Managing Applications in Kubernetes With the Carvel Kapp Controller


Any usual organization-grade application deployed on Kubernetes includes numerous API means that have to have to be deployed together. For case in point, the WordPress software, which is 1 of the case in point applications obtainable on the Kubernetes GitHub repository, incorporates:

  • a wordpress frontend pod,
  • a wp-pv-assert persistent quantity declare mounted to the frontend pod,
  • a wordpress-mysql MySQL databases pod,
  • a mysql-pv-assert persistent quantity declare mounted to the MySQL database pod,
  • two persistent volumes: wordpress-pv-1 and wordpress-pv-2 to serve the persistent quantity claims,
  • providers for the databases and frontend pods.

Software (or application) is not a native construct in Kubernetes. Nevertheless, handling programs is the main concern of the developers and functions. Software shipping on Kubernetes involves upgrading, downgrading, and customizing the unique API methods. Kubernetes permits you to restrict the distribute of your software sources by namespaces this kind of that you can deploy an whole application in a namespace that can be deleted or established. Having said that, a advanced software may well consist of means unfold across namespaces, and in this kind of scenarios answering the adhering to issues may well be a obstacle:

  • How numerous applications are working in a namespace?
  • Was the upgrade of all the assets of the app prosperous?
  • Which varieties of resources are linked with my app?

The kapp Instrument

The kapp (Kubernetes Application) instrument is 1 of the instruments readily available in the Carvel toolkit. kapp permits consumers to team a established of methods (assets with the identical label) as an application. In addition, kapp manages the individual methods of the application so that the buyers only have to run at the amount of programs via the kapp CLI.

Helm (particularly Helm3) is an option to the kapp resource. Just like Helm, kapp can complete a stateful improve of the software. Even so, Kapp does not have templating abilities like Helm, a ability that is fulfilled by the YTT instrument in the Carvel toolkit. By combining kapp with the kapp-controller, you can make neat GitOps workflows, which takes it significantly further than the capabilities of Helm.

To fully grasp kapp much better, let us install it on our technique with the following command:

wget -O- > set
sudo bash set

The former command will install all the equipment of the Carvel toolkit, these types of as YTTl, imgpkg, etc., on your method. Please investigate the usage of these instruments on your have. Every single software of the Carvel toolkit is impartial and normally takes tiny area on your program.

I wrote a simple .Web application that contains a frontend and a Redis database that we will use for screening kapp. You should experience free of charge to use the application for your demos or understanding purposes. You can use the all-in-a single.yaml manifest to put in all the components of the software to your cluster. Let’s now use kapp to install the parts in the sort of an application named az-vote.

kapp deploy --application=az-vote -f person.yaml

On executing the command, you will see the checklist of the elements that kapp will generate as follows:

List of Changes Identified by kapp
Record of changes determined by kapp

If you sort “y”, kapp will annotate your resources to url them to the app and later on control them. Dependent on the difference between the actual and ideal condition, it will update or delete methods and give you the overall standing of your application. The next screenshot demonstrates the annotations extra by kapp to the pods of the application:

Labels Applied by kapp
Labels applied by kapp

You can check out the applications in the cluster utilizing the kapp record command as follows:

List Apps in The Cluster
List applications in the cluster

To check out the record of resources that make up the software, use the kapp examine command as follows:

Inspecting Application
Inspect application

You will notice that some of the objects such as Endpoints and EndpointSlices that Kubernetes produces below the hood when you determine a Services are involved in the command output. The output structure would make it straightforward to study the achievement and failure states of all the application assets.

We will now introduce the kapp-controller to automate the deployment of the software. Having said that, right before we do so, delete the application by jogging the next command:

kapp delete --app=az-vote

The kapp-controller

When you bundle your application as a established of atomically managed means with perfectly-defined names (e.g., the components of the Azure Vote App), you to some degree make a CustomResourceDefinition (CRD). CRDs are usually utilized by Kubernetes operators that allow Kubernetes to act in the ‘right’ way when the assets are created, ruined, or up to date. In fact, you can currently discover a number of CRDs mounted on your Kubernetes clusters, which can be outlined by employing the kubectl get crd command.

The kapp-controller tool can take a kapp software and augments it with automation capabilities this kind of as GitOps. The tool allows you to create a entirely automatic software system that can routinely deploy and regulate 100s of applications without having necessitating any handbook or automated enter. Tools these as Helm and kapp-controller eradicate the need to have to build operators to regulate the lifecycle of most of the business enterprise applications which really don’t have to have to interact with the Kubernetes API.

Let us install kapp-controller to the cluster by making use of kapp itself as follows:

kapp deploy -a kc -f
kapp deploy -a default-ns-rbac -f

The to start with command installs the kapp-controller as an software on the cluster. The next command results in an RBAC rule to give total manage of the default namespace to the kapp-controller. The permissions enable the kapp-controller to generate, update, and delete Kubernetes sources in the namespace.

Now that our kapp-controller is jogging in the cluster, we can use it to automatically deploy our software from the specs readily available in our Git repository. To do so, we will build a kapp CR to determine our software. The kapp CR is effectively comprehended by the kapp-controller. The specification structure of a kapp software is offered on the Carvel docs web site.

Generate an application specification for the Azure Vote App, which will operate as a thoroughly kapp controlled software as follows:

type: Application
  identify: azure-vote-application
  namespace: default
  serviceAccountName: default-ns-sa
    - git:
        ref: origin/primary
        subPath: kubernetes-manifests/
    - ytt:
    - kapp: 

Pulling the application specs from Git is one of the supported approaches to produce an software. You can also instruct the kapp-controller to fetch the software specs from an HTTP locale, a Helm chart, a container registry, etc. By default, the kapp-controller will look at the resource of the application (HTTP site, Helm chart, and so forth.) just about every moment and consider to reconcile the state of the software in the cluster with the point out specified in the resource.

Deploy the software to the cluster by managing the command kubectl use -f . Afterward, operate the kapp list command to listing the apps in your cluster as follows:

List of Apps in The Cluster
Listing of apps in the cluster

Let us examine the assets of the software as follows:

List of Resources of the azure-vote-app-ctrl App
List of assets of the azure-vote-app-ctrl app

We have now integrated our software into a CI/CD system that can be managed solely inside of Kubernetes. We can now assign the accountability to post and maintain the CRDs for the applications to the builders and let the kapp controller operator deploy and manage the programs in the default namespace.

The kapp-controller put in the CRDs for the kapp applications in the cluster for us, which now empower us to use kubectl instructions to manipulate the purposes just like other native Kubernetes objects as follows:

Use kubectl with kapp
Use kubectl with kapp


We labored with two of the most well-known tools in the Carvel toolkit: kapp, and kapp-controller, which support us do the job with apps as an alternative of their individual factors. In addition, we noted that the kapp-controller can assistance us package our programs in a stateful way without the need of demanding us to publish any customized operators. Ultimately, we made use of the kapp instruments to carry out an operator-like deployment design of the Azure Vote App and realized that the kapp-controller can provide most of the rewards of the operators in a fraction of the time it can take to apply an operator.


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