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# Send out pieces of each reply until they are all sent. elif event & select.POLLOUT: response = responses.pop(sock) n = sock.send(response) if n < len(response): responses[sock] = response[n:] else: poll.modify(sock, select.POLLIN) requests[sock] = ''
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The main loop in this program is controlled by the poll object, which is queried at the top of every iteration. The poll() call is a blocking call, just like the recv() call in our simple server; so the difference is not that our first server used a blocking operating system call and that this second server is somehow avoiding that. No, this server blocks too; the difference is that recv() has to wait on one single client, while poll() can wait on dozens or hundreds of clients, and return when any of them shows activity. You can see that everywhere that the original server had exactly one of something one client socket, one question string, or one answer ready to send this event-driven server has to keep entire arrays or dictionaries, because it is like a poker dealer who has to keep cards flying to all of the players at once. The way poll() works is that we tell it which sockets we need to monitor, and whether each socket interests us because we want to read from it or write to it. When one or more of the sockets are ready, poll() returns and provides a list of the sockets that we can now use. To keep things straight when reading the code, think about the lifespan of one particular client and trace what happens to its socket and data. 1. The client will first do a connect(), and the server s poll() call will return and declare that there is data ready on the main listening socket. That can mean only one thing, since as we learned in 3 actual data never appears on a stream socket that is being used to listen(): it means that a new client has connected. So we accept() the connection and tell our poll object that we want to be notified when data becomes available for reading from the new socket. To make sure that the recv() and send() methods on the socket never block and freeze our event loop, we call the setblocking() socket method with the value False (which means blocking is not allowed ). When data becomes available, the incoming string is appended to whatever is already in the requests dictionary under the entry for that socket. (Yes, sockets can safely be used as dictionary keys in Python!) We keep accepting more data until we see a question mark, at which point the Launcelot question is complete. The questions are so short that, in practice, they probably all arrive in the very first recv() from each socket; but just to be safe, we have to be prepared to make several recv() calls until the whole question has arrived. We then look up the appropriate answer, store it in the responses dictionary under the entry for this client socket, and tell the poll object that we no longer want to listen for more data from this client but instead want to be told when its socket can start accepting outgoing data. Once a socket is ready for writing, we send as much of the answer as will fit into one send() call on the client socket. This, by the way, is a big reason send() returns a length: because if you use it in non-blocking mode, then it might be able to send only some of your bytes without making you wait for a buffer to drain back down.
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