewer machines, because at any instant one primary and one backup is needed (although when a backup is put into service as a primary, a new backup is needed instantly). On the downside, it works poorly in the presence of Byzantine failures in which the primary erroneously claims to be working perfectly. Also, recovery from a primary failure can be complex and time consuming.. 3. Is the system synchronous or asynchronous?. However, the sender of the last message does not know if the last message arrived. If it did not, the protocol did not complete and the other general will not attack. Thus the sender of the last message cannot know if the war is scheduled or not, and hence cannot safely commit his troops. Since the receiver of the last message knows the sender cannot be sure, he will not risk certain death either, and there is no agreement. Even with nonfaulty processors (generals), agreement between even two processes is not possible in the face of unreliable communication.. MethodCommentsWrite throughWorks, but does not affect write trafficDelayed writeBetter performance but possibly ambiguous semanticsWrite on closeMatches session semanticsCentralized controlUNIX semantics, but not robust and scales poorly. EventAction taken by a cache in response to its own CPU’s operationAction taken by a cache in response to a remote CPU’s operationRead missFetch data from memory and store in cache(No action)Read hitFetch data from local cache(No action)Write missUpdate data in memory and store in cache(No action)Write hitUpdate memory and cacheInvalidate cache entry. The third job of the kernel is to handle interprocess communication. Two forms of communication are provided: point-to-point communication and group communication. These are closely integrated to make them similar.. 10.1.2. Goals of DCE. Thread scheduling is similar to process scheduling, except that it is visible to the application. The scheduling algorithm determines how long a thread may run, and which thread runs next. Just as with process scheduling, many thread scheduling algorithms are possible.. 15. Give the X.500 name for the sales department at Kodak in Rochester, NY..