Job templating in Kubernetes

This short writeup addresses a need to add an additional label to Workbench sessions started in a kubernetes environment. Specifically this was for an environemnt where an external Workbench server was launching sessions into a Kubernetes cluster. The recommended way to do this is to use job templating.

Job templating for custom labels for Posit Workbench with k8s

These are the granular steps for taking advantage of kubernetes object templating that would work with an external Workbench instance launching into k8s:

1. Add to launcher.kubernetes.conf, use-templating = 1
2. Use the launcher CLI to create the job.tpl and service.tpl files (see below for an example)
3. Confirm that the version called out in job.tpl and service.tpl (using head job.tpl and head service.tpl for example) are compatible with your workbench version (version 2.3.1 is latest)
4. Create rstudio-library-templates-data.tpl - for now this is a helm-only construct so it will need to be copied from the helm output (helm repo) (see example below for doing this with the helm template command)
5. Copy job.tpl, service.tpl, and rstudio-library-templates-data.tpl into the launcher scratch directory on the host machine (in this case your Workbench server outside of k8s) (see below for default for where this is located) and make sure that rstudio-server has read/write access

For adding the new labels/values, here are good options:

• Modify rstudio-library-templates-data.tpl directly (either in the header bit, or lower under values/labels)
• Modify the values in the helm values file (example)
• Recommended: Add the values at time of making the template, IE with --set launcher.templateValues.job.labels.test=value \ (see below)

From talking with a colleague they shared this example using helm to create the template with the added label values:

helm template my-release rstudio/rstudio-workbench \
 --set launcher.useTemplates=true \
 --set launcherPem=test \
 --set session.defaultConfigMount=false \
 --set launcher.templateValues.job.labels.test=value \
 --set launcher.templateValues.job.labels.other=something-else \
 | less

In that helm example:

• session.defaultConfigMount=false prevents defining volumes and volumeMounts that are only relevant for the helm chart
• launcherPem=test speeds up templating
• launcher.useTemplates=true turns on templating (instead of job json overrides)

What if things go wrong?

The rstudio-kubernetes-launcher CLI can verify templates now!

Playing with the rstudio-kubernetes-launcher CLI:

/usr/lib/rstudio-server/bin/rstudio-kubernetes-launcher --help
/usr/lib/rstudio-server/bin/rstudio-kubernetes-launcher --generate-templates --scratch-path=/tmp/hello

Default scratch path:

 # mount into the default scratch-path
 - name: session-templates
  mountPath: "/var/lib/rstudio-launcher/Kubernetes/rstudio-library-templates-data.tpl"
  subPath: "rstudio-library-templates-data.tpl"
 - name: session-templates
  mountPath: "/var/lib/rstudio-launcher/Kubernetes/job.tpl"
  subPath: "job.tpl"
 - name: session-templates
  mountPath: "/var/lib/rstudio-launcher/Kubernetes/service.tpl"
  subPath: "service.tpl"

Another approach using json over-rides on Posit Workbench with k8s

Potentially json over-rides could work but my understanding is that this would overwrite ALL labels.

use this to add a label:

/etc/rstudio/launcher.kubernetes.profiles.conf

job-json-overrides="/spec/template/spec/labels":"/etc/rstudio/labels"

/etc/rstudio/labels

[
  {
    "applicationid": "workbench",
    "label": ["value"]
  }
]