Run a Flask app
In this guide we create and deploy a Flask web server. To run this example, follow these steps:
-
Install the
kraft
CLI tool and a container runtime engine, e.g. Docker. -
Clone the
examples
repository andcd
into theexamples/http-python3.12-flask3.0/
directory:
git clone https://github.com/kraftcloud/examplescd examples/http-python3.12-flask3.0/
Make sure to log into Unikraft Cloud by setting your token and a metro close to you.
We use fra0
(Frankfurt, 🇩🇪) in this guide:
# Set Unikraft Cloud access tokenexport UKC_TOKEN=token# Set metro to Frankfurt, DEexport UKC_METRO=fra0
When done, invoke the following command to deploy this application on Unikraft Cloud:
kraft cloud deploy -M 512 -p 443:8080 .
The output shows the instance URL and other details:
[●] Deployed successfully! │ ├────────── name: http-python312-flask30-bxwxm ├────────── uuid: 3ff1ebad-2639-4214-bab4-ed35c4c32fa4 ├───────── state: running ├─────────── url: https://damp-sunset-azd6dtyt.fra0.kraft.host ├───────── image: http-python312-flask30@sha256:d6c8e4c5a4f44e1d642d8eaeaa1d820b2841194dd6c5d4a872ae0a895c767da9 ├───── boot time: 222.27 ms ├──────── memory: 512 MiB ├─────── service: damp-sunset-azd6dtyt ├── private fqdn: http-python312-flask30-bxwxm.internal ├──── private ip: 172.16.6.5 └────────── args: /usr/bin/python3 /app/server.py
In this case, the instance name is http-python312-flask30-bxwxm
and the URL is https://damp-sunset-azd6dtyt.fra0.kraft.host
.
They are different for each run.
Use curl
to query the Unikraft Cloud instance of the Python-based HTTP web server:
curl https://young-night-5fpf0jj8.fra0.kraft.host
Hello, World!
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 TIMEhttp-python312-flask30-bxwxm damp-sunset-azd6dtyt.fra0.kraft.host running 1 minute ago http-python312-flask30@sha256:d6c8e... 512 MiB /usr/bin/python3 /app/server.py 222273us
When done, you can remove the instance:
kraft cloud instance remove http-python312-flask30-bxwxm
Customize your Application
To customize the application, update the files in the repository, listed below:
server.py
: the actual Python HTTP serverKraftfile
: the Unikraft Cloud specificationDockerfile
: the Docker-specified application filesystem
from flask import Flaskapp = Flask(__name__)
@app.route('/')def hello():return "Hello, World!\n"
if __name__ == '__main__':app.run(host='0.0.0.0', port=8080)
spec: v0.6
runtime: python:3.12
rootfs: ./Dockerfile
cmd: ["/usr/bin/python3", "/app/server.py"]
FROM python:3.12-bookworm AS base
WORKDIR /app
COPY requirements.txt /app
RUN pip3 install -r requirements.txt --no-cache-dir
FROM scratch
COPY --from=base /usr/local/lib/python3.12 /usr/local/lib/python3.12COPY ./server.py /app/server.py
Lines in the Kraftfile
have the following roles:
-
spec: v0.6
: The currentKraftfile
specification version is0.6
. -
runtime: python:3.12
: The Unikraft runtime kernel to use is Python 3.12. -
rootfs: ./Dockerfile
: Build the application root filesystem using theDockerfile
. -
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 thescratch
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 application:
-
If only updating the implementation in the
server.py
source file, no other change is required. -
If new files are added, these have to be copied in the application filesystem, using the
COPY
command in theDockerfile
. -
If a new Python source files is added, update the
cmd
line in theKraftfile
and replaceserver.py
to run that file when creating the instance. -
More extensive changes may require expanding the
Dockerfile
with additionalDockerfile
commands. This includes the use of Python frameworks and the use ofpip
, as shown in the next section.
Using pip
pip
is a package manager for Python.
It is used to install dependencies for Python applications.
pip
uses the requirements.txt
file to list required dependencies (with versions).
To create an pip
-based app:
-
Add the
requirements.txt
file used bypip
. -
Add framework-specific source files. In our case, this means the
server.py
file. -
Update the
Dockerfile
to:-
COPY
the local files. -
RUN
thepip3 install
command to install dependencies. -
COPY
of the resulting and required files (/usr/local/lib/pyhon3.12
andserver.py
) in the application filesystem, using thescratch
container.
-
The files are listed below:
from flask import Flaskapp = Flask(__name__)
@app.route('/')def hello(): return "Hello, World!\n"
if __name__ == '__main__': app.run(host='0.0.0.0', port=8080)
flask>=3.0,<3.1
spec: v0.6
runtime: python:3.12
rootfs: ./Dockerfile
cmd: ["/usr/bin/python3", "/app/server.py"]
FROM python:3.12-bookworm AS base
WORKDIR /app
COPY requirements.txt /app
RUN pip3 install -r requirements.txt --no-cache-dir
FROM scratch
COPY --from=base /usr/local/lib/python3.12 /usr/local/lib/python3.12COPY ./server.py /app/server.py
The requirements.txt
file lists the flask
dependency.
The Kraftfile
is the same one used for http-python3.12
.
For Dockerfile
newly added lines have the following roles:
-
FROM python:3.12-bookworm AS base
: Use the base image of thepython:3.12-bookworm
container. This provides thepip3
binary and other Python-related components. Name the current imagebase
. -
WORKDIR /app
: Use/app
as working directory. All other commands in theDockerfile
run inside this directory. -
COPY requirements.txt /app
: Copy the package configuration file to the Docker filesystem. -
RUN pip3 install ...
: Installpip
components listed inrequirements.txt
. -
COPY --from=base ...
: Copy generated Python files in the newbase
image in thescratch
-based image.
Similar actions are required for other pip3
-based applications.
See also the http-python3.12-django5.0
example.
Learn More
Use the --help
option for detailed information on using Unikraft Cloud:
kraft cloud --help
Or visit the CLI Reference.