[clx75]

At work we are using Metacontroller to implement our "operators". Quoted because these are not real operators but rather Metacontroller plugins, written in Python. All the watch and update logic - plus the resource caching - is outsourced to Metacontroller (which is written in Go). We define - via its CompositeController or DecoratorController CRDs - what kind of resources it should watch and which web service it should call into when it detects a change. The web service speaks plain HTTP (or HTTPS if you want).

In case of a CompositeController, the web service gets the created/updated/deleted parent resource and any already existing child resources (initially none). The web service then analyzes the parent and existing children, then responds with the list of child resources whose existence and state Metacontroller should ensure in the cluster. If something is left out from the response compared to a previous response, it is deleted.

Things we implemented using this pattern:

- Project: declarative description of a company project, child resources include a namespace, service account, IAM role, SMB/S3/FSX PVs and PVCs generated for project volumes (defined under spec.volumes in the Project CR), ingresses for a set of standard apps

- Job: high-level description of a DAG of containers, the web service works as a compiler which translates this high-level description into an Argo Workflow (this will be the child)

- Container: defines a dev container, expands into a pod running an sshd and a Contour HTTPProxy (TCP proxy) which forwards TLS-wrapped SSH traffic to the sshd service

- KeycloakClient: here the web service is not pure - it talks to the Keycloak Admin REST API and creates/updates a client in Keycloak whose parameters are given by the CRD spec

So far this works pretty well and makes writing controllers a breeze - at least compared to the standard kubebuilder approach.

https://metacontroller.github.io/metacontroller/intro.html

[JeffMcCune]

As other sibling comments suggest these use cases are better solved with a generator.

The rendered manifest pattern is a simpler alternative. Holos [1] is an implementation of the pattern using well typed CUE to wrap Helm and Kustomize in one unified solution.

It too supports Projects, they’re completely defined by the end user and result in the underlying resource configurations being fully rendered and version controlled. This allows for nice diffs for example, something difficult to achieve with plain ArgoCD and Helm.

[1]: https://holos.run/docs/overview/

[Kinrany]

The rendered manifests pattern is a great read by itself: https://akuity.io/blog/the-rendered-manifests-pattern

[ec109685]

Curious why using controller for these aspects versus generating the K8s objects as part of your deployment pipeline that you just apply? The latter gives you versioned artifacts you can roll forward and back and independent deployment of these supporting pieces with each app.

Is there runtime dynamism that you need the control loop to handle beyond what the built-in primitives can handle?

[clx75]

Some of the resources are short-lived, including jobs and dev containers. The corresponding CRs are created/updated/deleted directly in the cluster by the project users through a REST API. For these, expansion of the CR into child resources must happen dynamically.

Other CRs are realized through imperative commands executed against a REST API. Prime example is KeycloakRealm and KeycloakClient which translate into API calls to Keycloak, or FSXFileSystem which needs Boto3 to talk to AWS (at least for now, until FSXFileSystem is also implemented in ACK).

For long-lived resources up-front (compile time?) expansion would be possible, we just don't know where to put the expansion code. Currently long-lived resource CRs are stored in Git, deployment is handled with Flux. When projects want an extra resource, we just commit it to Git under their project-resources folder. I guess we could somehow add an extra step here - running a script? - which would do the expansion and store the children in Git before merging desired state into the nonprod/prod branches, I'm just not clear on how to do this in a way that feels nice.

Currently the entire stack can be run on a developer's laptop, thanks to the magic of Tilt. In local dev it comes really handy that you can just change a CRs and the children are synced immediately.

Drawbacks we identified so far:

If we change the expansion logic, child resources of existing parents are (eventually) regenerated using the new logic. This can be a bad thing - for example jobs (which expand into Argo Workflows) should not change while they are running. Currently the only idea we have to mitigate this problem is storing the initial expansion into a ConfigMap and returning the original expansion from this "expansion cache" if it exists at later syncs.

Sometimes the Metacontroller plugin cannot be a pure function and executing the side effects introduces latency into the sync. This didn't cause any problems so far but maybe will as it goes against the Metacontroller design expressed in the docs.

Python is a memory hog, our biggest controllers can take ~200M.

[remram]

The choice is always between a controller and a generator.

The advantage of a controller is that it can react to external conditions, for example nodes/pods failing, etc. The is great for e.g. a database where you need to failover and update endpointslices. The advantage of a generator is that it can be tested easier, it can be dry-runned, and it is much simpler.

All of your examples seem to me like use cases that would be better implemented with a generator (e.g. Helm, or any custom script outputting YAML) than a controller. Any reason you wrote these as controllers anyway?

[KGunnerud]

I've seen different aproaches to controllers, some times it should have been a generator instead, but the problem with generators is that they don't allow (in the same sense) for abstractions at the same level of controllers.

E.g. at one company I worked, they made a manifest to deploy apps that, in v1 was very close to Deployment. It felt owerkill. As they iterated, suddenly you got ACLs that changed NetworkPolicy in Calico (yes can be done with generator), then they added Istio manifests, then they added App authroizations for EntraID - Which again provisioned EntraID client and injected certificate into pods. All I did was add: this app, in this namespace, can talk to me and I got all this for "free". They code in the open so some of the documentation is here: https://docs.nais.io/explanations/nais/

One day, they decided to change from Istio to LinkerD. We users changed nothing. The point is, the controller was 2 things: 1: for us users to have a golden path and 2: for the plattform team themselves to have an abstraction over some features of kube. Although I do see that it might be easy to make poor abstractions as well, e.g. just because you don't create a Deployment (its done for you), you still have to own that Deployment and all other kube constructs.

I'm currently in a org that does not have this and I keep missing it every, every day.

[Kinrany]

Even if a controller is necessary, wouldn't you still want to have a generator for the easy stuff?

Kinda like "functional core, imperative shell"?

[fsniper]

At work we are using nolar/kopf for writing controllers that provisions/manages our kubernetes clusters. This also includes managing any infrastructure related apps that we deploy on them.

We were using whitebox controller at the start, which is also like metacontroller that runs your scripts on kubernetes events. That was easy to write. However not having full control on the lifecycle of the controller code gets in the way time to time.

Considering you are also writing Python did you review kopf before deciding on metacontroller?

[clx75]

Yes, we started with Kopf.

As we understood it, Kopf lets you build an entire operator in Python, with the watch/update/cache/expansion logic all implemented in Python. But the first operator we wrote in it just didn't feel right. We had to talk to the K8S API from Python to do all the expansions. It was too complex. We also had aesthetic issues with the Kopf API.

Metacontroller gave us a small, Go binary which takes care of all the complex parts (watch/update/cache). Having to write only the expansion part in Python felt like a great simplification - especially now that we have Pydantic.