Xpra - X Persistent Remote Applications

	Xpra (X Persistent Remote Applications) ... is a remote display solution for forwarding applications and desktop screens. Applications forwarded by Xpra appear on the local desktop as normal windows (seamless mode), rather than being all "trapped in a box together" (full desktop mode) like with VNC. Xpra uses a custom protocol that is self-tuning and relatively latency-insensitive, and thus is usable also over worse links. Xpra allows disconnection and reconnection without disrupting the forwarded application. Xpra is an open-source software for multiple platforms (Linux, Windows, MaxOS, etc.). Homepage: ​http://www.xpra.org Wiki: ​https://www.xpra.org/trac/

Preparation

Download & Installation Xpra-client

Xpra can be downloaded for Windows, Linux and Mac from the official Xpra site:
​http://www.xpra.org/trac/wiki/Download

Authentication

Xpra must communicate with JURECA via Secure Shell (SSH) and JURECA requires authentication to be password-less using SSH key-pair.
Please test your system before you proceed.

Execute the command with your JURECA-userid on your local workstation.
It must return the word 'success' for a successful SSH connection to JURECAVIS.

Linux/MacOSX:
ssh <YOUR-USERID>@juwelsvis.fz-juelich.de echo "success"

Windows/Putty:
"C:\Program Files (x86)\PuTTY\plink.exe" -ssh <YOUR-USERID>@juwelsvis.fz-juelich.de echo "success"
Ensure you have loaded your key in PuTTY´s SSH key agent called Pagent before!

General approach using Xpra requires three steps.

1. Start application in Xpra-server on a JUWELS login node.
2. Connect with Xpra-client from workstation to Xpra-server
3. Check if Xpra-server stopped on the JUWELS login node.

1. Start application in Xpra-server

Login to any JUWELS login node and start application with Xpra-server. If you want hardware accelerated rendering, you need to login to a vis-login node.

In the following example we use juwelsvis02.fz-juelich.de:

Before you can use Xpra, you must load the needed modules, which are, as of November 2020:

module --force purge
module use otherstages
ml Stages/Devel-2020  GCCcore/.9.3.0 xpra/4.0.4-Python-3.8.5

The you can a application via the "xpra" command:

• simple X11 application

  jwvis02> xpra start --start-child=xterm --min-quality=90
  

• OpenGL application

  jwvis02> xpra start --start-child="vglrun glxgears" --min-quality=90
  

  • using VirtualGL for hardware accelerated server-side rendering only works, if a GPU with a running X-server is available
  • currently only on JUWELSVIS login nodes (juwelsvis00.fz-juelich.de to juwelsvis03.fz-juelich.de)

Take a note of the <DISPLAYID> and the <LOGFILE> Xpra returns:

> Actual display used: :<DISPLAYID>
> Actual log file name is now: <LOGFILE>

2. Connect with Xpra-client from workstation to Xpra-server

Start the Xpra-client on the workstation to connect to the Xpra-server on JUWELS.

• Connect using Xpra command line (primarily with Linux or MacOSX)

  client> xpra attach ssh:<USERID>@<LOGINNODE>:<DISPLAYID>
  

• Connect using Xpra GUI (primarily with Windows)

  client> xpra_launcher
  

  

Mode = SSH
<USERID>@juwelsvis02.fz-juelich.de:22:<DISPLAYID>

3. Check if Xpra-server stopped

The Xpra-server shuts down automatically, when the application is closed.
Check if that is the case and stop it manually if required.

jwvis02> xpra list
jwvis02> xpra stop :<DISPLAYID>

Attention:
If you stop the Xpra-client without closing your application, your application keeps on running!
This can use cpu and memory resources others might need.

Real-case scenarios with Xpra

ParaView with full serverside rendering support using VirtualGL

client> xpra start ssh:<USERID>@juwelsvis02.fz-juelich.de --start-child=xterm --min-quality=90

Now start load the necessary modules and start ParaView in the xterm:

xterm> module --force purge
xterm> ml Stages/2020  GCC/9.3.0  ParaStationMPI/5.4.7-1
xterm> ml ParaView/5.8.1-Python-3.8.5
xterm> paraview

Atom
If you want to avoid to manually switch to another stage or load extra modules, you can follow this example.
Thanks goes to Johannes Hörmann.

A suitable xpra wrapper script can do the extra work before you program starts:

#!/bin/bash -x

module --force purge
module load Stages/Devel-2020  GCCcore/.9.3.0
module load xpra/4.0.4-Python-3.8.5
module load Atom/1.52.0

exec $@

• Add the executable bit (chmod +x xpra_wrapper)
• Add the line exec-wrapper = /absolute/path/to/xpra_wrapper to ~/.xpra/xpra.conf
  • if you have used xpra before, this file should already exist

Now you can bring up a remote Atom with this single command

client> xpra start ssh:<USERID>@juwelsvis02.fz-juelich.de --start-child="atom -w" --min-quality=90 --dpi=96

In order not to detach Atom from the calling process it needs to be started with -w option -- otherwise the xpra_wrapper would return immediately after initiating Atom and the xpra server shuts down.

More on Xpra

Full documentation of Xpra command: ​http://xpra.org/manual.html
Configuration Options: ​https://xpra.org/trac/wiki/Configuration

Usefull examples:

• List all running Xpra-server:

  xpra list
  

• Get all infos of a running Xpra-server:

  xpra info :<DISPLAYID>
  

• Start Xpra-server with larger/smaller fonts

  xpra start --start-child="xterm" --dpi=96 --min-quality=90
  

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any feedback welcomed - h.zilken@…, j.goebbert@…