Writing a tiny operator

Let's look at a simple operator
It does have:
- a control loop
- resource lifecycle management
- basic logging
It doesn't have:
- CRDs (and therefore, resource versioning, conversion webhooks...)
- advanced observability (metrics, Kubernetes Events)

Use case

When I push code to my source control system, I want that code to be built into a container image, and that image to be deployed in a staging environment. I want each branch/tag/commit (depending on my needs) to be deployed into its specific Kubernetes Namespace.

The last part requires the CI/CD pipeline to manage Namespaces
...And permissions in these Namespaces
This requires elevated privileges for the CI/CD pipeline

(read: cluster-admin)
If the CI/CD pipeline is compromised, this can lead to cluster compromise
This can be a concern if the CI/CD pipeline is part of the repository

(which is the default modus operandi with GitHub, GitLab, Bitbucket...)

Proposed solution

On-demand creation of Namespaces
Creation is triggered by creating a ConfigMap in a dedicated Namespace
Namespaces are set up with basic permissions
Credentials are generated for each Namespace
Credentials only give access to their Namespace
Credentials are exposed back to the dedicated configuration Namespace
Operator implemented as a shell script

An operator in shell... Really?

About 150 lines of code

(including comments + white space)
Performance doesn't matter
- operator work will be a tiny fraction of CI/CD pipeline work
- uses watch semantics to minimize control plane load
Easy to understand, easy to audit, easy to tweak

Show me the code!

GitHub repository and documentation:

https://github.com/jpetazzo/nsplease
Operator source code:

https://github.com/jpetazzo/nsplease/blob/main/nsplease.sh

Main loop

  info "Waiting for ConfigMap events in $REQUESTS_NAMESPACE..."
  kubectl --namespace $REQUESTS_NAMESPACE get configmaps \
    --watch --output-watch-events -o json \
    | jq --unbuffered --raw-output '[.type,.object.metadata.name] | @tsv' \
    | while read TYPE NAMESPACE; do

      debug "Got event: $TYPE $NAMESPACE"

--watch to avoid active-polling the control plane
--output-watch-events to disregard e.g. resource deletion, edition
jq to process JSON easily

Resource ownership

Check out the kubectl patch commands
The created Namespace "owns" the corresponding ConfigMap and Secret
This means that deleting the Namespace will delete the ConfigMap and Secret
We don't need to watch for object deletion to clean up
Clean up will we done automatically even if operator is not running

Why no CRD?

It's easier to create a ConfigMap

(e.g. kubectl create configmap --from-literal= one-liner)
We don't need the features of CRDs

(schemas, printer columns, versioning...)
“This CRD could have been a ConfigMap!”

(this doesn't mean all CRDs could be ConfigMaps, of course)

Discussion

A lot of simple, yet efficient logic, can be implemented in shell scripts
These can be used to prototype more complex operators
Not all use-cases require CRDs

(keep in mind that correct CRDs are a lot of work!)
If the algorithms are correct, shell performance won't matter at all

(but it will be difficult to keep a resource cache in shell)
Improvement idea: this operator could generate events

(visible with kubectl get events and kubectl describe)

???

:EN:- How to write a simple operator with shell scripts :FR:- Comment écrire un opérateur simple en shell script

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