[lug] X11 Protocol, migrating windows to remote displays

[Michael D. Hirsch]

On Sunday 18 May 2003 07:01 pm, D. Stimits wrote:
> Does anyone here know the X11 protocol well enough to say how easy it
> would be to write an app (or modify xterm/konsole/gnome-terminal) to
> allow migrating an X11 terminal to a remote display *after* the app has
> started? I have an integrated app that I want to run entirely on one
> machine, but I want the multiple windows it produces to be able to
> migrate to/from a remote display. If I set $DISPLAY ahead of time, the
> entire app goes to the remote display; even if I wanted that, it would
> not allow me to migrate the window back to the original machine at a
> later time. I'd like to be able to take an ID from a visible window or
> widget, and tell X11 to redisplay on some other machine.

I know enough to know it would be quite hard.  There are two parts to the 
difficulty.  The first part is the stateful nature of an X connection.  
The X server remembers a lot data and later X requests can refer to that 
data.  To migrate a window from one server to another means that you have 
to migrate all that data, too.  Another problem is the issues of different 
bit depths and fonts.  X uses a particularly primitive color system in 
which each application needs to be able to support each different color 
depth.  Once it has connected to one server at one color-depth, it will be 
unable to talk to another server at another color depth.  

I think the easiest way to do this would be to make a vnc server that could 
serve just one window.  That actually doesn't sound that impossible.  It 
wouldn't actually send the native X protocol to the remote machine, but 
the effect would be the same.

You might look at the CCF project I was involved in at Emory U 
<http://www.mathcs.emory.edu/ccf/>.  It's defunct now, but one tool we 
built was a virtual desktop that allows multiple users to share X 
applications.  If you are willing to restrict yourself to only running 
your programs inside a virtual desktop, then it might be of use to you.  
(There are a copule of other programs out there that try to do this.  As 
of 1999 when I left the project, this was the most advanced.)

We did not implement the ability to make the applications display like any 
other window in your desktop.  It would have been probably easier than 
what we did--displaying in a virtual desktop, a la vncviewer--but it did 
not fit our model of how it was going to be used.

To do what you asked for, you you could start with our ccfx code and cut 
out the desktop parts.  Be warned that this is not a trivial task--there 
is a lot of poorly documented code.  Then, before starting the application 
in question start your hacked ccfx and make the DISPLAY variable point to 
its port.  Then start you app.

ccfx acts as an X proxy for the app, and it will display the app on your 
desktop.  When another ccfx connects they can both get the window.  The 
first user should, in principle, be able to close their view of the app.

I think the single window VNC server would work better and easier.

Good luck,

Michael