Create canonical Service object about stackset-controller

[muaazsaleem]

It seems there are some questions around this feature:

What are the use cases for it?
I only see one use case for this. If someone is switching from using a normal service to StackSet, their application would go from having a stable cluster.local endpoint like
some-service.default.svc.cluster.local to only having per version service endpoints i.e some-service-v*.default.svc.cluster.local.

This might break callers that were using the original cluster.local endpoint. For example, services that use UI/API Gateways e.g tailor or zuul usually tend to use the cluster.local endpoints.

Which Stack version would this service point to?
Since we can't really traffic switch on service endpoints, which Stack version should this service point to. Should it always point to the latest version or should that be configurable?

One solution could be that we have a service key in the spec that refers to a stack version, something like:

apiVersion: zalando.org/v1
kind: StackSet
metadata:
  name: my-app
spec:
  #... optional igress definition
  # define which Stack service should the cluster.local endpoint point to 
 service:
    version: v1 # direct 100% of the cluster.local traffic to this version of the Stack.

People could also achieve the same effect by defining a service explicitly so the value of doing this is not clear.

Another solution would be to have the convention that this service always points to the Stack version that has the most traffic.

We could also decide that it would only work if a stack version is getting a 100% of the traffic. But then what should the endpoint refer to during traffic switching when there are multiple version.

But in all of the above solutions traffic switching is essentially "broken" for traffic coming from inside the cluster.

[mikkeloscar]

Good idea!

They can't be traffic switched so the question is, what should be the selector?

stackset: <name> could be the selector. Each stack has this.

[linki]

stackset: could be the selector.

Right, but that would mean 100% on a particular version. Wrong if you have a 20/80% split on the Ingress level but probably good enough.

[linki]

@muaazsaleem Thanks for the detailed explanation.

[linki]

We decided that people have to provide their own service.yaml when they need the unversioned cluster.local dns name.