Write your first Kubernetes charm for a FastAPI app

Imagine you have a FastAPI 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 FastAPI application in no time. Let’s get started!

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

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:

• Set things up
• Create the FastAPI application
• Run the FastAPI application locally
• Pack the FastAPI application into a rock
• Create the charm
• Deploy the FastAPI application
• Enable juju config fastapi-hello-world greeting=<greeting>
• Integrate with a database
• Expose the app using ingress
• Clean up environment

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 --channel latest/edge --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

MicroK8s is required to deploy the FastAPI 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 FastAPI application
sudo microk8s enable registry
# Required to expose the FastAPI application
sudo microk8s enable ingress

See more: ingress^

Juju is required to deploy the FastAPI 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 fastapi-hello-world
cd fastapi-hello-world

Create the FastAPI application

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

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

fastapi[standard]

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

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def root():
    return {"message": "Hello World"}

Run the FastAPI 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 FastAPI application to verify that it works:

fastapi dev app.py --port 8080

Test the FastAPI 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:8080

The FastAPI application should respond with {"message":"Hello World"}. The FastAPI application looks good, so we can stop for now using ctrl + c.

Pack the FastAPI application into a rock

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

rockcraft init --profile fastapi-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 fastapi-hello-world. Ensure that platforms includes the architecture of your host. For example, if your host uses the ARM architecture, include arm64 in platforms.

Pack the rock:

ROCKCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=true rockcraft pack

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

ls *.rock -l

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:fastapi-hello-world_0.1_amd64.rock \
   docker://localhost:32000/fastapi-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 FastAPI application. Charmcraft will automate the creation of these files by using the fastapi-framework profile:

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

The files will automatically be created in your working directory.

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

CHARMCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=true charmcraft fetch-libs
CHARMCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=true charmcraft pack

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

ls *.charm -l

Deploy the FastAPI application

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

juju add-model fastapi-hello-world

Now the FastAPI application can be deployed using Juju:

juju deploy ./fastapi-hello-world_amd64.charm fastapi-hello-world \
   --resource app-image=localhost:32000/fastapi-hello-world:0.1

The FastAPI 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
fastapi-hello-world  dev-controller  microk8s/localhost  3.5.4    unsupported  13:45:18+10:00

App                  Version  Status  Scale  Charm                Channel  Rev  Address        Exposed  Message
fastapi-hello-world           active      1  fastapi-hello-world             0  10.152.183.53  no       

Unit                    Workload  Agent  Address      Ports  Message
fastapi-hello-world/0*  active    idle   10.1.157.75

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 FastAPI app:

juju deploy nginx-ingress-integrator
juju integrate nginx-ingress-integrator fastapi-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=fastapi-hello-world path-routes=/

Monitor juju status until everything has a status of active. Use curl http://fastapi-hello-world --resolve fastapi-hello-world:80:127.0.0.1 to send a request via the ingress. It should return the {"message":"Hello World"} greeting.

Configure the FastAPI application

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

import os

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def root():
    return {"message": os.getenv("APP_GREETING", "Hello World")}

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

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

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

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

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

juju refresh fastapi-hello-world \
   --path=./fastapi-hello-world_amd64.charm \
   --resource app-image=localhost:32000/fastapi-hello-world:0.2

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

Running curl http://fastapi-hello-world --resolve fastapi-hello-world:80:127.0.0.1 shows that the response is still {"message":"Hello, world!"} as expected. The greeting can be changed using Juju:

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

curl http://fastapi-hello-world --resolve fastapi-hello-world:80:127.0.0.1 now returns the updated {"message":"Hi!"} greeting.

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 fastapi-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 ... Create 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()

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 FastAPI 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
from typing import Annotated

from fastapi import FastAPI, Header
import psycopg2

app = FastAPI()
DATABASE_URI = os.environ["POSTGRESQL_DB_CONNECT_STRING"]


@app.get("/")
async def root(user_agent: Annotated[str | None, Header()] = None):
    with psycopg2.connect(DATABASE_URI) as conn, conn.cursor() as cur:
        timestamp = datetime.datetime.now()

        cur.execute(
            "INSERT INTO visitors (timestamp, user_agent) VALUES (%s, %s)",
            (timestamp, user_agent)
        )
        conn.commit()

    return {"message": os.getenv("APP_GREETING", "Hello World")}


@app.get("/visitors")
async 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 {"count": total_visitors}

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

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

The FastAPI app now requires a database which needs to be declared in the charmcraft.yaml file. Go back into the charm directory using cd charm. 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_ENABLE_EXPERIMENTAL_EXTENSIONS=true charmcraft pack and refresh the deployment using Juju:

juju refresh fastapi-hello-world \
   --path=./fastapi-hello-world_amd64.charm \
   --resource app-image=localhost:32000/fastapi-hello-world:0.3

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

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

Wait for juju status to show that the App is active again. curl http://fastapi-hello-world --resolve fastapi-hello-world:80:127.0.0.1 should still return the {"message":"Hi!"} greeting. To check the total visitors, use curl http://fastapi-hello-world/visitors --resolve fastapi-hello-world:80:127.0.0.1 which should return {"count":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://fastapi-hello-world --resolve fastapi-hello-world:80:127.0.0.1, curl http://fastapi-hello-world/visitors --resolve fastapi-hello-world:80:127.0.0.1 will return {"count":2}.

Clean up environment

We’ve reached the end of this tutorial. We have created a FastAPI 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 __pycache__
# delete all the files created during the tutorial
rm fastapi-hello-world_0.1_amd64.rock fastapi-hello-world_0.2_amd64.rock \
   fastapi-hello-world_0.3_amd64.rock rockcraft.yaml app.py \
   requirements.txt migrate.py
# Remove the juju model
juju destroy-model fastapi-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

Contributors: @econley, @jdkandersson, @javierdelapuente

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