ApplicationSet generator CLI tool to validate the resource output

[agaudreault]

Description

One of the problem we have with ApplicationSet is the ability to test the generators before to commit/apply the yaml in the cluster.

Context

We have the apps-of-appsets pattern where we have a global application-sync responsible to sync a folder in a Git repository with multiple ApplicationSet manifests. Before a user commit to the Git repository, we would like to perform a validation of what the generators will return.

This is quite important, especially when you do not use preserveResourcesOnDeletion, because an error or a typo in the generator would mean that your Application is deleted, and all associated resources with it.

A solution to prevent that is to have --policy create-update option enabled, but this causes a lot of manual actions whenever the generator result changes and would not be suitable for us at scale.

Solution

A solution would be to give developers a tool to validate the result of their generator

Example

With the following file,

# ./test-appset.yaml
apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: test-appset
  annotations:
    argocd.argoproj.io/sync-options: Prune=false
spec:
  syncPolicy:
    preserveResourcesOnDeletion: false
  generators:
    - clusters:
        selector:
          matchExpressions:
            - key: environment
              operator: In
              values: [dev, staging]
    - clusters:
        selector:
          matchExpressions:
            - key: tenant
              operator: In
              values: [foo]
  template:
# truncated

A user (or a CI) would be able to run a CLI command, curl or something like that locally, and it would return the list of item that the generator would iterate upon, such as

> argocd appset get-items -f ./test-appset.yaml
foo-dev-cluster-1
foo-dev-cluster-2
foo-staging-cluster-1

Another feature could be to have a command similar to helm template to be able to generate the resulting YAML. With the example above, it would generate the template of the 3 applications, with the values interpolated if any.

> argocd appset template -f ./test-appset.yaml
---
# resulting yaml here

[agaudreault]

Updating this since that #9584 is merged

[crenshaw-dev]

Extended discussion here: #10895