Using ApplicationSet with Matrix Generator and define individual Namespaces

OpenShift GitOps

- Thomas Jungbauer Thomas Jungbauer ( Lastmod: 2025-04-17 ) - 6 min read

During my day-to-day business, I am discussing the following setup with many customers: Configure App-of-Apps. Here I try to explain how I use an ApplicationSet that watches over a folder in Git and automatically adds a new Argo CD Application whenever a new folder is found. This works great, but there is a catch: The ApplicationSet uses the same Namespace default for all Applications. This is not always desired, especially when you have different teams working on different Applications.

Recently I was asked by the customer if this can be fixed and if it is possible to define different Namespaces for each Application. The answer is yes, and I would like to show you how to do this.

The Current Situation

Currently, I am (or was) using the following ApplicationSet to watch over a folder in Git. The ApplicationSet uses the Matrix Generator to create a new Argo CD Application for each folder found in the Git repository. It also uses the list operator to define the targetCluster:

    generatormatrix:
          # Git: Walking through the specific folder and take whatever is there.
          - git:
              directories:
                - path: clusters/management-cluster/*
              repoURL: *repourl
              revision: *branch
          # List: simply define the targetCluster. The name of the cluster must be known by Argo CD
          - list:
              elements:
                  # targetCluster is important, this will define on which cluster it will be rolled out.
                  # The cluster name must be known in Argo CD
                - targetCluster: *mgmtclustername

This will create a new Application for any subfolder found in the clusters/management-cluster/ folder any every Application in Argo CD will be configured with the same target namespace: default.

Technically, this is not a problem, as I define the exact namespace in the different Helm Charts, but it is not always desired.

I personally recommend defining the Namespace in the Helm Charts, since especially for the cluster configuration, sometimes there is no clear target Namespace or multiple Namespaces are modified.

What did not work

The first idea was to use the Matrix generator and define the targetNamespace in list.elements and if the namespace is not defined, use a default one. So similar like this:

    generatormatrix:
          # Git: Walking through the specific folder and take whatever is there.
          - git:
              directories:
                - path: clusters/management-cluster/*
              repoURL: *repourl
              revision: *branch
          # List: simply define the targetCluster. The name of the cluster must be known by Argo CD
          - list:
              elements:
                  # targetCluster is important, this will define on which cluster it will be rolled out.
                  # The cluster name must be known in Argo CD
                - targetCluster: *mgmtclustername
                  path: clusters/management-cluster/cert-manager
                  targetNamespace: cert-manager
                - targetCluster: *mgmtclustername
                  path: clusters/management-cluster/*
                  targetNamespace: default

To make is short: This does not work

The matrix operator walks over all folders and creates a cartesian product of the elements. This means, it will create a new Application for each folder and each element in the list. So if you have 10 folders and 2 elements in the list, you will end up with 20 Applications. This is not what we want. We want to create a new Application for each folder and define the targetNamespace in the Git repository.

The second test was the use of the Merge generator. This did not work as well, as it was not possible to define a default Namespace.

The Solution

The solution is to use the Git FILES generator and define the targetNamespace in the Git repository. This is done by creating a file called config.json in each subfolder. The content of the file is simple:

{
  "namespace": "default",
  "environment": "in-cluster"
}

The advantage is that it is possible to define multiple parameters in that file. However, the disadvantage is that this file must be created, otherwise the ApplicationSet will ignore the folder and will not create a new Application. I think this is a small disadvantage, and the file is easy to maintain.

Bringing everything together now opens two possibilities:

This will require helper-argocd version 2.0.41 or higher.

Option 1: Keep Matrix Generator and use Git File sub-generator

The first option simply replaces the git directory generator with the git file generator. The rest of the ApplicationSet remains unchanged.

I like this option somehow better than the second one, because I can keep everything as I had it before, the only thing is to create the config.json file in each subfolder and change two lines in the ApplicationSet.
    # Switch to set the namespace to '.namespace' ... must be defined in config.json
    use_configured_namespace: true (1)

    # Definition of Matrix Generator. Only 2 generators are supported at the moment
    generatormatrix:
      # Git: Walking through the specific folder and take whatever is there.
      - git:
          files: (2)
            - path: clusters/management-cluster/**/config.json (3)
          repoURL: *repourl
          revision: *branch
      # List: simply define the targetCluster. The name of the cluster must be known by Argo CD
      - list:
          elements:
              # targetCluster is important, this will define on which cluster it will be rolled out.
              # The cluster name must be known in Argo CD
            - targetCluster: *mgmtclustername
1Switch to use the configured namespace. This is important, otherwise the namespace is set to "default". This was added for backward compatibility.
2The git file generator is used instead of the git directory generator.
3The path is changed to the config.json file. The ** is important, as it defines to look into every subfolder.

The config.json can be shortened to:

{
  "namespace": "default"
}

Option 2: Switch to plain Git File Generator

The second option is to switch to the plain Git generator. This removes the Matrix generator, but also requires defining the targetCluster in the config.json file. This is not a problem, as the config.json file can be used to define multiple parameters.

    generatorgit: (1)
      # Git: Walking through the specific folder and take whatever is there.
      - files:
          - clusters/management-cluster/**/config.json
        repourl: *repourl
        revision: *branch
1No Matrix but Git generator instead.

Here the full config.json file is required, otherwise the targetCluster is not defined:

{
  "namespace": "default",
  "environment": "in-cluster"
}

Full working example

Source: https://github.com/tjungbauer/openshift-clusterconfig-gitops/blob/main/base/argocd-resources-manager/values.yaml

applicationsets:
  ######################################
  # MATRIX GENERATOR EXAMPLE Git Files #
  ######################################
  # The idea behind the GIT Generate (File) is to walk over a folder, for example /clusters/management-cluster and fetch a config.json from each folder.
  # This is more or less similar as the Matrix generator (see below), but reqires a bit more configuration ... the config.json.
  # The advantage is that you can configure individual namespaces for example in this config.json and provide an additional information
  mgmt-cluster-matrix-gitfiles:
    enabled: true

    # Description - always usful
    description: "ApplicationSet that Deploys on Management Cluster Configuration (using Git Generator)"
    # Any labels you would like to add to the Application. Good to filter it in the Argo CD UI.
    labels:
      category: configuration
      env: mgmt-cluster

    # Using go text template. See: https://argo-cd.readthedocs.io/en/stable/operator-manual/applicationset/GoTemplate/
    goTemplate: true
    argocd_project: *mgmtclustername

    environment: *mgmtclustername

    # preserve all resources when the application get deleted. This is useful to keep that workload even if Argo CD is removed or severely changed.
    preserveResourcesOnDeletion: true

    # Switch to set the namespace to '.namespace' ... must be defined in config.json
    use_configured_namespace: true

    # Definition of Matrix Generator. Only 2 generators are supported at the moment
    generatormatrix:
      # Git: Walking through the specific folder and take whatever is there.
      - git:
          files:
            - path: clusters/management-cluster/**/config.json
          repoURL: *repourl
          revision: *branch
      # List: simply define the targetCluster. The name of the cluster must be known by Argo CD
      - list:
          elements:
              # targetCluster is important, this will define on which cluster it will be rolled out.
              # The cluster name must be known in Argo CD
            - targetCluster: *mgmtclustername

    syncPolicy:
      autosync_enabled: false

    # Retrying in case the sync failed.
    retries:
      # number of failed sync attempt retries; unlimited number of attempts if less than 0
      limit: 5
      backoff:
        # the amount to back off. Default unit is seconds, but could also be a duration (e.g. "2m", "1h")
        # Default: 5s
        duration: 5s
        # a factor to multiply the base duration after each failed retry
        # Default: 2
        factor: 2
        # the maximum amount of time allowed for the backoff strategy
        # Default: 3m
        maxDuration: 3m

Conclusion

In this blog post I have shown you how to use the ApplicationSet with the Matrix generator and define individual Namespaces for each Application. This is done by using the Git File generator and defining a config.json file in each subfolder. The config.json file can be used to define multiple parameters, but it is required to create the file in each subfolder. This is a small disadvantage, but I think it is worth the effort. The advantage is that you can define individual Namespaces for each Application, and you can use the same ApplicationSet for all your Applications.

I hope this blog post was helpful and you learned something new. If you have any questions or comments, please feel free to reach out to me. I am happy to help you.

See Also:

  • Introducing AdminNetworkPolicies –

    Classic Kubernetes/OpenShift offer a feature called NetworkPolicy that allows users to control the traffic to and from their assigned Namespace. NetworkPolicies are designed to give project owners or tenants the ability to protect their own namespace. Sometimes, however, I worked with customers where the cluster administrators or a dedicated (network) team need to enforce these policies. Since the NetworkPolicy API is namespace-scoped, it is not possible to enforce policies across namespaces. The only solution was to create custom (project) admin and edit roles, and remove the ability of creating, modifying or deleting NetworkPolicy objects. Technically, this is possible and easily done. But shifts the whole network security to cluster administrators. Luckily, this is where AdminNetworkPolicy (ANP) and BaselineAdminNetworkPolicy (BANP) comes into play.

  • Using Kustomize to post render a Helm Chart –

    Lately I came across several issues where a given Helm Chart must be modified after it has been rendered by Argo CD. Argo CD does a helm template to render a Chart. Sometimes, especially when you work with Subcharts or when a specific setting is not yet supported by the Chart, you need to modify it later …​ you need to post-render the Chart. In this very short article, I would like to demonstrate this on a real-live example I had to do. I would like to inject annotations to a Route objects, so that the certificate can be injected. This is done by the cert-utils operator. For the post-rendering the Argo CD repo pod will be extended with a sidecar container, that is watching for the repos and patches them if required.

  • Managing Certificates using GitOps approach –

    The article SSL Certificate Management for OpenShift on AWS explains how to use the Cert-Manager Operator to request and install a new SSL Certificate. This time, I would like to leverage the GitOps approach using the Helm Chart cert-manager I have prepared to deploy the Operator and order new Certificates. I will use an ACME Letsencrypt issuer with a DNS challenge. My domain is hosted at AWS Route 53. However, any other integration can be easily used.

  • Update Cluster Version using GitOps approach –

    During a GitOps journey at one point, the question arises, how to update a cluster? Nowadays it is very easy to update a cluster using CLI or WebUI, so why bother with GitOps in that case? The reason is simple: Using GitOps you can be sure that all clusters are updated to the correct, required version and the version of each cluster is also managed in Git. All you need is the channel you want to use and the desired cluster version. Optionally, you can define the exact image SHA. This might be required when you are operating in a restricted environment.

  • Multiple Sources for Applications in Argo CD –

    Argo CD or OpenShift GitOps uses Applications or ApplicationSets to define the relationship between a source (Git) and a cluster. Typically, this is a 1:1 link, which means one Application is using one source to compare the cluster status. This can be a limitation. For example, if you are working with Helm Charts and a Helm repository, you do not want to re-build (or re-release) the whole chart just because you made a small change in the values file that is packaged into the repository. You want to separate the configuration of the chart with the Helm package. The most common scenarios for multiple sources are (see: Argo CD documentation): Your organization wants to use an external/public Helm chart You want to override the Helm values with your own local values You don’t want to clone the Helm chart locally as well because that would lead to duplication and you would need to monitor it manually for upstream changes. This small article describes three different ways with a working example and tries to cover the advantages and disadvantages of each of them. They might be opinionated but some of them proved to be easier to use and manage.

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