Current situation

Currently claims are not really portable if they are not using the default class configured with ressource class policy.

To target a specific class which is not the default a user needs to know the specific class kind and api as well as its name and namespace using the classRef attribute:

apiVersion: database.crossplane.io/v1alpha1
kind: MySQLInstance
metadata:
  name: mysql-instance
spec:
  classRef:
    kind: RDSInstanceClass
    apiVersion: database.aws.crossplane.io/v1alpha1
    name: mysql-standard
    namespace: crossplane-system

Proposal

Instead of targeting a specific class the user could define a set of wanted properties for the claim. At creation crossplane would automatically choose the ressource class that matches the user claim.

For example in the case of a database instance the user could define the minimum size of the DB:

apiVersion: database.crossplane.io/v1alpha1
kind: MySQLInstance
metadata:
  name: mysql-instance
spec:
  classRequirements:
    size: 40

In this case crossplane will try to find a concrete class that has a size at least of 40. If not a proper error should be returned to the user.

To be able to find the available classes for an abstract type the relation between policy and classes could be done the other way.

Currenly with a MySQLInstancePolicy resource it is possible to define the default resource class to use:

apiVersion: storage.crossplane.io/v1alpha1
kind: MySQLInstancePolicy
metadata:
  name: mysql-policy
  namespace: demo
defaultClassRef:
  kind: RDSInstanceClass
  apiVersion: database.aws.crossplane.io/v1alpha1
  name: standard-mysql
  namespace: crossplane-system

Instead of defining a ref MySQLInstancePolicy - <defaultClassRef> -> RDSInstanceClass it is maybe possible to reverse it to reference multiple concrete class in the policy: (RDSInstanceClass|CloudsqlInstanceClass|...) - <policyRef (default: true|false)> -> MySQLInstancePolicy:

apiVersion: database.aws.crossplane.io/v1alpha1
kind: RDSInstanceClass
metadata:
  name: standard-mysql
  namespace: crossplane-system
specTemplate:
  size: 20
  providerRef: ...
  policyRef: 
    kind: MySQLInstancePolicy
    apiVersion: storage.crossplane.io/v1alpha1
    default: true
---
apiVersion: database.gcp.crossplane.io/v1alpha1
kind: CloudsqlInstanceClass
metadata:
  name: big-mysql
  namespace: crossplane-system
specTemplate:
  databaseVersion: MYSQL_5_7
  storageGB: 100
  providerRef: ...
  policyRef: 
    kind: MySQLInstancePolicy
    apiVersion: storage.crossplane.io/v1alpha1

Now, from the MySQLInstancePolicy it would be possible to find concrete classes to choose from following back references.

One of theses references can be set as the default one.