EchoServer

Now, let's look at a further feature of the POSIX environment. It also supports writing network programs, that communicate over the Internet using sockets. A socket can be thought of as a two-way communication channel, consisting of both an RFile and a WFile.

Two network programs, which execute on different machines, may connect over the network and communicate using a socket. Typically, one of the programs will be a server and the other a client. Here is a trivial server, the EchoServer. This server waits indefinitely for clients to connect on port 12345. When a client connects, the server starts a session, where client input rows are echoed back to the client, prefixed by a line number. Several clients may be served concurrently and line numbering is independent for each client.

module EchoServer where

import POSIX

port = Port 12345

root w = do
    env = new posix w
    env.inet.tcp.listen port (server env.stdout)


server logfile sock = class

   n := 1

   p = show sock.remoteHost

   log str = logfile.write ('[':str ++ "]\n")

   echo str = action
      sock.outFile.write (show n ++"> "++str)
      n := n+1

   close = request
      log (p ++ " closing")
      result ()

   neterror str = action log ("Neterror: "++str)

   established = action
      log ("Connected from " ++ p)
      sock.inFile.installR echo

   result Connection{..}

Comments to the code:

• The root procedure just installs a listening callback in the environment. Such a callback has type Socket -> Class Connection where
  • a Socket object has selectors for the resources from the environment needed by the serving object: the remoteHost and remotePort and the inFile and outFile parts of the connection.
  • a Connection implements the services provided by the server to the environment: an initial action to call when the connection is established, an action to call with an error message when a neterror occurs and a method to call when the remote client wants to close the connection.
• The echoing behaviour is achieved by installing, when connection is established, the echo method with the inFile of the socket.
• Additionally, the server logs its connections and errors, using the file interface that it gets as parameter (which leads to stdout of the environment).

In order to let you run the server, we provide a simple client, TCPClient. We do not list the code here; you may review it in TCPClient.t. The client accepts a host name and a port number as command line arguments and connects to a server on that address. When a connection is established, the client accepts user input lines, sends them to the server and displays replies obtained on the screen.