> On the other hand, most operators I've seen are just k8s manifest templates implemented in Go.
> I'm a fan of metacontroller [1], which is a tool that allows you to write operators without actually writing a lot of imperative code that interacts with the k8s API, but instead just provide a general JSON->JSON transformer,
That seems... surprising, to me. It's not clear to me how a JSON->JSON transformer (which is essentially a pure function on UTF-8 strings to UTF-8 strings, i.e. an operation without side effect) can actually modify the state of the world to bring your requested resources to life. If the only thing the Operator is being used for is pure computation, then I agree it's overkill.
An example use case for an Operator would be a Pod running on the cluster that is able to receive YAML documents/resource objects describing what kind of x509 certificate is desired, fulfill an ACME certificate order, and populate a Secret resource on the cluster containing the x509 certificate requested. It's not strictly JSON to JSON, from "certificate" custom resource to Secret resource - there's a bunch of side-effecting that needs to take place to, for instance, respond to DNS01 or HTTP01 challenges by actually creating a publicly accessible artifact somewhere. That's what Operators are for.
Metacontroller is actually quite easy to learn. It comes with good examples too. Including a re-implementation of the Stateful Set controller, all done with iterations of an otherwise pure computation. The trick is obviously that the state lives in the k8s api server, from which the inputs of the subsequent invocation of your pure function come.
> I'm a fan of metacontroller [1], which is a tool that allows you to write operators without actually writing a lot of imperative code that interacts with the k8s API, but instead just provide a general JSON->JSON transformer,
That seems... surprising, to me. It's not clear to me how a JSON->JSON transformer (which is essentially a pure function on UTF-8 strings to UTF-8 strings, i.e. an operation without side effect) can actually modify the state of the world to bring your requested resources to life. If the only thing the Operator is being used for is pure computation, then I agree it's overkill.
An example use case for an Operator would be a Pod running on the cluster that is able to receive YAML documents/resource objects describing what kind of x509 certificate is desired, fulfill an ACME certificate order, and populate a Secret resource on the cluster containing the x509 certificate requested. It's not strictly JSON to JSON, from "certificate" custom resource to Secret resource - there's a bunch of side-effecting that needs to take place to, for instance, respond to DNS01 or HTTP01 challenges by actually creating a publicly accessible artifact somewhere. That's what Operators are for.