ApplicationSets and Multi-Cluster GitOps

1. 1

   Create and register a second cluster

   argocd cluster add creates a ServiceAccount in the target cluster and stores its credentials as a Secret in the argocd namespace. Labels on that Secret drive the cluster generator. On EKS, use IAM-based auth instead of long-lived tokens. With kind, use the Docker network IP for the API server so Argo's pods can reach it.

   terminalcopy cmd

   $ kind create cluster --name shoplite-prod

   argocd cluster add kind-shoplite-prod --name prod-mumbai --label env=prod --label region=ap-south-1 --yes

   argocd cluster set in-cluster --label env=dev

   argocd cluster list

   ── expected output ──

   SERVER NAME VERSION STATUS MESSAGE

   https://kubernetes.default.svc in-cluster 1.34 Successful

   https://172.18.0.3:6443 prod-mumbai 1.34 Successful

2. 2

   One ApplicationSet for every service × cluster

   The MATRIX generator combines a Git directory generator (every apps/* folder is a service) with a cluster generator (every cluster with an env label). Each pair becomes an Application whose path uses the cluster's env label to choose the overlay. goTemplate gives proper templating with missingkey=error, so a typo fails loudly.

   apiVersion: argoproj.io/v1alpha1
   kind: ApplicationSet
   metadata:
     name: shoplite-services
     namespace: argocd
   spec:
     goTemplate: true
     goTemplateOptions: ["missingkey=error"]
     generators:
       - matrix:
           generators:
             - git:
                 repoURL: https://github.com/<you>/shoplite-gitops.git
                 revision: main
                 directories:
                   - path: apps/*
             - clusters:
                 selector:
                   matchExpressions:
                     - { key: env, operator: In, values: [dev, prod] }
     template:
       metadata:
         name: '{{.path.basename}}-{{.name}}'
         labels: { service: '{{.path.basename}}', env: '{{index .metadata.labels "env"}}' }
       spec:
         project: shoplite
         source:
           repoURL: https://github.com/<you>/shoplite-gitops.git
           targetRevision: main
           path: 'apps/{{.path.basename}}/overlays/{{index .metadata.labels "env"}}'
         destination:
           server: '{{.server}}'
           namespace: 'shoplite-{{index .metadata.labels "env"}}'
         syncPolicy:
           automated: { prune: true, selfHeal: true }
           syncOptions: [CreateNamespace=true]

3. 3

   Roll out cluster by cluster

   By default an ApplicationSet change updates every generated app at once. The progressive sync strategy (RollingSync) updates apps in groups by label: dev first, then prod, and each group must be Healthy before the next starts. Add this to the ApplicationSet's spec (it needs --enable-progressive-syncs on the controller).

     strategy:
       type: RollingSync
       rollingSync:
         steps:
           - matchExpressions: [{ key: env, operator: In, values: [dev] }]
           - matchExpressions: [{ key: env, operator: In, values: [prod] }]
             maxUpdate: 50%

4. 4

   Adopt it and delete the hand-written apps

   Adding a new service is now: create apps/cart/base and its overlays, commit. The ApplicationSet discovers the folder and creates cart-in-cluster and cart-prod-mumbai. Adding a region is argocd cluster add with labels.

   terminalcopy cmd

   $ git rm argocd/apps/web-dev.yaml argocd/apps/web-prod.yaml

   git add . && git commit -m 'appset: services × clusters' && git push

   argocd app list -l service -o name

   ── expected output ──

   argocd/cart-in-cluster

   argocd/cart-prod-mumbai

   argocd/checkout-in-cluster

   argocd/checkout-prod-mumbai

   argocd/web-in-cluster

   argocd/web-prod-mumbai

Make each change, run the command, and read the error before revealing the diagnosis. Recognising these messages on sight is what makes you fast on a real team. Undo the change afterwards.