Using and Creating ServiceTemplates: Creating and Deploying Applications#

Deploying an application, like deploying a cluster, involves applying a template to the management cluster. Rather than a ClusterTemplate, however, applications are deployed using a ServiceTemplate.

You can find more information on Bringing Your Own Templates in the Template Guide, but this section gives you an idea of how to create a ServiceTemplate and use it to deploy an application to a k0rdent child cluster.

The basic sequence looks like this:

Define the source of the Helm chart that defines the service. The source object must have the label k0rdent.mirantis.com/managed: "true". For example, this YAML describes a custom Source object of kind HelmRepository:

apiVersion: source.toolkit.fluxcd.io/v1
kind: HelmRepository
metadata:
  name: custom-templates-repo
  namespace: kcm-system
  labels:
    k0rdent.mirantis.com/managed: "true"
spec:
  insecure: true
  interval: 10m0s
  provider: generic
  type: oci
  url: oci://ghcr.io/external-templates-repo/charts

Create the ServiceTemplate

A template can either define a Helm chart directly using the template's spec.helm.chartSpec field or reference its location using the spec.helm.chartRef field.

For example:
```
apiVersion: k0rdent.mirantis.com/v1alpha1
kind: ServiceTemplate
metadata:
  name: project-ingress-nginx-4.11.3
  namespace: my-target-namespace
spec:
  helm:
    chartSpec:
      chart: ingress-nginx
      version: 4.11.3
      interval: 10m0s
      sourceRef:
        kind: HelmRepository
        name: k0rdent-catalog
```
In this case, we're creating a ServiceTemplate called ingress-nginx-4.11.3 in the my-target-namespace namespace. It references version 4.11.3 of the ingress-nginx chart located in the k0rdent-catalog Helm repository.

For more information on creating templates, see the Template Guide.
Create a ServiceTemplateChain

To let k0rdent know where this ServiceTemplate can and can't be used, create a ServiceTemplateChain object, as in:
```
apiVersion: k0rdent.mirantis.com/v1alpha1
kind: ServiceTemplateChain
metadata:
  name: project-ingress-nginx-4.11.3
  namespace: my-target-namespace
spec:
  supportedTemplates:
    - name: project-ingress-nginx-4.11.3
    - name: project-ingress-nginx-4.10.0
      availableUpgrades:
        - name: project-ingress-nginx-4.11.3
```
Here you see a template called project-ingress-nginx-4.11.3 that is meant to be deployed in the my-target-namespace namespace. The .spec.helm.chartSpec specifies the name of the Helm chart and where to find it, as well as the version and other important information. The ServiceTempateChain shows that this template is also an upgrade path from version 4.10.0.

If you wanted to deploy this as an application, you would first go ahead and add it to the cluster in which you were working, so if you were to save this YAML to a file called project-ingress.yaml you could run this command on the management cluster:
```
kubectl apply -f project-ingress.yaml -n my-target-namespace
```

Adding a Service to a ClusterDeployment

To add the service defined by this template to a cluster, you would simply add it to the ClusterDeployment object when you create it, as in:

apiVersion: k0rdent.mirantis.com/v1alpha1
kind: ClusterDeployment
metadata:
  name: my-cluster-deployment
  namespace: tenant42
spec:
  config:
    clusterLabels: {}
  template: aws-standalone-cp-0-1-0
  credential: aws-credential
  serviceSpec:
    services:
      - template: project-ingress-nginx-4.11.3
        name: ingress-nginx
        namespace: my-target-namespace
    priority: 100

As you can see, you're simply referencing the template in the .spec.serviceSpec.services[].template field of the ClusterDeployment to tell k0rdent that you want this service to be part of this cluster.

If you wanted to add this service to an existing cluster, you would simply patch the definition of the ClusterDeployment, as in:

kubectl patch clusterdeployment my-cluster-deployment -n my-target-namespace --type='merge' -p '{"spec":{"services":[{"template":"project-ingress-nginx-4.11.3","name":"ingress-nginx","namespace":"my-target-namespace"}]}}'

For more information on creating and using ServiceTemplate objects, see the User Guide.