Supporting conditional configurations in the parameters hierarchy

Some components can benefit from being able to provide different defaults values for a parameter based on the value of another component parameter.

For example, this can be very useful to dynamically set Helm chart values based on another component parameter.

It’s considered best practice to use constant parameters to implement lookup tables for such cases.

Another common use case for lookup tables is to dynamically select dependency versions based on a component parameter. See component openshift4-terraform for an example lookup table to select the Terraform module version based on the cloud provider for which the component is configured.

Example

In this example we want to configure a Helm chart to use PodSecurityPolicies on RKE and SecurityContextConstraints on OpenShift 4. We get the information about the distribution for which to configure the Helm chart from component parameter distribution.

We can implement a lookup table in the component parameters using the following structure:

parameters:
  my_component:
    # Component defaults to distribution "rke"
    distribution: rke (1)
    =_dist_helm_values: (2)
      rke:
        # Use PSPs on RKE
        psps: true
        sccs: false
      openshift4:
        # Use SCCs on OpenShift
        psps: false
        sccs: true
    helm_values:
      psps: ${my_component:_dist_helm_values:${my_component:distribution}:psps}
      sccs: ${my_component:_dist_helm_values:${my_component:distribution}:sccs}

1	Parameter which indicates the distribution for which to configure the Helm chart
2	Helm value lookup table as a constant parameter, so users can’t accidentally overwrite the lookup table contents.