How can I make a dockerized Flask app write output to file?

Question

I'm trying to make the server which hosts a (dockerized) Flask app "log" requests that are sent to it, simply by writing them to a text file.

My docker-compose.yml looks like

version: '2'

services:
  writer:
    build: writer/
    ports: 
      - 5000:5000
    container_name: writer

and inside the writer directory I have the writer.py file as follows, which is supposed to get the request and write it to a local file feedback.txt :

from flask import Flask, request, jsonify

app = Flask(__name__, static_url_path='')

@app.route('/feedback', methods=['POST']) 
def log_feedback():
    with open("feedback.txt","a") as fo:
        fo.write(request.data.decode("utf-8"))
        print(request.data.decode("utf-8"))
        fo.write('\n')
    return 'Got it!'

if __name__ == '__main__':
    app.run(host='0.0.0.0', debug = False)

For reference, my Dockerfile in the same writer dir is:

FROM python:3.6-slim
WORKDIR /app
ADD . /app
RUN pip install -r requirements.txt
ENTRYPOINT [ "python", "writer.py" ]

requirements.txt is simply

flask

And a sample request is

curl -H "Content-Type:application" -X POST -d '{"Content":"Hello world"}' http://0.0.0.0:5000/feedback

Unfortunately, no feedback.txt gets written locally (eg outside the container). Can you hint to me the modifications I should perform eg in docker-compose.yml (eg with volumes?) in order to get the feedback.txt written and accessible on the server?

Answer 1

You have to add a volume in the docker-compose.yml. The path in the container should contain the folder that contains the written file. The path to local folder should contain a folder on your system where you want to see the written file.

version: '2'

services:
  writer:
    build: writer/
    ports: 
      - 5000:5000
    container_name: writer
    volumes:
      - /PATH/TO/YOUR/LOCAL_FOLDER:/PATH/TO/FOLDER/IN/CONTAINER