set spec hostname to get headless service to work

[kagesenshi]

Notebook pod hostname could not be resolved from another pod through headless service. Setting pod.spec.hostname to pod name will get it to work.

Other people affected:

https://discourse.jupyter.org/t/how-to-run-single-user-pod-in-headless-mode-so-that-i-can-run-spark-jobs-on-an-external-hadoop-cluster/13806

https://gitter.im/jupyterhub/jupyterhub?at=5e316125f301780b83439232

https://groups.google.com/g/jupyter/c/UxeDkxAmuTI/m/tV0jN5ZxAwAJ

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[consideRatio]

Thanks @kagesenshi!

    pod.spec.hostname = name
    return pod

I'm not yet confident enough to merge this, thinking about questions like this:

• Does hostname part of the k8s Pod specification has restrictions on what we can specify, for example only a certain set of characters? If so, is what we assign to it always an acceptable string as well?
• Does setting hostname risk influencing things outside the pod, or is it just influencing the internals of containers running in the pod?
• Do we perceive a risk that other software stops working when we now start declaring a hostname?

Unless we are confident this is a good default, I suggest we instead recommend that users running into this use extra_pod_config to configure hostname.

[kagesenshi]

unfortunately i don't think i have an answer for that. How I found out the fix is by comparing the containers that was created in k8s through k8s yml component yml file and finding the key that was missing, the most obvious missing key was this.

hostname afaik is limited to valid DNS domain characters and subject to the usual DNS character limits.

iirc when trying to solve this, extra_pod_config was unable to set this property there are no way to know what is the generated container name, closest method that can override this attribute is through modify_pod_hook.

[dolfinus]

You can use extra_pod_config with name: "jupyter-{username}--{servername}" where "jupyter-{username}--{servername}" is a value of c.KubeSpawner.pod_name_template. Kubespawner will resolve substitutions before updating the spec

[kagesenshi]

@dolfinus

the issue here is not about setting pod_name_template, but rather this pod name is not being applied as container hostname itself, causing internal DNS resolution of services to not work correctly.

[dolfinus]

Sorry, I mean that you can use extra_pod_config to set hostname: "jupyter-{username}--{servername}" in pod's spec, without adding potentially breaking changes to the KubeSpawner

[manics]

It sounds like extra_pod_config or modify_pod_hook can handle this, so should we close this?