Jupyterhub SwarmSpawner select worker node - docker

Question

I have set a docker swarm with multiple worker nodes.
My current Jupyterhub setup with SwarmSpawner works fine, I am able to deploy single-user docker images based on user-selected image before spawning the image, using _options_form_default in my jupyterhub_config.py.
What I would like now is to give users the possibility to select the swarm worker node name (hostname) on which he would like to spawn his single-user JupyterHub image, because our worker nodes have different types of hardware specs (GPUs, RAM, processors etc) and users know in advance the name of the host he would like to use.
Is it possible to determine the node on which to spawn the image ?
My current swarm has for example 1 master node: "master" and 3 worker nodes: "node1", "node2", "node3" (those are their hostnames, as it appears in the column HOSTNAME in the output of the command docker node ls on the master node).
So what I would like is that, just as it appears in the image below, users have a dropdown selection of the swarm worker nodes hostnames on which they would like to spawn their jupyterhub image, with a question such as: "Select the server name".

Answer 1

Ok so I actually figured out how to do that.
Here is the relevant part in my jupyterhub_config.py:
class CustomFormSpawner(SwarmSpawner):
# Shows frontend form to user for host selection
# The option values should correspond to the hostnames
# that appear in the `docker node ls` command output
def _options_form_default(self):
return """
<label for="hostname">Select your desired host</label>
<select name="hostname" size="1">
<option value="node1">node1 - GPU: RTX 2070 / CPU: 40</option>
<option value="node2">node2 - GPU: GTX 1080 / CPU: 32</option>
</select>
"""
# Retrieve the selected choice and set the swarm placement constraints
def options_from_form(self, formdata):
options = {}
options['hostname'] = formdata['hostname']
hostname = ''.join(formdata['hostname'])
self.extra_placement_spec = { 'constraints' : ['node.hostname==' + hostname] }
return options
c.JupyterHub.spawner_class = CustomFormSpawner