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Running the server under VNC (Unix)

A nice way to deploy the SWI-Prolog based servers is by using VNC. We refer to the Wikipedia page because there are various implementations of VNC around. We use TightVNC, but the others will probably work fine as well. VNC is a virtual desktop to which you can connect using a thin client called vncviewer. Starting the server is automatically done in two steps:

  1. initd starts a VNC server under an unprivileged user.
  2. the .vnc/xtartup file of this user creates terminals in which the service(s) run.

The nice part is that you can connect to the server using vncviewer, and access the Prolog toplevel to reload or debug the server. You can even run the graphical debugger, although tracing will be complicated on a heavily loaded server.

Setting up VNC on Debian/Ubuntu

Unfortunately, the setup requires root privileges and some Unix skills. Below, we outline the steps for Debian and Debian derived systems (e.g., Ubuntu). This can serve as a starting point for all Unix-based systems. We assume TightVNC, root privileges and these steps create a user wwwswi.

  1. Install VNC if not already done. On a Debian/Ubuntu system:
    % sudo apt-get install tightvncserver xtightvncviewer
  2. Make a new user. Either using the GUI in modern systems or using useradd. E.g.
    % sudo useradd -c "SWI-Prolog HTTP servers" -m -r -s /bin/bash wwwswi
  3. Setup VNC:
    % sudo -iu wwwswi bash
    $ unset XAUTHORITY
    $ vncserver
    You will require a password to access your desktops.
    Would you like to enter a view-only password (y/n)? n
    New 'X' desktop is ct:1

    The unset XAUTHORITY is not really needed, but avoids a long timeout trying to establish X11 authorization with the original user.

  4. Connect to the server from any system using the command below. The 1 is the display returned by New 'X' desktop is ct:1
    % vncviewer <host>:1

    alert.gif Verify security and access restrictions. VNC uses port 5900+display. Login verification is challenge-response based, but further communication is not encrypted. There are many ways to improve that, e.g., by using tunneling over SSH.

    alert.gif Since a long time, the default Gnome desktop in VNC has an annoying bug: typing a d closes all windows. Open "System/Preferences/Keyboard shortcuts" and disable or change the shortcut. I needed to stop and restart the server. Stopping can be done from the shell in (3) using vncserver -kill :1

  5. Install the server application in the new environment using the VNC desktop, for example installing ClioPatria.
    % git clone git://www.swi-prolog.org/home/pl/git/ClioPatria.git
    % mkdir test
    % cd test
    % ../ClioPatria/configure

    Optionally, you may wish to add a run script to restart the server in the case that it crashes. E.g.

    cd $HOME/test
    while true; do
        sleep 1
  6. Further configure the application (may also be done later)
  7. Add the application to the startup of the server. You do this by editing $HOME/.vnc/xstartup, adding a line
    gnome-terminal -t "Demo Server" -e $HOME/test/run

    You can now test the restart by going to the shell you created in (3) and running these commands to stop and start the server. You may need to adjust the display in the first command.

    % vncserver -kill :1
    % vncserver

    If everything works fine, the server nicely restarts and you can access it from your browser (ClioPatria runs on port 3020 by default, so the address is http://localhost:3020). You can also contact the desktop using vcnviewer to type commands in the Prolog window.

  8. This last step makes the server start at boot-time. For that you have to play with the system init scripts. Unfortunately, the details vary a lot between systems. We'll limit this description to the three steps that are necessary in most of these systems. In the script fragments we use two environment variables: $VNCUSER is the user and $VNC_DISPLAY is the desired (fixed) display number.
    1. Status: detect whether a server is running. We use a simple script called lsvnc for that. Here is the content:
      ps aux | \
              grep Xtightvnc | \
              grep ' :[0-9][0-9]* ' | \
              sed 's/\(^[a-z][a-z]*\).* \(:[0-9][0-9]*\) .*/\1 \2/g'

      Using this, we can verify that the server is running using this bash fragment:

      { /usr/local/bin/lsvnc | grep -qw $VNCUSER
    2. Start: Start the server. The first line removes stale VNC named pipes that sometimes survive on hard system crashes. The name varies a bit between VNC versions.
      rm -f /tmp/.X11-unix/X$VNC_DISPLAY
      su $VNCUSER -c "cd /home/$VNCUSER && bin/server :$VNC_DISPLAY"
    3. Stop: Stop the server.
      su $VNC_USER -c "vncserver -kill :$VNC_DISPLAY"
See also
- Example startup script used in Debian Squeeze
- ServerInetd.txt for using the inet superdaemon
- You probably want your server to appear on port 80, here is how.