Comments on the Client/Server Command Line Processor in Visual Studio .NET

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This solution includes a number of features as well as limitations that will be addressed in later chapters
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Multiple client processes can connect with the server and perform concurrent requests; each client has a dedicated server (or worker) thread allocated from the thread pool The server and clients can be run from separate command prompts or can be run under control of JobShell (Program 6-3) If all the named pipe instances are in use when a client attempts to connect, the new client will wait until a different client disconnects on receiving a $Quit command, making a pipe instance available for another client Several new clients may be attempting to connect concurrently and will race to open the available instance; any threads that lose this race will need to wait again Each server thread performs synchronous I/O, but some server threads can be processing requests while others are waiting for connections or client requests Extension to networked clients is straightforward, subject to the limitations of named pipes discussed earlier in this chapter Simply change the pipe names in the header file, or add a client command line parameter for the server name Each server worker thread creates a simple connection thread, which calls ConnectNamedPipe and terminates as soon as a client connects This allows a worker thread to wait, with a time-out, on the connection thread handle and test the global shutdown flag periodically If the worker threads blocked on ConnectNamedPipe, they could not test the flag and the server could not shut down For this reason, the server thread performs a CreateFile on the named pipe in order to force the connection thread to resume and shut down An alternative would be to use asynchronous I/O ( 14) so that an event could be associated with the ConnectNamedPipe call The comments in the book's Web site source file provide additional alternatives and information Without this solution, connection threads might never terminate by themselves, resulting in resource leaks in DLLs This subject is discussed in 12 There are a number of opportunities to enhance the system For example, there could be an option to execute an in-process server by using a DLL that implements some of the commands This enhancement is added in 12 The number of server threads is limited by the WaitForMultipleObjects call in the main thread While this limitation is easily overcome, the system here is not truly scalable; too many threads will impair performance, as we saw in 10 14 uses asynchronous I/O ports to address this issue
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Mailslots
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A Windows mailslot, like a named pipe, has a name that unrelated processes can use for communication Mailslots are a broadcast mechanism, based on datagrams (described in 12), and behave differently from named pipes, making them useful in some important but limited situations Here are the significant characteristics of mailslots
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A mailslot is one-directional A mailslot can have multiple writers and multiple readers, but frequently it will be one-to-many of one form or the other A writer (client) does not know for certain that all, some, or any readers (servers) actually received the message Mailslots can be located over a network domain Message lengths are limited
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Using a mailslot requires the following operations
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Each server creates a mailslot handle with CreateMailslot The server then waits to receive a mailslot message with a ReadFile call A write-only client should open the mailslot with CreateFile and write messages with WriteFile The open will fail (name not found) if there are no waiting readers
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A client's message can be read by all servers; all of them receive the same message There is one further possibility The client, in performing the CreateFile, can specify a name of this form: \\*\mailslot\mailslotname
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In this way, the * acts as a wildcard, and the client can locate every server in the name domain, a networked group of systems assigned a common name by the network administrator
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