Label selector for services

[arthurdm]

I've come across a scenario where an application needs to use a service, but there could be many instances of that service (with different configurations), dynamically added anytime before or after the application is up and running.

Depending on the request that comes into the application it would need to call a different instance of the service, so the ideal scenario would be for the application to have all of the available service instances mounted so it can browse through them and call the correct one.

If we added a labelSelector to the ServiceBinding.spec.service element we could achieve this, since all of the service instances would share a common label.

Otherwise I would need to create a way (probably another CR) that declares the binding intent from my app to any instance of the given service, and have some processor dynamically create a new ServiceBinding CR for each service instance that comes in.

[scothis]

There are a few impacts of this change that should be considered:

• many-to-many relationships are more difficult to logically understand and manage. Since service bindings are a form of access control, we should seek to avoid user surprise.
• any time a service matching the label is created/removed the application workload will be restarted (strictly speaking this is a side effect of the implementation and could be worked around, but the work arounds have other more serious impacts)
• the selector would still be limited to a single apiVersion/kind
• the .spec.type and .spec.provider fields become ambiguous
• the name of each binding in a container needs to be unique

[scothis]

Otherwise I would need to create a way (probably another CR) that declares the binding intent from my app to any instance of the given service

This seems like the right move. In the same way that Deployments orchestrate the rollout of ReplicaSets

[baijum]

From yesterday's meeting:

• Value symmetry between application and service
• Since filesystem name of binding is from ServiceBinding name (or spec override) you’ll never be able to select two
• We can enforce that only a single item can be selected from each
• Need to call out removal behavior ( Ref. #170 )
• Ready for a PR ( cc. @arthurdm )

[arthurdm]

Closing in favour of #184