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Open OnDemand (Beta) FAQ

General information

Q. I don't see some of the tabs or buttons described in the documentation. Where is this stuff?

A. In a mobile browser or narrow browser window, the top navigation bar is hidden behind a hamburger (≡) button.

Q. The Web application is acting strangely, including giving error 422. Is it down?

A. Try clearing your browser cache and cookies for Open OnDemand, and restarting your Web server.

Q. What do you mean my Web server?

A. When you log into Open OnDemand, you are assigned a per-user NGINX instance which runs as a process belonging to your HPC user, can execute scripts with your permissions, submits Slurm jobs in your name, and works with data in your home directory. It can be restarted using the button under the 'Help' drop-down menu in the top right without impacting any other user.

Interactive sessions

Q. Where are the logs for my interactive sessions located?

A. You can access the logs of your sessions, the batch script and connection details either in the Web interface or your home directory. You can find these files via the Web interface by following these steps:

  1. Log into ood.hpc.tu-dresden.de
  2. My Interactive Sessions
  3. Locate the corresponding session card
  4. Click the Session ID link

If you are using the filesystem more directly, you can find the session data in your home directory at:

/home/<USER>/ondemand/data/sys/dashboard/batch_connect/sys/<APP>/output/<SESSION_ID>

Replace:

<USER> - with your username

<APP> - with the software name (e.g. matlab)

<SESSION_ID> - with the Session ID you find on the session card, for example:

/home/marie/ondemand/data/sys/dashboard/batch_connect/sys/matlab/output/84977e1b-6b5d-4564-b30e-d99647df7d6c

Q. What information is logged from my interactive sessions?

A. The output from the Slurm job is collected in output.log. For VNC sessions, you will also have a vnc.log and often a websockify.log, which contain only the logs from the VNC server and Websockify respectively. These are useful for diagnosing display and connectivity issues. Other application-specific logs and cache files may be created in the usual ways and locations, exactly as if you had submitted your job directly to Slurm.

Q. My sessions never start (turn green, have a 'Connect' button) even though they are allocated resources (turn dark blue). Is something broken in Open OnDemand?

A. Open OnDemand performs normal Slurm job submission via your HPC user account. Anything that interferes with normal login, ssh, module operations or Slurm operations on your HPC user account will interfere with your Open OnDemand jobs in the same way. This may include issues with your own configurations as well as system-wide issues. Check if you have received any announcement emails recently and if you can submit jobs normally with other tools.

Q. My session gave me an "Internal Server Error". What can I do?

A. Try restarting your Web server with the button under 'Help' in the top right of the page. If this is impossible or doesn't help, try renaming your ~/ondemand/data directory to clear out any corrupted session data the application may be encountering, then restart again. A new ~/ondemand/data will be created to replace it for your subsequent sessions.

Applications

Desktops

Q. My desktop icons are missing or don't work. Is it supposed to be like that?

A. No, probably something is wrong with the desktop configuration in ~/.config/xfce4. This will be regenerated if you delete it.

VNC Applications

Q. When I launch a VNC application or desktop on certain nodes, I just get a black screen. What do I do?

A. VNC keeps a local record of your sessions in ~/.vnc, and sometimes these are not cleaned up correctly when a session is canceled, leading to issues with future sessions on the same node. Emptying ~/.vnc should fix it.

TensorBoard

Q. I submitted a TensorBoard job. The session card turned green and gave me a 'Connect' button, but when I clicked it it opened a page that said 'Could not connect' in huge letters. What's the problem?

A. TensorBoard server has started, is still loading. You can just wait a few seconds and refresh the page.

MATLAB (Web)

Q. I launched the MATLAB Web app with a version from before 2025. When I first connected to it, authentication failed. Should I open a ticket?

A. You may be able to click submit or refresh the page to continue the session normally. This bug occurs in the 2022-2024 versions of the MATLAB Web app, and the initial connection attempt is affected about 1 time in 6 for these versions. A real authentication failure will show a clear message on screen (messages in the logs can be misleading for these versions). It is normal for the application to take some time to launch.

Q. I launched MATLAB with a 2025 version and it shows a warning about Fluxbox. Is this a problem?

A. This warning is expected in the 2025 versions. It was later removed.

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