Run Python Flask with SQLite

In this guide we create and deploy a Python Flask application using SQLite. To run this example, follow these steps:

Install the kraft CLI tool and a container runtime engine, e.g. Docker.
Clone the examples repository and cd into the examples/python3.12-flask3.0-sqlite/ directory:

git clone https://github.com/kraftcloud/examples
cd examples/python3.12-flask3.0-sqlite/

Make sure to log into KraftCloud by setting your token and a metro close to you. We use fra0 (Frankfurt, 🇩🇪) in this guide:

# Set KraftCloud access token
export KRAFTCLOUD_TOKEN=token
# Set metro to Frankfurt, DE
export KRAFTCLOUD_METRO=fra0

When done, invoke the following command to deploy this application on KraftCloud:

kraft cloud deploy -p 443:8080 -M 512 .

[●] Deployed successfully!
 │
 ├────────── name: python312-flask30-sqlite-qodkd
 ├────────── uuid: e00e7aca-962d-409c-87c2-c245ca08b54b
 ├───────── state: running
 ├─────────── url: https://lingering-orangutan-840mmdvd.fra0.kraft.host
 ├───────── image: python312-flask30-sqlite@sha256:bdb0bf35a9675b9b3836cbb626606da0606334d91768c7ba31195c3062d6f517
 ├───── boot time: 166.25 ms
 ├──────── memory: 512 MiB
 ├─ service group: lingering-orangutan-840mmdvd
 ├── private fqdn: python312-flask30-sqlite-qodkd.internal
 ├──── private ip: 172.16.3.3
 └────────── args: /usr/bin/python3 /app/server.py

In this case, the instance name is python312-flask30-sqlite-qodkd and the URL is https://lingering-orangutan-840mmdvd.fra0.kraft.host. They are different for each run.

Use curl to query the KraftCloud instance of the Python-based HTTP web server:

curl https://young-night-5fpf0jj8.fra0.kraft.host

<!doctype html>
<html lang="en">
    [...]
    <h1> Welcome to FlaskBlog </h1>

        <a href="/1">
            <h2>First Post</h2>
        </a>
        <span class="badge badge-primary">2024-02-15 22:01:07</span>
        <a href="/1/edit">
            <span class="badge badge-warning">Edit</span>
        </a>
        <hr>

        <a href="/2">
            <h2>Second Post</h2>
        </a>
</html>

At any point in time, you can list information about the instance:

kraft cloud instance list

NAME                            FQDN                                           STATE    CREATED AT    IMAGE                                                  MEMORY   ARGS                             BOOT TIME
python312-flask30-sqlite-qodkd  lingering-orangutan-840mmdvd.fra0.kraft.host  running  1 minute ago  razvan.unikraft.io/python312-flask30-sqlite@sha256...  512 MiB  /usr/bin/python3 /app/server.py  166250us

When done, you can remove the instance:

kraft cloud instance remove python312-flask30-sqlite-qodkd

Implementation Details

The application uses several files, listed below:

schema.sql: SQL schema for the database
init_db.py: script to initialized the database file from schema.sql in /app/database.db
server.py + templates/: the actual Flask-based implementation: Python source code file and HTML template files
requirements.txt: pip configuration file to install required packages: Flask and SQLite
Kraftfile: the KraftCloud specification
Dockerfile: the Docker-specified application filesystem

DROP TABLE IF EXISTS posts;

CREATE TABLE posts (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    created TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    title TEXT NOT NULL,
    content TEXT NOT NULL
);

import sqlite3

connection = sqlite3.connect('/app/database.db')


with open('schema.sql') as f:
    connection.executescript(f.read())

cur = connection.cursor()

cur.execute("INSERT INTO posts (title, content) VALUES (?, ?)",
            ('First Post', 'Content for the first post')
            )

cur.execute("INSERT INTO posts (title, content) VALUES (?, ?)",
            ('Second Post', 'Content for the second post')
            )

connection.commit()
connection.close()

[...]
@app.route('/<int:id>/delete', methods=('POST',))
def delete(id):
    post = get_post(id)
    conn = get_db_connection()
    conn.execute('DELETE FROM posts WHERE id = ?', (id,))
    conn.commit()
    conn.close()
    flash('"{}" was successfully deleted!'.format(post['title']))
    return redirect(url_for('index'))


if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8080)

flask>=3.0,<3.1
pysqlite3

spec: v0.6

runtime: python:3.12

rootfs: ./Dockerfile

cmd: ["/usr/bin/python3", "/app/server.py"]

Lines in the Kraftfile have the following roles:

spec: v0.6: The current Kraftfile specification version is 0.6.
runtime: python:3.12: The Unikraft runtime kernel to use is Python 3.12.
rootfs: ./Dockerfile: Build the application root filesystem using the Dockerfile.
cmd: ["/usr/bin/python3", "/app/server.py"]: Use /usr/bin/python3 /app/server.py as the starting command of the instance.

Lines in the Dockerfile have the following roles:

FROM scratch: Build the filesystem from the scratch container image, to create a base image.
COPY rules result in the copying of required files. The requirements.txt file is copied before running pip3 install. Other files (including schema.sql, init_db.py) are copied for the initialization of the database.
RUN commands trigger actions such as installing Python packages and initializing the database.
The relevant contents required by the application are copied to the new scratch image:
- the SQLite dynamic library: /usr/lib/x86_64-linux-gnu/libsqlite3.so.0
- the Python package files: /usr/local/lib/python3.12
- the /app directory

Customize your Application

To customize the application, update application files in the repository:

schema.sql: Update the database schema.
server.py, templates/: Update the Flask application..
requirements.txt, Dockerfile: Update the list of Python packages used by the application.
Kraftfile: Update the command line used to start the application.

The following options are available for customizing the application:

If only updating the implementation in the server.py source file or template/ directory, and the database schema in schema.sql, no other change is required.
If new files are added, these have to be copied in the application filesystem, using the COPY command in the Dockerfile.
If new dependencies are added in requirements.txt, the RUN pip3 install in the Dockerfile command should take care of everything. It may be the case that other files, such as the /usr/lib/x86_64-linux-gnu/libsqlite3.so.0 for SQLite, are required to be copied via COPY commands in the Dockerfile. More extensive changes may require expanding the Dockerfile with additional Dockerfile commands.
If a new Python source file is added that’s running the main() function, update the cmd line in the Kraftfile and replace server.py to run that file when creating the instance.

Learn More

Use the --help option for detailed information on using KraftCloud:

kraft cloud --help

Or visit the CLI Reference.