recho.txt
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Sockets again

 Tonight: the internals of internet services - sockets and select
  culminating in chat server and client
 Most services have clients and server - asymmetric
  server always available, handles many clients who come and go
  each client connects for short-lived session
 sockets - basic API for connecting to services
  Berkeley sockets ca 1980, Unix and C, now similar in most OS and languages
  universal glue for connecting diverse programs on same or different hosts
   good exercise: code client, server in different languages
 Server and Client socket work differently




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recho Server and Client

 Service identified by host (name or number) and socket number
 Server has single persistent listener socket, possibly many client sockets
 accept call returns a new client socket for each client session
 Each client session may have many send/receive 

 Server's listener socket: 
  socket, bind, listen, [  accept                      ]  close
                            |
                            | returns client socket
                            V
   server's client socket: [     receive, send, ... ] close
                            :      :       :
                            :      :       :
 Client:         socket, connect [send, receive  ] close
 
 [...] indicates loop, : indicates synchronization



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Socket behaviors

  Server accept blocks (waits) until a client connects
  Client connect fails (raises exception) if no server etc. etc.
  send, recv communicate via buffers and intermediate hosts

    send -->  [...] --> [...] --> [...] --> [...] --> recv

   send blocks until *some* room available in output buffer
    send might not send whole arg, returns n of bytes sent
   recv(bufsize) specifies maximum amount to receive
    recv blocks until *some* data available in input buffer
    sent data might not appear in recv buffer all at once,
    recv returns data actually read, may be less than bufsize
      http://docs.python.org/howto/sockets.html
      http://blogs.oracle.com/jag/resource/Fallacies.html
   recv returns 0 when partner executes close