app-of-app legacy

In this blog post we’re going to setup Argo CD on a Kubernetes cluster. We’ll install it with Helm, create an application to use the app-of-apps pattern, set Argo CD up so that it can update itself, and install Prometheus via Argo CD as an example.

A picture of the Argo CD web UI showing the details view of an application

All files mentioned in this blog post are available in a Git repository on GitHub.

What is Argo CD?

Argo CD is a GitOps tool to automatically synchronize the cluster to the desired state defined in a Git repository. Each workload is defined declarative through a resource manifest in a YAML file. Argo CD checks if the state defined in the Git repository matches what is running on the cluster and synchronizes it if changes were detected.

For example, instead of manually running CLI commands to update Kubernetes resources with kubectl apply or helm upgrade, we would update a YAML file in our Git repository that contains an Application manifest. Argo CD periodically checks this manifest for changes and will automatically synchronize resources that are defined in it with the ones that are running on our cluster.

A connection to the cluster, either from the developers laptop or from a CI/CD system, is no longer needed as changes are pulled from the Git repository by a Kubernetes Operator running inside the cluster.

Requirements

To follow this tutorial you’ll need the following. The version number shows what I’ve used for this tutorial:

  • A Kubernetes cluster (1.23)

  • kubectl (1.23.5)

  • Helm (3.8.1)

  • A public git repository

Create a Git repository

Our application manifests are stored in a Git repository. For this tutorial I’m using a public Github repo:

$ gh repo create argotest --public --clone
$ cd argotest

Creating an umbrella Helm chart

We’ll use Helm to install Argo CD with the official chart from argoproj/argo-helm. We create a Helm umbrella chart that pulls in the original Argo CD chart as a dependency.

Using this approach we have the possibility to bundle extra resources with the chart. For example, we can install credentials that are used to authenticate with private Git or Helm repositories by placing them in the chart template/ directory.

To create the umbrella chart we make a directory in our Git repository and place two files in it:

$ mkdir -p charts/argo-cd

charts/argo-cd/Chart.yaml

apiVersion: v2
name: argo-cd
version: 1.0.0
dependencies:
  - name: argo-cd
    version: 4.2.2
    repository: https://argoproj.github.io/argo-helm

charts/argo-cd/values.yaml

argo-cd:
  dex:
    enabled: false
  server:
    extraArgs:
      - --insecure
    config:
      repositories: |
        - type: helm
          name: argo-cd
          url: https://argoproj.github.io/argo-helm

All available options can be found in the chart values.yaml file. But keep in mind that for our subchart all values must be set below the argo-cd: key.

For this tutorial we override the following values:

  • We disable the dex component that is used for integration with external auth providers

  • We start the server with the --insecure flag to serve the Web UI over http (This is assuming we’re using a local k8s server without TLS setup)

  • We add the Argo CD Helm repository to the repositories list to be used by applications

  • The password for the admin user is set to argocd

Before we install the chart we need to generate a Chart.lock file:

$ helm repo add argo-cd https://argoproj.github.io/argo-helm
$ helm dep update charts/argo-cd/

This will generate two files:

  • Chart.lock

  • charts/argo-cd-4.2.2.tgz

The tgz file is the downloaded dependency and not required in our Git repository, we can therefore exclude it. Argo CD will download the dependencies by itself based on the Chart.lock file.

We exclude it by creating a .gitignore file in the chart directory:

$ echo "charts/" > charts/argo-cd/.gitignore

The chart is now ready to push to our Git repository:

$ git add charts/argo-cd
$ git commit -m 'add argo-cd chart'
$ git push

Installing our Argo CD Helm chart

We install Argo CD manually via the Helm CLI:

$ helm install argo-cd charts/argo-cd/

Accessing the Web UI

The Helm chart doesn’t install an Ingress by default, to access the Web UI we have to port-forward to the argocd-server service:

$ kubectl port-forward svc/argo-cd-argocd-server 8080:443

We can then visit http://localhost:8080 to access it.

The default username is admin. The password is auto-generated and we can get it with:

$ kubectl get secret argocd-initial-admin-secret -o jsonpath="{.data.password}" | base64 -d

After logging in we’ll see the following screen:

A picture of the Argo CD Web UI after logging in for the first time

In practice Argo CD applications could be added through the Web UI or CLI, but since we want to manage everything declaratively we’ll write Application manifests in YAML and put them into our Git repo.

The root app

To add an application to Argo CD we need to add an Application resource to Kubernetes. It specifies the Git repository and the file path under which to find the manifests.

For example, if we wanted to deploy Prometheus we would write an Application manifest for it. It would specify what Helm chart to use and what values to set. We would then apply the Application manifest with kubectl and wait for the resource to be created in the cluster.

However, applying the manifests with kubectl is a manual step that’s error prone and insecure. We would also need to repeat it for every application, not just when adding applications but also when updating them.

With Argo CD there is a way to automate adding Applications by creating an application that implements the app of apps pattern. We call this the “root” application.

The root application has one task: it generates Application manifests for other applications. Argo CD will watch the root application and synchronize any applications that it generates.

With this setup we only have to add one application manually: the root application.

Creating the root app

For the root application we’ll use Helm and create a Helm chart that has Application manifests as templates.

We create it in an apps/ directory and put a Chart.yaml file and an empty values.yaml file in it. In our git repo we run:

$ mkdir -p apps/templates
$ touch apps/values.yaml

apps/Chart.yaml

apiVersion: v2
name: root
version: 1.0.0

We create the Application manifest for our root application in apps/templates/root.yaml. This allows us to do any updates to the root application itself through Argo CD:

apps/templates/root.yaml:

apiVersion: argoproj.io/v1alpha1
kind: Application
metadata:
  name: root
  finalizers:
  - resources-finalizer.argocd.argoproj.io
spec:
  destination:
    server: https://kubernetes.default.svc
    namespace: default
  project: default
  source:
    path: apps/
    repoURL: https://github.com/arthurk/argocd-example-install.git
    targetRevision: HEAD
  syncPolicy:
    automated:
      prune: true
      selfHeal: true

The above Application watches the Helm chart under apps/ (our root application) and synchronizes it if changes were detected.

How does Argo CD know our application is a Helm chart? It looks for a Chart.yaml file under path in the Git repository.

Note: Argo CD will not use helm install to install charts. It will render the chart with helm template and then apply the output with kubectl. This means we can’t run helm list on a local machine to get all installed releases.

To deploy our root application we need to push the files to our Git repository and apply the manifest:

$ git add apps
$ git commit -m 'add root app'
$ git push

$ helm template apps/ | kubectl apply -f -

In the Web UI we can now see that the root application was created successfully:

Argo CD Web UI showing root application

Letting Argo CD manage itself

We previously installed Argo CD with helm install which means that updates to Argo CD itself would require us to run helm upgrade manually. To avoid doing this we can create an Application resource for Argo CD and let it manage itself.

With this approach any updates to our Argo CD deployment can be made by modifying files in our Git repository rather than running manual commands.

We put the application manifest in apps/templates/argo-cd.yaml:

apps/templates/argo-cd.yaml:

apiVersion: argoproj.io/v1alpha1
kind: Application
metadata:
  name: argo-cd
  namespace: default
  finalizers:
  - resources-finalizer.argocd.argoproj.io
spec:
  destination:
    server: https://kubernetes.default.svc
    namespace: default
  project: default
  source:
    path: charts/argo-cd
    repoURL: https://github.com/arthurk/argocd-example-install.git
    targetRevision: HEAD
  syncPolicy:
    automated:
      prune: true
      selfHeal: true

Then push the file to our Git repository:

$ git add apps/templates/argo-cd.yaml
$ git commit -m 'add argo-cd application'
$ git push

In the Web UI we should now see the root application being OutOfSync and Syncing.

If it doesn’t show the application immediately, click the “Refresh” button on the root application. By default it checks for changes in the Git repository every 3 minutes.

Argo CD Web UI overview after the Argo CD application has been created

Once the Argo CD application is synced it can now manage itself and we can delete the previously manually installed (via helm install) installation. The following command will not delete Argo CD from the cluster, only let Helm know that it is not managing Argo CD anymore:

$ kubectl delete secret -l owner=helm,name=argo-cd

When listing helm releases it should now show an empty list:

$ helm list

NAME    NAMESPACE   REVISION    UPDATED STATUS  CHART   APP VERSIONCE   REVISION

Example: Installing Prometheus

To demonstrate how to deploy a Helm chart with Argo CD, we’ll add Prometheus to our cluster.

First we create an Application manifest in apps/templates/prometheus.yaml that uses the Prometheus helm chart.

apps/templates/prometheus.yaml

apiVersion: argoproj.io/v1alpha1
kind: Application
metadata:
  name: prometheus
  namespace: default
  finalizers:
  - resources-finalizer.argocd.argoproj.io
spec:
  destination:
    server: https://kubernetes.default.svc
    namespace: default
  project: default
  source:
    chart: prometheus
    helm:
      values: |
        pushgateway:
          enabled: false
    repoURL: https://prometheus-community.github.io/helm-charts
    targetRevision: 15.6.0
  syncPolicy:
    automated:
      prune: true
      selfHeal: true

Compared to our previously created Argo CD umbrella chart, the differences are:

  • We’re using chart instead of path to install a Helm chart from a different Helm repository

  • The targetRevision is the specific chart version that we want to install

  • The repoURL is set to the prometheus-community Helm chart repository

  • We’re overriding the chart default values to disable the pushgateway

To deploy the application all we have to do is push the manifest to our Git repository:

$ git add apps/templates/prometheus.yaml
$ git commit -m 'add prometheus'
$ git push

Prometheus should show up in the Web UI after the next refresh.

Argo CD Web UI showing application overview after prometheus application has been added

Example: Uninstall Prometheus

To uninstall Prometheus we just have to delete the previously added prometheus.yaml file from out Git repo:

$ git rm apps/templates/prometheus.yaml
$ git commit -m 'remove prometheus'
$ git push

The application will be removed from the cluster after the next refresh.

Conclusion

In this tutorial we’ve installed Argo CD with Helm and set it up so that it can manage itself. Updates to Argo CD can be done by modifying the manifest inside the Git repository and don’t require any manual steps.

We’ve created a root application that uses the app-of-apps pattern to manage our applications in a declarative way.

Applications can be added, updated or removed with Git. As an example we’ve installed Prometheus in our cluster.

All files mentioned in this blog post are available in a Git repository on GitHub.

More details about Argo CD can be found on the project page and the GitHub repository.

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