This guide explains how to create and deploy a Flask web server. To run this example, follow these steps:

1. Install the CLI. Use the unikraft CLI or the legacy kraft 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.

   Note: The unikraft CLI is the current standard, while kraft is the legacy version. Choose one of the CLIs below and only run the commands associated with it for the rest of this guide.

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

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

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

Using the unikraft CLI (Recommended)

unikraft login

or

Using the legacy kraft CLI

# Set Unikraft Cloud access token
export UKC_TOKEN=token
# Set metro to Frankfurt, DE
export UKC_METRO=fra

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

Using the unikraft CLI (Recommended)

unikraft build . --output <my-org>/httpserver-python312-flask30:latest
unikraft run --metro fra \
  -m 512M \
  -p 443:8080/tls+http \
  --scale-to-zero policy=on,cooldown-time=1000 \
  --image <my-org>/httpserver-python312-flask30:latest

or

Using the legacy kraft CLI

kraft cloud deploy \
  -M 512Mi \
  -p 443:8080/tls+http \
  --scale-to-zero on \
  --scale-to-zero-cooldown 1s \
  .

The output shows the instance address and other details:

Using the unikraft CLI (Recommended)

metro:        fra
name:         httpserver-python312-flask30-bxwxm
uuid:         3ff1ebad-2639-4214-bab4-ed35c4c32fa4
state:        starting
image:        <my-org>/httpserver-python312-flask30
resources:
  memory:     512MiB
  vcpus:      1
service:
  uuid:       2021ca1e-5e47-a35c-bde2-7190f3815c07
  name:       damp-sunset-azd6dtyt
  domains:
  - fqdn:     damp-sunset-azd6dtyt.fra.unikraft.app
networks:
- uuid:       4dc3b272-4fa2-ddb4-7ce0-5e66e8112a61
  private-ip: 10.0.6.5
  mac:        12:b0:66:99:b3:3f
timestamps:
  created:    just now

or

Using the legacy kraft CLI

[●] Deployed successfully!
 │
 ├───────── name: httpserver-python312-flask30-bxwxm
 ├───────── uuid: 3ff1ebad-2639-4214-bab4-ed35c4c32fa4
 ├──────── metro: https://api.fra.unikraft.cloud/v1
 ├──────── state: starting
 ├─────── domain: https://damp-sunset-azd6dtyt.fra.unikraft.app
 ├──────── image: oci://unikraft.io/<my-org>/httpserver-python312-flask30@sha256:d6c8e4c5a4f44e1d642d8eaeaa1d820b2841194dd6c5d4a872ae0a895c767da9
 ├─────── memory: 512 MiB
 ├────── service: damp-sunset-azd6dtyt
 ├─ private fqdn: httpserver-python312-flask30-bxwxm.internal
 └─── private ip: 10.0.6.5

In this case, the instance name is httpserver-python312-flask30-bxwxm and the address is https://damp-sunset-azd6dtyt.fra.unikraft.app. They're different for each run.

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

curl https://damp-sunset-azd6dtyt.fra.unikraft.app

Hello, World!

You can list information about the instance by running:

Using the unikraft CLI (Recommended)

unikraft instances list

METRO  NAME                                STATE    IMAGE                                  ARGS  MEMORY  VCPUS  FQDN                                   CREATED
fra    httpserver-python312-flask30-bxwxm  running  <my-org>/httpserver-python312-flask30        512MiB  1      damp-sunset-azd6dtyt.fra.unikraft.app  2 minutes ago

or

Using the legacy kraft CLI

kraft cloud instance list

NAME                                FQDN                                   STATE    STATUS        IMAGE                                                               MEMORY   VCPUS  ARGS  BOOT TIME
httpserver-python312-flask30-bxwxm  damp-sunset-azd6dtyt.fra.unikraft.app  running  1 minute ago  oci://unikraft.io/<my-org>/httpserver-python312-flask30@sha256:...  512 MiB  1            222.27 ms

When done, you can remove the instance:

Using the unikraft CLI (Recommended)

unikraft instances delete httpserver-python312-flask30-bxwxm

or

Using the legacy kraft CLI

kraft cloud instance remove httpserver-python312-flask30-bxwxm

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

• server.py: the actual Python HTTP server
• 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", "/src/server.py"]: Use /usr/bin/python3 /src/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 ./server.py /app/server.py: Copy the server implementation file (server.py) in the Docker filesystem (in /app/server.py).

The following options are available for customizing the app:

• If you only update the implementation in the server.py source file, 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.

• More extensive changes may require extending the Dockerfile (see Dockerfile syntax reference). This includes the use of Python frameworks and the use of pip, as shown in the next section.

pip is a package manager for Python. It's used to install dependencies for Python apps. pip uses the requirements.txt file to list required dependencies (with versions).

To create an pip-based app:

1. Add the requirements.txt file used by pip.

2. Add framework-specific source files. In this case, this means the server.py file.

3. Update the Dockerfile to:

   3.1. COPY the local files.

   3.2. RUN the pip3 install command to install dependencies.

   3.3. COPY of the resulting and required files (/usr/local/lib/python3.12 and server.py) in the app filesystem, using the scratch container.

The following lists the files:

The requirements.txt file lists the flask dependency.

The Kraftfile is the same one used for httpserver-python3.12.

For Dockerfile newly added lines have the following roles:

• FROM python:3.12-bookworm AS base: Use the base image of the python:3.12-bookworm container. This provides the pip3 binary and other Python-related components. Name the current image base.

• WORKDIR /app: Use /app as working directory. All other commands in the Dockerfile run inside this directory.

• COPY requirements.txt /app: Copy the package configuration file to the Docker filesystem.

• RUN pip3 install ...: Install pip components listed in requirements.txt.

• COPY --from=base ...: Copy generated Python files in the new base image in the scratch-based image.

Similar actions apply to other pip3-based apps.

Use the --help option for detailed information on using Unikraft Cloud:

Using the unikraft CLI (Recommended)

unikraft --help

or

Using the legacy kraft CLI

kraft cloud --help

Or visit the CLI Reference or the legacy CLI Reference.