Heroku+Flask

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

• 2021-01-18: Natural Language Processing on Heroku
• 2021-10-14: FastAPI is also an option and should be considered (this time I tried FastAPI for a bit and then went back to Flask).

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 python

from flask import Flask

app = Flask(__name__)

def create_app():
    return app

@app.route('/')
def root():
    return "OK"

- Minimal server implementation
- I had a habit of doing app.run with `if __name__ == "__main__"`, but I don't do that anymore, even in official Flask tutorials.
    - Specify the module to be started in the environment variable, and start it with flask run.
        - [explanation on the heroku side](https://devcenter.heroku.com/articles/flask-memcache) does that too.
        - Setting environment variables may seem tedious, but you can use dotenv
        - The official explanation uses [Flask Blueprint](/en/Flask%20Blueprint), but I decided that's not necessary at the minimum.
    - 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 :

FLASK_APP=server
FLASK_ENV=development

• $ pip install python-dotenv
• $ flask run
  • OK
  • If I look at http://127.0.0.1:5000/ in my browser it says OK

Deploy to heroku

• $ git init
• create ".gitignore" :

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" :

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.
  • Or error at build time: [mecab no such file or directory: /usr/local/etc/mecabrc https://medium.com/@jiraffestaff/mecabrc-%E3%81%8C%E8%A6%8B%E3 81%A4%E3%81%8B%E3%82%89%E3%81%AA%E3%81%84%E3%81%84%E3%81%86%E3%82%A8%E3%83%A9%E3%83%BC-b3e278e9ed07]
• By default, a route only answers to GET requests.doc
  • @app.route('/', methods=['GET', 'POST'])
• JSON
  • property json: Optional[Any] doc: Incoming Request Data

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

• If you call from other origins: Flask-CORS is required

"No web processes running"

• I typo'd the file name in the Procfile.
• The build itself goes through...

----old memo Heroku+Flask 2019-6-07 memo

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