Developing Applications


In the application development guide we demonstrate how one can integrate functionalities into the Kaapana platform, that do not fit into the structure of processing pipelines. For this purpose we show how we can host a general flask web application within Kaapana.

Deploy a Flask Application on the platform

Aim: In this chapter we deploy a Flask application within the Kaapana platform.

Step 1: Create and run our Flask app locally

As a starting point, we first develop a Flask application and run it locally. The source code of the Hello-World Flask application can be found in the templates_and_examples/examples/services/hello-world/docker/files! In case you have never worked with Flask this tutorial will get you started!

First of all install the requirements.

pip install -r requirements.txt

Now you can try to run the Flask application locally:

flask run

When you go to http://localhost:5000 you should see a hello message!

Since flask run is only for development, we use Gunicorn to run in production. Gunicorn is started via the bash script. To try it please run:

SCRIPT_NAME=/hello-world gunicorn -b :5000 -e SECRET_KEY='test' -e HELLO_WORLD_USER='klaus' -e APPLICATION_ROOT='/hello-world' run:app

Now the application can be accessed via http://localhost:5000/hello-world. As you can see we adapted the application to run on the prefix path /hello-world. A requirement for any application running on our platform is that it runs within its own subpath, otherwise, it is only possible to serve it via http on a specific port.

Step 2: Create a Docker container with the Flask application

First of all we build the docker container with:

sudo docker build -t <docker-registry><docker-repo>/hello-world:0.1.0 .


Depending on your docker registry docker-repo might be not defined: docker-repo='' or the name of the docker repository!

Then check locally if the docker container works as expected:

sudo docker run -p 5000:5000 -e SECRET_KEY='some-secret-key' -e HELLO_WORLD_USER='Kaapana' -e APPLICATION_ROOT='/hello-world' <docker-registry><docker-repo>/hello-world:0.1.0

Again you should be able to access the application via http://localhost:5000/hello-world

Now, we need to push the docker image to the docker registry.

If not already done, log in to the docker registry:

sudo docker login <docker-registry>

and push the docker image with:

sudo docker push <docker-registry><docker-repo>/hello-world:0.1.0

Step 3: Write the Kubernetes deployments

Since the Kaapana platform runs in Kubernetes, we will create a Kubernetes deployment, service and ingress in order to get the application running inside the platform. The following steps will show how to to that:

  • Copy the folder hello-word-chart from kaapana/templates_and_examples/examples/services/hello-world/hello-world-chart to the instance where the platform is running.

  • Log in to the server and go to the templates directory.

  • Inside the hello-world-chart/templates/deployment.yaml file comment the lines containing templated arguments e.g. namespace: "{{ }}"

  • Each of these lines has a commented version with a specific value right next to it enable these lines.

  • Now replace <docker-registry><docker-repo>/hello-world:<version-tag> with your docker registry and the version tag you used, e.g. 0.1.0.

Repeat the same procedure with the service.yaml, you will not need to fill in your registry here. Now you should be able to deploy the platform. Go to the server directory containing the hello-world-chart folder and execute:

kubectl apply -f hello-world-chart/templates/

If everything works well the docker container is started inside the Kubernetes cluster. When going to /hello-world on your platform, you should see the hello kaapana page again. Furthermore you should also be able to see the application running on the port 5000. This is because we specified a NodePort in the service.yaml file.

If the pod started successfully you can also execute:

kubectl get pods -A

Then you should see your pod starting or running!

In order to remove the deployment again execute:

kubectl delete -f hello-world-chart/templates/

Step 4: Write a helm chart

For local testing you can go to the hello-world directory and build the helm chart locally with:

helm package hello-world-chart

This will generate a file called hello-world-chart-0.1.0.tgz, which can be installed on the platform with:

helm install hello-world-chart hello-world-chart-0.1.0.tgz

Alternatively, you can also set your own private registry for helm Install

helm install --set-string global.registry_url=<private-registry> --set-string global.credentials_registry_username=<username> --set-string global.credentials_registry_password=<password>  hello-world-chart hello-world-chart-0.1.0.tgz

Either way, you should have the same result as before when you created the deployment with kubectl. With helm ls you can view all helm releases that are currently running.

In order to remove the chart execute:

helm uninstall hello-world-chart --no-hooks
  • To push the helm chart to a repository you can use the open container interface (oci):

helm push hello-world-chart-0.1.0.tgz oci://<registry><repository>
  • You can install a helm chart from a repository py pulling the image and installing it to your cluster afterwards:

helm pull oci://<registry><repository>/hello-world-chart --version 0.1.0
helm install hello-world-chart hello-world-chart-0.1.0.tgz

Also here the chart can be deleted again with:

helm uninstall hello-world-chart

Step 5: Provide the application as an extension

You can also add the Flask application as an extension to the Kaapana platform. To do so follow the steps described in Option 2: Add the extension to the file system of the host machine or Option 3: (Persistent alternative) Build the platform.