Serving Sockets... The Salad

I haven't had the opportunity to do much with Dart lately, due to scheduling. However this afternoon I had a little chance to write some more. I decided that I wanted to modify the EchoServer to maintain the connection and close the connection only when receiving a specific command. I also wanted to add a command to shutdown the server itself completely. In order to properly accommodate this, I needed to refactor the server code a little. I wrapped the ServerSocket in a manager class:

#import('dart:io');

class ServerManager {
  ServerSocket _listenServer;
  
  ServerManager() {
    _listenServer = new ServerSocket("127.0.0.1", 5700, 0);
    
    _listenServer.connectionHandler = this._handleConn;
  }

  void _handleConn(Socket conn) {
    StringInputStream clientIn = new StringInputStream(conn.inputStream);
    
    clientIn.lineHandler = () {
      String input = clientIn.readLine();
      print("Received: $input");
      if(input.toLowerCase() == 'stop') {
        String cls = "** Stopping Server. Closing connection **\n";
        conn.writeList(cls.charCodes(), 0, cls.length);
        conn.close();
        _listenServer.close();
        print("** Stopping Server! **");
      } else if(input.toLowerCase() == 'exit') {
        String cls = "** Closing connection to client **\n";
        conn.writeList(cls.charCodes(), 0, cls.length);
        conn.close();
        print("** Closing connection to client. **");
      } else {
        String output = "${input.toUpperCase()}\n";
        conn.writeList(output.charCodes(), 0, output.length);
        print("Sent: $output");
      }
    };
    
  }
}

void main() {
  ServerManager sMan = new ServerManager();
 
}

So as you can see I made a few changes from my original EchoServer. As mentioned above I wrapped the server in a manager class, this enables me to easily close the server socket without using a global variable. In addition I added a couple clauses to check for the 'stop' or 'exit' commands which will stop the server or just close the client connection respectively. And finally I stopped pulling the output stream of the sockets directly, and instead use the writeList methods directly on the socket itself. I wasn't gaining any real benefit by creating an additional variable for the socket's OutputStream, so I just dropped it altogether.

Now as is, the above will run and accept connections and echo any new lines until the stop or exit commands are received. If the exit command is received, then the server will close the connection to that socket. If stop is received it will close the connection to that socket and then tell the server to close. However because of the event driven nature of the server, the Sockets and ServerSockets are not blocking. That is, even without adding any additional threads (Isolates), we can accept connections from multiple sources. If you open up multiple telnet connections to the host, you can see how you can send data and receive responses on each connection independent of the other.

But this also leaves us with a small issue. If we tell the server to stop from one telnet session while the other is still active.. the server will accept the stop command, and it will schedule the ServerSocket to be stopped, but not until the other socket has been closed. Try it out and you will see that the connection in which we issue the stop command is disconnected, and the console will indicate that the server is stopping. But the other telnet session will remain active until we issue an exit or stop command. Only once the 2nd session is closed will the server stop. And if for some reason the other session does not terminate properly (for instance connection drops or the telnet application is closed before issuing an exit/stop command) then the server will hang, not accepting new connections but not terminating either (assuming eventually the socket will time out but potentially not since I do not have those error handlers in place either).

This may be the desired situation with some servers to shut them down gracefully for instance, however in our EchoServer we want it to shut down immediately if it receives the stop command. So we'll need to keep a list of active connections and iterate through them and close each one, then stop the server. So I ended up with the following:

#import('dart:io');

class ServerManager {
  ServerSocket _listenServer;
  List _socketList;
  
  ServerManager() {
    _socketList = new List();
    _listenServer = new ServerSocket("127.0.0.1", 5700, 0);
    
    _listenServer.connectionHandler = this._handleConn;
  }
  
  void sendStops() {
    List cls = "** Server received stop request. Closing connection to client **\n".charCodes();
    
    while(!_socketList.isEmpty()) {
      Socket conn = _socketList.removeLast();
      conn.writeList(cls, 0, cls.length);
      conn.close();
    }
  }
  
  void _handleConn(Socket conn) {
    _socketList.add(conn);
    StringInputStream clientIn = new StringInputStream(conn.inputStream);
    
    clientIn.lineHandler = () {
      String input = clientIn.readLine();
      print("Received: $input");
      
      if(input.toLowerCase() == 'stop') {
        sendStops();
        _listenServer.close();
        print("** Stopping Server! **");
      } else if(input.toLowerCase() == 'exit') {
        String cls = "** Closing connection to client **\n";
        conn.writeList(cls.charCodes(), 0, cls.length);
        int sockInd = _socketList.indexOf(conn);
        _socketList.removeRange(sockInd, 1);
        conn.close();
        print("** Closing connection to client: $sockInd **");
      } else {
        String output = "${input.toUpperCase()}\n";
        conn.writeList(output.charCodes(), 0, output.length);
        print("Sent: $output");
      }
    };
    
  }
}

void main() {
  ServerManager sMan = new ServerManager();
 
}

As you can see I also added a method sendStops just to iterate through all the sockets, popping them out of list and sending them the stop notice and disconnecting them. I made this separate from the actual stopping of the server in case it should ever be required for any other reason as well. Initially I tried using a Set to hold just unique connections, and provide easier way of removing elements however I found out that there's an issue with Set's in that any values stored in a set must implement Hashable. This wasn't added in the API documentation and it was only after a little digging through the DartBug page and Newsgroup that I found this is 'expected' behaviour. As such, I had to use the list. For a specific 'exit' command I have to get the index of the value and remove it from the list with removeRange with a size of 1 element. I also setup the broadcast message directly to a List of Int's immediately just to avoid having to convert it multiple times as I iterate through the connections. While still missing any error handling, etc. I'm rather pleased with how the server is progressing and in some ways it conjures up images of the old school MUD's. Maybe a project to play with?