I am facing this problem: services deployed with Knative serving shall be registered with an API gateway when installed (i.e. kn service create some-service ...) and un-registered when the service is removed (i.e. kn service delete some-service).
Since the first part can easily be implemented in the start procedure of the service code, this is not an issue. But detecting when a service is removed altogether, not just scaled down, seems very tricky. At least I could not find any working solution. Kubernetes lifecycle hooks don't do the job because they are called when a pod (not the service) is stopped/killed.
Does anyone have an idea how the service deletion can be intercepted to get a custom command to execute? Thanks for any help.
If you want to do this server-side, rather than with a client script, you'll probably need to implement a small Kubernetes controller. There's are also two halfway solutions, each with some trade-offs.
If you implement a controller, you'll basically have a single process (Pod/StatefulSet if running it on Kubernetes) which performs a Watch on all Knative Services. It could handle both the create and delete aspects if you want, or you could keep your current creation code.
A second option is to add a finalizer to each service and run a periodic job to look for all Services with a deletionTimestamp and your finalizer set. For reach of these Services, the job can unregister the Service with the API registry then remove the finalizer. This is similar to the controller approach, but instead of a constantly-running Watch, the finalizer delays the delete until the cleanup job runs. The drawback is that this means it may take up to N minutes (the frequency of the job running) for deletes to happen.
The last option is to use a validating admission webhook or an audit webhook to be notified of delete events. Each of these has trade-offs:
Two more non-controller solutions that popped to mind:
Add a "heartbeat" check on the API registration that includes the URL of the having resource. If the registration has a having URL and the object no longer exists, remove the registration. This may be fairly simple to implement.
You can use the apiserver source to set up a trigger on Knative Services. You'd be watching for the dev.knative.apiserver.ref.delete or dev.knative.apiserver.resource.delete events, and the could have a single Knative Service (or other URL) handle these POSTs and do a cleanup of the resources. Unfortunately, I'm not sure there's a good example of this on the web yet.