Write your first Kubernetes charm for a Flask app

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Imagine you have a Flask application backed up by a database such as PostgreSQL and need to deploy it. In a traditional setup, this can be quite a challenge, but with Juju you’ll find yourself deploying, configuring, scaling, integrating, monitoring, etc., your Flask application in no time. Let’s get started!

In this tutorial we will build a rock and Kubernetes charm for a Flask application using the charm SDK, so we can have a Flask application up and running with Juju in about 90 minutes.

:open_book: rock
An Ubuntu LTS-based OCI compatible container image designed to meet security, stability, and reliability requirements for cloud-native software.

:open_book: charm

A package consisting of YAML files + Python code that will automate every aspect of an application’s lifecycle so it can be easily orchestrated with Juju.

:open_book: Juju
An orchestration engine for software operators that enables the deployment, integration and lifecycle management of applications using charms.

What you’ll need:

  • A workstation, e.g., a laptop, with amd64 or arm64 architecture which has sufficient resources to launch a virtual machine with 4 CPUs, 4 GB RAM, and a 50 GB disk
  • Familiarity with Linux

What you’ll do:

At any point, to give feedback or ask for help:

Don’t hesitate to get in touch on Matrix or Discourse (or follow the “Help improve this document in the forum” on the bottom of this doc to comment directly on the doc).

Set things up

Install Multipass.

See more: Multipass | How to install Multipass

Use Multipass to launch an Ubuntu VM with the name charm-dev from the 24.04 blueprint:

multipass launch --cpus 4 --disk 50G --memory 4G --name charm-dev 24.04

Once the VM is up, open a shell into it:

multipass shell charm-dev

In order to create the rock, you’ll need to install Rockcraft:

sudo snap install rockcraft --classic

LXD will be required for building the rock. Make sure it is installed and initialised:

sudo snap install lxd
lxd init --auto

In order to create the charm, you’ll need to install Charmcraft:

sudo snap install charmcraft --channel latest/edge --classic

This tutorial requires version 3.0.0 or later of Charmcraft. Check the version of Charmcraft using charmcraft --version If you have an older version of Charmcraft installed, use sudo snap refresh charmcraft --channel latest/edge to get the latest edge version of Charmcraft.

MicroK8s is required to deploy the Flask application on Kubernetes. Install MicroK8s:

sudo snap install microk8s --channel 1.31-strict/stable
sudo adduser $USER snap_microk8s
newgrp snap_microk8s

Wait for MicroK8s to be ready using sudo microk8s status --wait-ready. Several MicroK8s add-ons are required for deployment:

sudo microk8s enable hostpath-storage
# Required to host the OCI image of the Flask application
sudo microk8s enable registry
# Required to expose the Flask application
sudo microk8s enable ingress

Juju is required to deploy the Flask application. Install Juju and bootstrap a development controller:

sudo snap install juju --channel 3.5/stable
mkdir -p ~/.local/share
juju bootstrap microk8s dev-controller

Finally, create a new directory for this tutorial and go inside it:

mkdir flask-hello-world
cd flask-hello-world

Create the Flask application

Start by creating the “Hello, world” Flask application that will be used for this tutorial.

Create a requirements.txt file, copy the following text into it and then save it:

Flask

In the same directory, copy and save the following into a text file called app.py:

import flask

app = flask.Flask(__name__)

@app.route("/")
def index():
    return "Hello, world!\n"

if __name__ == "__main__":
    app.run()

Run the Flask application locally

Install python3-venv and create a virtual environment:

sudo apt-get update && sudo apt-get install python3-venv -y
python3 -m venv .venv
source .venv/bin/activate
pip install -r requirements.txt

Now that we have a virtual environment with all the dependencies, let’s run the Flask application to verify that it works:

flask run -p 8000

Test the Flask application by using curl to send a request to the root endpoint. You may need a new terminal for this; if you are using Multipass use multipass shell charm-dev to get another terminal:

curl localhost:8000

The Flask application should respond with Hello, world!. The Flask application looks good, so we can stop for now using Ctrl + c.

Pack the Flask application into a rock

First, we’ll need a rockcraft.yaml file. Rockcraft will automate its creation and tailoring for a Flask application by using the flask-framework profile:

rockcraft init --profile flask-framework

The rockcraft.yaml file will automatically be created and set the name based on your working directory. Open the file in a text editor and check that the name is flask-hello-world. Ensure that platforms includes the architecture of your host. For example, if your host uses the ARM architecture, include arm64 in platforms.

For this tutorial, we’ll use the name “flask-hello-world” and assume you are on the amd64 platform. Check the architecture of your system using dpkg --print-architecture. Choosing a different name or running on a different platform will influence the names of the files generated by Rockcraft.

Pack the rock:

rockcraft pack

Depending on your system and network, this step can take a couple of minutes to finish.

Once Rockcraft has finished packing the Flask rock, you’ll find a new file in your working directory with the .rock extension:

ls *.rock -l

If you changed the name or version in rockcraft.yaml or are not on an amd64 platform, the name of the .rock file will be different for you.

The rock needs to be copied to the Microk8s registry so that it can be deployed in the Kubernetes cluster:

rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false \
   oci-archive:flask-hello-world_0.1_amd64.rock \
   docker://localhost:32000/flask-hello-world:0.1

See more: skopeo^

Create the charm

Create a new directory for the charm and go inside it:

mkdir charm
cd charm

We’ll need a charmcraft.yaml, requirements.txt and source code for the charm. The source code contains the logic required to operate the Flask application. Charmcraft will automate the creation of these files by using the flask-framework profile:

charmcraft init --profile flask-framework --name flask-hello-world

The files will automatically be created in your working directory. Pack the charm:

charmcraft pack

Depending on your system and network, this step can take a couple of minutes to finish.

Once Charmcraft has finished packing the charm, you’ll find a new file in your working directory with the .charm extension:

ls *.charm -l

If you changed the name in charmcraft.yaml or are not on the amd64 platform, the name of the .charm file will be different for you.

Deploy the Flask application

A Juju model is needed to deploy the application. Let’s create a new model:

juju add-model flask-hello-world

If you are not on a host with the amd64 architecture, you will need to include a constraint to the Juju model to specify your architecture. For example, for the arm64 architecture, use juju set-model-constraints -m flask-hello-world arch=arm64. Check the architecture of your system using dpkg --print-architecture.

Now the Flask application can be deployed using Juju:

juju deploy ./flask-hello-world_ubuntu-22.04-amd64.charm \
   flask-hello-world --resource \
   flask-app-image=localhost:32000/flask-hello-world:0.1

It will take a few minutes to deploy the Flask application. You can monitor the progress using juju status --watch 5s. Once the status of the App has gone to active, you can stop watching using ctrl + c.

See more: Command ‘juju status’

The Flask application should now be running. We can monitor the status of the deployment using juju status which should be similar to the following output:

Model              Controller      Cloud/Region        Version  SLA          Timestamp
flask-hello-world  dev-controller  microk8s/localhost  3.1.8    unsupported  17:04:11+10:00

App           Version  Status  Scale  Charm              Channel  Rev  Address         Exposed  Message
flask-hello-world      active      1  flask-hello-world             0  10.152.183.166  no       

Unit             Workload  Agent  Address      Ports  Message
flask-hello-world/0*  active    idle   10.1.87.213

The deployment is finished when the status shows active. Let’s expose the application using ingress. Deploy the nginx-ingress-integrator charm and integrate it with the Flask app:

juju deploy nginx-ingress-integrator --channel=latest/edge
juju integrate nginx-ingress-integrator flask-hello-world

The hostname of the app needs to be defined so that it is accessible via the ingress. We will also set the default route to be the root endpoint:

juju config nginx-ingress-integrator \
   service-hostname=flask-hello-world path-routes=/

Monitor juju status until everything has a status of active. Test the deployment using curl http://flask-hello-world --resolve flask-hello-world:80:127.0.0.1 to send a request via the ingress to the root endpoint. It should still be returning the Hello, world! greeting.

The --resolve flask-hello-world:80:127.0.0.1 option to the curl command is a way of resolving the hostname of the request without setting a DNS record.

Configure the Flask application

Now let’s customise the greeting using a configuration option. We will expect this configuration option to be available in the Flask app configuration under the keyword GREETING. Go back out to the root directory of the project using cd .. and copy the following code into app.py:

import flask

app = flask.Flask(__name__)
app.config.from_prefixed_env()


@app.route("/")
def index():
    greeting = app.config.get("GREETING", "Hello, world!")
    return f"{greeting}\n"


if __name__ == "__main__":
    app.run()

Open rockcraft.yaml and update the version to 0.2. Run rockcraft pack again, then upload the new OCI image to the MicroK8s registry:

rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false \
   oci-archive:flask-hello-world_0.2_amd64.rock \
   docker://localhost:32000/flask-hello-world:0.2

Change back into the charm directory using cd charm. The flask-framework Charmcraft extension supports adding configurations to charmcraft.yaml which will be passed as environment variables to the Flask application. Add the following to the end of the charmcraft.yaml file:

config:
  options:
    greeting:
      description: |
        The greeting to be returned by the Flask application.
      default: "Hello, world!"
      type: string

Configuration options are automatically capitalised and - are replaced by _. A FLASK_ prefix will also be added which will let Flask identify which environment variables to include when running app.config.from_prefixed_env() in app.py.

Run charmcraft pack again. The deployment can now be refreshed to make use of the new code:

juju refresh flask-hello-world \
   --path=./flask-hello-world_ubuntu-22.04-amd64.charm \
   --resource flask-app-image=localhost:32000/flask-hello-world:0.2

For the refresh command, the --constraints option is not required if you are not running on an amd64 host as Juju will remember the constraint for the life of the application deployment.

Wait for juju status to show that the App is active again. Verify that the new configuration has been added using juju config flask-hello-world | grep -A 6 greeting: which should show the configuration option.

The grep command extracts a portion of the configuration to make it easier to check whether the configuration option has been added.

Using curl http://flask-hello-world --resolve flask-hello-world:80:127.0.0.1 shows that the response is still Hello, world! as expected. The greeting can be changed using Juju:

juju config flask-hello-world greeting='Hi!'

curl http://flask-hello-world --resolve flask-hello-world:80:127.0.0.1 now returns the updated Hi! greeting.

It might take a short time for the configuration to take effect.

Integrate with a database

Now let’s keep track of how many visitors your application has received. This will require integration with a database to keep the visitor count. This will require a few changes:

  • We will need to create a database migration that creates the visitors table
  • We will need to keep track how many times the root endpoint has been called in the database
  • We will need to add a new endpoint to retrieve the number of visitors from the database

The charm created by the flask-framework extension will execute the migrate.py script if it exists. This script should ensure that the database is initialised and ready to be used by the application. We will create a migrate.py file containing this logic.

Go back out to the tutorial root directory using cd ... Open the migrate.py file using a text editor and paste the following code into it:

import os

import psycopg2


DATABASE_URI = os.environ["POSTGRESQL_DB_CONNECT_STRING"]


def migrate():
    with psycopg2.connect(DATABASE_URI) as conn, conn.cursor() as cur:
        cur.execute("""
            CREATE TABLE IF NOT EXISTS visitors (
                timestamp TIMESTAMP NOT NULL,
                user_agent TEXT NOT NULL
            );
        """)
        conn.commit()


if __name__ == "__main__":
    migrate()

The charm will pass the Database connection string in the POSTGRESQL_DB_CONNECT_STRING environment variable once postgres has been integrated with the charm.

Open the rockcraft.yaml file in a text editor and update the version to 0.3.

To be able to connect to postgresql from the Flask app the psycopg2-binary dependency needs to be added in requirements.txt. The app code also needs to be updated to keep track of the number of visitors and to include a new endpoint to retrieve the number of visitors to the app. Open app.py in a text editor and replace its contents with the following code:

import datetime
import os

import flask
import psycopg2

app = flask.Flask(__name__)
app.config.from_prefixed_env()

DATABASE_URI = os.environ["POSTGRESQL_DB_CONNECT_STRING"]


@app.route("/")
def index():
    with psycopg2.connect(DATABASE_URI) as conn, conn.cursor() as cur:
        user_agent = flask.request.headers.get('User-Agent')
        timestamp = datetime.datetime.now()
        
        cur.execute(
            "INSERT INTO visitors (timestamp, user_agent) VALUES (%s, %s)",
            (timestamp, user_agent)
        )
        conn.commit()


    greeting = app.config.get("GREETING", "Hello, world!")
    return f"{greeting}\n"


@app.route("/visitors")
def visitors():
    with psycopg2.connect(DATABASE_URI) as conn, conn.cursor() as cur:
        cur.execute("SELECT COUNT(*) FROM visitors")
        total_visitors = cur.fetchone()[0]

    return f"{total_visitors}\n"


if __name__ == "__main__":
    app.run()

Run rockcraft pack and upload the newly created rock to the MicroK8s registry:

rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false \
   oci-archive:flask-hello-world_0.3_amd64.rock \
   docker://localhost:32000/flask-hello-world:0.3

Go back into the charm directory using cd charm. The Flask app now requires a database which needs to be declared in the charmcraft.yaml file. Open charmcraft.yaml in a text editor and add the following section to the end:

requires:
  postgresql:
    interface: postgresql_client
    optional: false

Pack the charm using charmcraft pack and refresh the deployment using Juju:

juju refresh flask-hello-world \
   --path=./flask-hello-world_ubuntu-22.04-amd64.charm \
   --resource flask-app-image=localhost:32000/flask-hello-world:0.3

Deploy postgresql-k8s using Juju and integrate it with flask-hello-world:

juju deploy postgresql-k8s --trust
juju integrate flask-hello-world postgresql-k8s

Wait for juju status to show that the App is active again. Running curl http://flask-hello-world --resolve flask-hello-world:80:127.0.0.1 should still return the Hi! greeting. To check the total visitors, use curl http://flask-hello-world/visitors --resolve flask-hello-world:80:127.0.0.1 which should return 1 after the previous request to the root endpoint and should be incremented each time the root endpoint is requested. If we perform another request to curl http://flask-hello-world --resolve flask-hello-world:80:127.0.0.1, curl http://flask-hello-world/visitors --resolve flask-hello-world:80:127.0.0.1 will return 2.

Clean up environment

We’ve reached the end of this tutorial. We have created a Flask application, deployed it locally, exposed it via ingress and integrated it with a database!

If you’d like to reset your working environment, you can run the following in the root directory for the tutorial:

# exit and delete the virtual environment
deactivate
rm -rf charm .venv __pycache__
# delete all the files created during the tutorial
rm flask-hello-world_0.1_amd64.rock flask-hello-world_0.2_amd64.rock \
   flask-hello-world_0.3_amd64.rock rockcraft.yaml app.py \
   requirements.txt migrate.py
# Remove the juju model
juju destroy-model flask-hello-world --destroy-storage

If you created an instance using Multipass, you can also clean it up. Start by exiting it:

exit

And then you can proceed with its deletion:

multipass delete charm-dev
multipass purge

Next steps

If you are wondering… visit…
“How do I…?” SDK How-to docs
“How do I debug?” Charm debugging tools
“What is…?” SDK Reference docs
“Why…?”, “So what?” SDK Explanation docs

Contributors: @econley, @jdkandersson , @tmihoc, @weii-wang

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