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Example apps

Flask and SQLite HTTP Server

This guide explains how to create and deploy a Python Flask app using SQLite. To run this example, follow these steps:

1. Install the unikraft CLI. You need a BuildKit builder. The easiest way to get one is via Docker. Alternatively, you can also directly set up and use BuildKit, see the quick start.

2. Clone the examples repository and cd into the examples/httpserver-python3.12-flask3.0-sqlite/ directory:

   Code
   git clone https://github.com/unikraft-cloud/examples
   cd examples/httpserver-python3.12-flask3.0-sqlite/

Make sure to log into Unikraft Cloud and pick a metro close to you. This guide uses fra (Frankfurt, 🇩🇪):

Code
unikraft login

When done, invoke the following command to deploy this app on Unikraft Cloud:

Code
unikraft build . --output <my-org>/httpserver-python312-flask30-sqlite:latest
unikraft run --metro fra \
  -m 768M \
  -p 443:8080/tls+http \
  --scale-to-zero policy=on,cooldown-time=1000 \
  --image <my-org>/httpserver-python312-flask30-sqlite:latest

The output shows the instance address and other details:

Code
metro:        fra
name:         httpserver-python312-flask30-sqlite-qodkd
uuid:         e00e7aca-962d-409c-87c2-c245ca08b54b
state:        starting
image:        <my-org>/httpserver-python312-flask30-sqlite
resources:
  memory:     768MiB
  vcpus:      1
service:
  uuid:       1a2b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
  name:       lingering-orangutan-840mmdvd
  domains:
  - fqdn:     lingering-orangutan-840mmdvd.fra.unikraft.app
networks:
- uuid:       2b3c4d5e-6f7a-8b9c-0d1e-2f3a4b5c6d7e
  private-ip: 10.0.3.3
  mac:        12:b0:8a:4f:2c:91
timestamps:
  created:    just now

In this case, the instance name is httpserver-python312-flask30-sqlite-qodkd and the address is https://lingering-orangutan-840mmdvd.fra.unikraft.app. They're different for each run.

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

Code
curl https://young-night-5fpf0jj8.fra.unikraft.app

Code
<!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>

You can list information about the instance by running:

Code
unikraft instances list

Code
METRO  NAME                                       STATE    IMAGE                                         ARGS  MEMORY  VCPUS  FQDN                                  CREATED
fra    httpserver-python312-flask30-sqlite-qodkd  running  <my-org>/httpserver-python312-flask30-sqlite        768MiB  1      lingering-orangutan-840mmdvd.fra....  2 minutes ago

When done, you can remove the instance:

Code
unikraft instances delete httpserver-python312-flask30-sqlite-qodkd

Implementation details

The app uses the following files:

• 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 Unikraft Cloud specification
• Dockerfile: the Docker-specified app filesystem

Lines in the Kraftfile have the following roles:

• spec: v0.7: The current Kraftfile specification version is 0.7.

• runtime: base-compat:latest: The runtime kernel to use is the base compatibility kernel.

• rootfs: Build the app root filesystem. source: ./Dockerfile means the filesystem is built using the Dockerfile. format: erofs means the filesystem type is EROFS.

• 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 copy required files. The process copies the requirements.txt file before running pip3 install. The process copies other files (including schema.sql, init_db.py) for the initialization of the database.

• RUN commands trigger actions such as installing Python packages and initializing the database.

• The new scratch image contains the relevant contents required by the app:

  • 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 app

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

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

The following options are available for customizing the app:

• If you only update the implementation in the server.py source file or template/ directory, and the database schema in schema.sql, you don't need to make any other changes.

• If you create any new source files, copy them into the app filesystem by using the COPY command in the Dockerfile.

• If you add new dependencies in requirements.txt, the RUN pip3 install in the Dockerfile command should take care of everything. It may be the case that you need to copy other files, such as the /usr/lib/x86_64-linux-gnu/libsqlite3.so.0 for SQLite, via COPY commands in the Dockerfile. More extensive changes may require extending the Dockerfile (see Dockerfile syntax reference).

• If you add a new Python source file 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 Unikraft Cloud:

Code
unikraft --help

Or visit the CLI Reference.