Write your first Kubernetes charm for a Flask app

doc
1

In this tutorial you will learn how to build a rock and Kubernetes charm for a Flask application using the charm SDK, so you can have your Flask application up and running with Juju in about an hour!

: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 working station, e.g., a laptop, with amd64 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. To fast-track the setup, launch a VM with the name charm-dev using the charm-dev blueprint:

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

For a more hands-on setup, use Multipass to launch an Ubuntu VM with the name charm-dev from the 22.04 blueprint:

multipass launch --cpus 4 --disk 50G --memory 4G --name charm-dev 22.04

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 1.30-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. The MicroK8s registry is required to host the OCI image of the Flask application and ingress is required to expose the Flask application. Enable both using:

sudo microk8s enable hostpath-storage
sudo microk8s enable dns
sudo microk8s enable registry
sudo microk8s enable ingress

Juju is required to deploy the Flask application. Install Juju and boostrap a develoment 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

A pop-up window may appear when installing python-venv. Select OK or press enter to proceed to the next step.

Now that you have a virtual environment with all the dependencies, run the Flask application to verify that it works:

flask run -p 8000 &

The & at the end of the command runs the Flask application in the background. You can continue to use your terminal as normal and will see all the output from the Flask application in your terminal. To stop the Flask application, you use the kill $! command shown below.

Test the Flask application by using curl to send a request to the root endpoint:

curl localhost:8000

The Flask application should respond with Hello, world!. The Flask application looks good, so you can stop for now:

kill $!

If you run any other commands in the background, the Flask application will not be stopped with kill $!. You may identify the process ID of the Flask application using ps | grep flask and kill the process using kill <process_ID>.

The ps command shows a list of processes and the grep command filters that list to commands that include flask.

Pack the Flask application into a rock

First, you’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 in your working directory. You can optionally open it in a text editor and customise the name, summary and description. Ensure that platforms includes the architecture of your host. For example, if your host uses the ARM architecture, include arm64 in platforms. If you aren’t sure what the architecture of your host is, use uname -m to find out. For example, x86_64 indicates amd64 and aarch64 indicates arm64.

For this tutorial, we’ll use the name “flask-hello-world” and assume you are on the amd64 platform. 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 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 .rockextension:

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

Create the charm

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

mkdir charm
cd charm

You’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

The files will automatically be created in your working directory. Open charmcraft.yaml in a text editor and change the name to flask-hello-world and optionally edit summary and description.

For this tutorial, we’ll use the name flask-hello-world.

The charm depends on several libraries. Download the libraries and pack the charm:

charmcraft fetch-libs
charmcraft pack

Depending on your 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. Create a new model:

juju add-model flask-hello-world

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

If you are not on a host with the amd64 architecture, you will need to include a constraint to the Juju deploy command. For example, for the arm64 architecture, use juju deploy ./flask-hello-world_ubuntu-22.04-arm64.charm flask-hello-world --resource flask-app-image=localhost:32000/flask-hello-world:0.1 --constraints='arch=arm64'.

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.

The Flask application should now be running. You can see 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

Test the deployment using curl to send a request to the root endpoint. The IP address is the Address listed in the Unit section of the juju status output (e.g., 10.1.87.213 in the sample output above):

$ curl 10.1.87.213:8000
Hello, world!

Configure the Flask application

Let’s say you want to 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 10.1.87.215:8000 (with the IP address shown in the Unit section of juju status) 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 10.1.87.215:8000 now returns the updated Hi! greeting.

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

Integrate with a database

This step of the tutorial only works for hosts with the amd64 architecture. For other architectures, skip this step.

Let’s say that you would like to 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 caontaining 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.

We will now need to ensure that the rock will contain the new files we have created. Open the rockcraft.yaml file in a text editor and update the version to 0.3 and add the following at the end:

parts:
  flask-framework/install-app:
    prime:
      - flask/app/app.py
      - flask/app/migrate.py

The flask/app prefix for each file is required because that is where the flask-framework extension expects the files to be located in the OCI image.

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 the 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. curl 10.1.87.215:8000 should still return the Hi! greeting. To check the total visitors, use curl 10.1.87.215:8000/visitors 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 10.1.87.215:8000, curl 10.1.87.215:8000/visitors will return 2.

Expose the app using ingress

This step of the tutorial only works for hosts with the amd64 architecture. For other architectures, skip this step.

As a final step, let’s expose the application using ingress. Deploy the nginx-ingress-integrator and integrate it with you Flask app:

juju deploy nginx-ingress-integrator
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. Use curl 127.0.0.1 -H "Host: flask-hello-world" to send a request via the ingress. It should still be returning the Hi! greeting.

The -H "Host: flask-hello-world" option to the curl command is a way of setting the hostname of the request without setting a DNS record.

Clean up environment

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

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
# 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.sh 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
“What is…?” SDK Reference docs
“Why…?”, “So what?” SDK Explanation docs

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

1 Like