Heroku+Flask

2020-09-15: Work note for running an API server for machine learning, natural language processing, etc. on heroku.

First, create a minimal server and run it locally.

$ python3 -m venv venv

$ source venv/bin/activate

create "server/__init__.py" and write minimal server

code:python

from flask import Flask

app = Flask(__name__)

def create_app():

return app

@app.route('/')

def root():

return "OK"

Minimal server implementation

Specify the module to be started in the environment variable, and start it with flask run.

Setting environment variables may seem tedious, but you can use dotenv

create_app is used to call from gunicorn later

$ python server/__init__.py

ModuleNotFoundError: No module named 'flask'

$ pip install --upgrade pip

$ pip install flask

OK

$ flask run

NG because environment variables are not set

Error: Could not locate a Flask application. You did not provide the "FLASK_APP" environment variable, and a "wsgi.py" or "app.py" module was not found in the current directory.

create ".env" file

$ code .env

code::

FLASK_APP=server

FLASK_ENV=development

$ pip install python-dotenv

$ flask run

OK

Deploy to heroku

$ git init

create ".gitignore"

code::

venv/

.env

*.pyc

__pycache__/

.env ignore?

If you don't IGNORE, you don't need to do $ heroku config:set FLASK_APP=server later

This is to avoid the common accident of putting API keys, etc. that should not be made public in .env and pushing them to the public repository.

$ git add .

$ git commit -m 'minimal Flask server'

$ heroku create foo-server

This is creating an app from a local console, but if you are already using it from a browser, just follow the instructions in the browser.

$ heroku git:remote -a foo-server

$ git remote

heroku

I'm going to push here.

$ git push --set-upstream heroku master

I can push, but build fails.

$ pip install gunicorn

$ pip freeze > requirements.txt

create "Procfile"

code::

web: gunicorn server:"create_app()"

name "server" points the module on "./server"

$ heroku config:set FLASK_APP=server

add and commit

$ git push heroku master

Make sure the build is successful

$ heroku open

Open it in your browser and make sure it says OK.

Implement the required API

View the build log when pushing and the error log when accessing

$ heroku logs --tail

You forgot to include the necessary files, and you get a runtime error when accessing the system.

@app.route('/', methods=['GET', 'POST'])

JSON

The parsed JSON data if mimetype indicates JSON (application/json, see is_json).

"No web processes running"

I typo'd the file name in the Procfile.

The build itself goes through...

----old memo

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