Seminar 5

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Seminar 5
2013-04-29 13:00:51

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  1. Proxies can support replication transparency by invoking each replica. Can(the server side of) an application be subject to a replicated calls?
    Yes: consider a replicated object A invoking another (nonreplicated) objectB. If Aconsists of k replicas, an invocation of B will be done by each replica.However, B should normally be invoked only once. Special measures areneeded to handle such replicated invocations.
  2. Constructing a concurrent server by spawning a process has some advantagesand disadvantages compared to multithreaded servers. Mention a few.
    An important advantage is that separate processes are protected againsteach other, which may prove to be necessary as in the case of a super-server handling completely independent services. On the other hand, pro-cess spawning is a relatively costly operation that can be saved when usingmultithreaded servers. Also, if processes do need to communicate, then us-ing threads is much cheaper as in many cases we can avoid having the kernelimplement the communication.
  3. Is a server that maintains a TCP/IP connection to a client stateful or state-less?
    Assuming the server maintains no other information on that client, one couldjusti?ably argue that the server is stateless. The issue is that not the server,but the transport layer at the server maintains state on the client. Whatthe local operating systems keep track of is, in principle, of no concern tothe server.
  4. Imagine a Web server that maintains a table in which client IP addresses aremapped to the most recently accessed Web pages. When a client connectsto the server, the server looks up the client in its table, and if found, returnsthe registered page. Is this server stateful or stateless?
    It can be strongly argued that this is a stateless server. The important issuewith stateless designs is not if any information is maintained by the server onits clients, but instead whether that information is needed for correctness.In this example, if the table is lost for what ever reason, the client and servercan still properly interact as if nothing happened. In a stateful design, suchan interaction