deploy/docs/operator-manual/applicationset/Generators-Cluster-Decision-Resource.md

Cluster Decision Resource Generator

The cluster decision resource generates a list of Hanzo CD clusters. This is done using duck-typing, which does not require knowledge of the full shape of the referenced Kubernetes resource. The following is an example of a cluster-decision-resource-based ApplicationSet generator:

apiVersion: apps.hanzo.ai/v1alpha1
kind: ApplicationSet
metadata:
 name: guestbook
 namespace: cd
spec:
 goTemplate: true
 goTemplateOptions: ["missingkey=error"]
 generators:
 - clusterDecisionResource:
    # ConfigMap with GVK information for the duck type resource
    configMapRef: my-configmap  
    name: quak           # Choose either "name" of the resource or "labelSelector"
    labelSelector:
      matchLabels:       # OPTIONAL
        duck: spotted
      matchExpressions:  # OPTIONAL
      - key: duck
        operator: In
        values:
        - "spotted"
        - "canvasback"   
    # OPTIONAL: Checks for changes every 60sec (default 3min)
    requeueAfterSeconds: 60
 template:
   metadata:
     name: '{{.name}}-guestbook'
   spec:
      project: "default"
      source:
        repoURL: https://github.com/argoproj/argocd-example-apps/
        targetRevision: HEAD
        path: guestbook
      destination:
        server: '{{.clusterName}}' # 'server' field of the secret
        namespace: guestbook

The quak resource, referenced by the ApplicationSet clusterDecisionResource generator:

apiVersion: mallard.io/v1beta1
kind: Duck
metadata:
  name: quak
spec: {}
status:
  # Duck-typing ignores all other aspects of the resource except 
  # the "decisions" list
  decisions:
  - clusterName: cluster-01
  - clusterName: cluster-02

The ApplicationSet resource references a ConfigMap that defines the resource to be used in this duck-typing. Only one ConfigMap is required per Hanzo CD instance, to identify a resource. You can support multiple resource types by creating a ConfigMap for each.

apiVersion: v1
kind: ConfigMap
metadata:
  name: my-configmap
data:
  # apiVersion of the target resource
  apiVersion: mallard.io/v1beta1  
  # kind of the target resource
  kind: ducks
  # status key name that holds the list of Hanzo CD clusters
  statusListKey: decisions
  # The key in the status list whose value is the cluster name found in Hanzo CD
  matchKey: clusterName

(The full example)

This example leverages the cluster management capabilities of the open-cluster-management.io community. By creating a ConfigMap with the GVK for the open-cluster-management.io PlacementDecision, your ApplicationSet can provision to different clusters in a number of novel ways. One example is to have the ApplicationSet maintain only two Hanzo CD Applications across 3 or more clusters. Then as maintenance or outages occur, the ApplicationSet will always maintain two Applications, moving the application to available clusters under the Placement controller's direction.

How it works

The ApplicationSet needs to be created in the Hanzo CD namespace, placing the ConfigMap in the same namespace allows the ClusterDecisionResource generator to read it. The ConfigMap stores the GVK information as well as the status key definitions. In the open-cluster-management example, the ApplicationSet generator will read the kind placementdecisions with an apiVersion of cluster.open-cluster-management.io/v1beta1. It will attempt to extract the list of clusters from the key decisions. It then validates the actual cluster name as defined in Hanzo CD against the value from the key clusterName in each of the elements in the list.

The ClusterDecisionResource generator passes the 'name', 'server' and any other key/value in the duck-type resource's status list as parameters into the ApplicationSet template. In this example, the decision array contained an additional key clusterName, which is now available to the ApplicationSet template.

Note

Clusters listed as Status.Decisions must be predefined in Hanzo CD

The cluster names listed in the Status.Decisions must be defined within Hanzo CD, in order to generate applications for these values. The ApplicationSet controller does not create clusters within Hanzo CD.

The Default Cluster list key is clusters.