    #!/usr/bin/env python

    """Fault tolerance system for WebKit.

    Contributed to Webware for Python by Jay Love.

    This module is intended to provide additional assurance that the
    AppServer continues running at all times. This module will be
    reponsible for starting the AppServer, and monitoring its health.
    It does that by periodically sending a status check message to the
    AppServer to ensure that it is responding. If it finds that the
    AppServer does not respond within a specified time, it will start a
    new copy of the AppServer, after killing the previous process.

    Use::

      $ python Monitor.py start
      $ python Monitor.py stop

    The default AppServer specified below will be used, or you can list
    the AppServer you would like after ``start``.

    You can have the whole process run as a daemon by specifying ``daemon``
    after ``start`` on the command line.

    To stop the processes, run ``Monitor.py stop``.

    """

    # Future:
    # Add ability to limit number of requests served. When some count is reached,
    # send a message to the server to save its sessions, then exit. Then start
    # a new AppServer that will pick up those sessions.

    # It should be possible on both Unix and Windows to monitor the AppServer
    # process in 2 ways:
    # 1) The method used here, ie can it service requests?
    # 2) is the process still running?

    # Combining these with a timer lends itself to load balancing of some kind.

    defaultServer = "ThreadedAppServer"
    monitorInterval = 10 # add to config if this implementation is adopted
    maxStartTime = 120

    """Module global:

    `defaultServer`:
        default ``"ThreadedAppServer"``. The type of AppServer
        to start up (as listed in ``Launch.py``)
    `monitorInterval`:
        default 10. Seconds between checks.
    `maxStartTime`:
        default 120. Seconds to wait for AppServer to start
        before killing it and trying again.

    """

    import os, sys, time, socket, signal
    from marshal import dumps

    # Initialize some more global variables

    serverName = defaultServer
    srvpid = 0
    addr = None
    running = False

    debug = True

    statstr = dumps({'format': 'STATUS'})
    statstr = dumps(len(statstr)) + statstr
    quitstr = dumps({'format': 'QUIT'})
    quitstr = dumps(len(quitstr)) + quitstr

    ## Start ##

    def createServer(setupPath=0):
        """Unix only, executed after forking for daemonization."""
        print "Starting Server..."

        import WebKit
        code = 'from WebKit.%s import main' % serverName
        exec code
        main(['start'])

    def startupCheck():
        """Make sure the AppServer starts up correctly."""
        count = 0
        print "Waiting for start..."
        time.sleep(monitorInterval/2) # give the server a chance to start
        while 1:
            if checkServer(False):
                break
            count += monitorInterval
            if count > maxStartTime:
                print "Couldn't start AppServer."
                print "Killing AppServer..."
                os.kill(srvpid, signal.SIGKILL)
                sys.exit(1)
            print "Waiting for start..."
            time.sleep(monitorInterval)

    def startServer(killcurrent=True):
        """Start the AppServer.

        If `killcurrent` is true or not provided, kill the current AppServer.

        """
        global srvpid
        if os.name == 'posix':
            if killcurrent:
                try:
                    os.kill(srvpid, signal.SIGTERM)
                except Exception:
                    pass
                try:
                    os.waitpid(srvpid, 0)
                except Exception:
                    pass
            srvpid = os.fork()
            if srvpid == 0:
                createServer(not killcurrent)
                sys.exit()

    def checkServer(restart=True):
        """Send a check request to the AppServer.

        If restart is 1, then attempt to restart the server
        if we can't connect to it.

        This function could also be used to see how busy an AppServer
        is by measuring the delay in getting a response when using the
        standard port.

        """
        try:
            sts = time.time()
            s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
            s.connect(addr)
            s.send(statstr)
            s.shutdown(1)
            resp = s.recv(9) # up to 1 billion requests!
            monwait = time.time() - sts
            if debug:
                print "Processed %s Requests." % resp
                print "Delay %s." % monwait
            return True
        except Exception:
            print "No Response from AppServer."
            if running and restart:
                startServer()
                startupCheck()
            else:
                return False

    def main(args):
        """The main loop.

        Starts the server with `startServer(False)`,
        checks it's started up (`startupCheck`), and does a
        loop checking the server (`checkServer`).

        """
        global running
        running = True

        file = open("monitor.pid", "w")
        if os.name == 'posix':
            file.write(str(os.getpid()))
        file.flush()
        file.close()
        startServer(False)
        try:
            startupCheck()

        except Exception, e:
            if debug:
                print "Startup check exception:", e
                print "Exiting monitor..."
            try:
                os.kill(srvpid, signal.SIGTERM)
            except Exception:
                pass
            sys.exit()

        while running:
            try:
                if debug:
                    print "Checking server..."
                checkServer()
                time.sleep(monitorInterval)
            except Exception, e:
                if debug:
                    print "Exception:", e
                if not running:
                    return
                print "Exiting Monitor..."
                try:
                    os.kill(srvpid, signal.SIGTERM)
                except Exception:
                    sys.exit(0)
                try:
                    os.waitpid(srvpid, 0) # prevent zombies
                except Exception:
                    sys.exit(0)

    def shutDown(signum, frame):
        """Shutdown handler.

        For when Ctrl-C has been hit, or this process is being cleanly killed.

        """
        global running
        print "Monitor Shutdown Called."
        sys.stdout.flush()
        running = False
        try:
            s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
            s.connect(addr)
            s.send(quitstr)
            s.shutdown(1)
            resp = s.recv(10)
            s.close()
            print "AppServer response to shutdown request:", resp
        except Exception, e:
            print e
            print "No Response to shutdown request, performing hard kill."
            os.kill(srvpid, signal.SIGINT)
            os.waitpid(srvpid, 0)
        sys.stdout.flush()
        sys.stderr.flush()
        return False

    signal.signal(signal.SIGINT, shutDown)
    signal.signal(signal.SIGTERM, shutDown)

    ## Stop ##

    def stop():
        """Stop the monitor.

        This kills the other monitor process that has been opened
        (from the PID file ``monitor.pid``).

        """
        pid = int(open("monitor.pid", "r").read())
        # this goes to the other running instance of this module
        os.kill(pid, signal.SIGINT)

    ## Command line interface ##

    def usage():
        print """
    This module serves as a watcher for the AppServer process.
    The required command line argument is one of:

      start: Starts the monitor and default appserver.

      stop: Stops the currently running Monitor process and the AppServer
            if is running. This is the only way to stop the process other
            than hunting down the individual process ID's and killing them.

    Optional arguments:

    "AppServer": The AppServer class to use (currently only ThreadedAppServer)
    daemon:      If "daemon" is specified, the Monitor will run
                 as a background process.

    """

    arguments = ["start", "stop"]
    servernames = ["ThreadedAppServer"]
    optionalargs = ["daemon"]

    if __name__ == '__main__':

        if os.name != 'posix':
            print "This service can only be run on Posix machines (UNIX)."
            sys.exit()

        if len(sys.argv) == 1:
            usage()
            sys.exit()

        args = sys.argv[1:]
        if args[0] not in arguments:
            usage()
            sys.exit()

        if True: # setup path:
            if '' not in sys.path:
                sys.path = [''] + sys.path
            try:
                import WebwarePathLocation
                wwdir = os.path.abspath(os.path.join(os.path.dirname(
                    WebwarePathLocation.__file__), '..'))
            except Exception, e:
                print e
                usage()
            if not wwdir in sys.path:
                sys.path.insert(0, wwdir)
            sys.path.remove('')
            try:
                sys.path.remove('.')
            except Exception:
                pass

        cfgfile = open(os.path.join(wwdir, 'WebKit',
            'Configs/AppServer.config'), 'rU').read()
        cfg = dict(WebwarePath=wwdir)
        if cfgfile.lstrip().startswith('{'):
            cfg = eval(cfgfile, cfg)
        else:
            exec cfgfile in cfg
        if not cfg.get('EnableMonitor'):
            print "Monitoring has not been enabled in AppServer.config!"
            sys.exit()
        host = cfg.get('Host', '127.0.0.1')
        port = cfg.get('MonitorPort', 8085)
        addr = (host, port)

        if 'stop' in args:
            stop()
            sys.exit()

        for i in servernames:
            if i in args:
                serverName = i

        if 'daemon' in args: # fork and become a daemon
            daemon = os.fork()
            if daemon:
                sys.exit()

        main(args)