Description

This repo shows how to migrate an existing Flask app to one that can use OpenShift's s2i tool to create a Docker image of the app.

s2i builds Docker images using a common two-stage build pattern: the source code is piped to a builder image, prepared, and then output into a final image. It's most interesting feature is that it can do all this without a Dockerfile.

NOTE: If starting off fresh without any existing, it may be better to use the s2i create <image name> <destination directory> command from the s2i CLI

Building the Docker image: `s2i build` and `docker run`

Assuming the final image name is hello-world-flask:v0:

In the terminal, change directories to the repo's flask-app/ folder
Run s2i build . centos/python-36-centos7 hello-world-flask:v0
Run docker run -p 8888:8080 hello-world-flask:v0

Migration Steps

The only required adjustments to the original Flask repo are:

Moving the Flask app's files into a folder ending in "-app"
Adding an .s2i/environment specifying the app start script e.g. the file used to run the app with the command python <file>
- NOTE: if app.py is the start script, then no need to add the .s2i/environment file

Details

Builder image + Python version

This Flask app uses the centos/python-36-centos7 image as the builder image.

Flask app file structure

If the original repo looks like this

flask-repo
|-- run.py
|-- sampleapp/
    |-- __init__.py
    |-- views.py

then the new repo would look like this

flask-repo
|-- flask-app               # new contents folder
    |-- run.py
    |-- sampleapp/
        |-- __init__.py
        |-- views.py
    |-- .s2i                # new folder
        |-- environment     # new file

Other notes

OpenShift

This repo cannot be directly deployed as a git repo to OpenShift 3; see an example HERE instead. This is because the .s2i/ folder and app files need to be directly in the repo folder rather than in any of the repo's child folders. To see the Flask app adapted for directly deploying a git repo to OpenShift and letting the OpenShift instance automatically build via s2i, again see the sister repo mentioned above.

The sister repo can be directly deployed on OpenShift Online / OpenShift Container Platform via Add to project > Browse Catalog > Python > 3.5 and pasting a fork of the git repository. NOTE: the current app succesfully runs on both 3.5 (as of 04-03-2018, 3.5 is the current latest version on OpenShift's online base images) and 3.6 (available via CentOS's Docker Hub).

geordgez/flask-s2i