Migrate an application instance from one Kind to another Kind

[devdattakulkarni]

Migrating an application instance from one Kind to another Kind is needed when using licenses. A license is associated with a Kind. Suppose that provider has two Kinds defined - KindTrialLicense and KindYearlyLicense. Suppose that the provider signs up two customers - Customer1 and Customer2. Suppose both of them start with the trial license of the application. So the provider creates two application instances of KindTrialLicense. Suppose that after the trial period is over, customer1 wants to sign up for the yearly license. At this point, it will be logical for the provider to migrate customer1's application instance from KindTrailLicense to KindYearlyLicense.

[devdattakulkarni]

We can implement the migration functionality as follows.

We define a special annotation: kubeplus/migrate-from:<Source Kind>.
This annotation is put on an instance that we want to migrate from <Source Kind> to the <Target Kind>. When we call kubectl apply on this instance, KubePlus will intercept this call. It will check if the above annotation is present on the instance. If it is then it performs the following steps:

1. Parse the annotation to get the <Source Kind>.
2. Because NS with the app instance name will already be present, KubePlus will be running a helm upgrade on the release. In this the helm release to use will be <Source Kind>-<App instance name>. The chart to be used will be corresponding to the <New Kind> and the overrides values will be corresponding to the Spec properties of the <New instance>.
3. In the status field of the <New instance> store the Helm release name, which will be made up of <Source Kind> and <Source Name>.
4. Remove the helm release name from the status field in the old application instance (<Source Kind>, <Source Name>). Note that this will make the app instance dangling (the app instance is present but there is no helm release corresponding to it). This will be only problematic when one tries to delete the application instance. So maybe KubePlus can just delete the old app instance as it is no longer needed. Another option is to let users delete this app instance.

[devdattakulkarni]

Note that the Spec properties of the application instance cannot change between the two Kinds. This means that the underlying Helm chart's values.yaml should remain same across the two Kinds. The application container image can change.