Why my Flask backend is unstable on Heroku?

Question

I created a small backend API for a game. When a user creates a game (a request to the API is done), Python creates a new instance of this game (to be more precise, I add a game in a dict). The user gets the game id in the response and can now play (the frontend calls several routes to update the state of this game).

It works perfectly locally, however on Heroku it is very unstable: I use polling and approximately 50% of the requests fail because the game id can not be found.

I can't figure out why the backend sometimes finds the game and sometimes not.

Does anybody have an idea of what went wrong?

Thank you very much.

Answer 1

This sounds like it could be due to the way you've implemented in-memory storage. If it's not thread-safe the app might work fully in development, but when deployed with a WSGI server like gunicorn with several worker processes/threads, each with their own memory, it could lead to strange behaviour as you descibe.

What's more, Heroku is quirky.

Here's the output of gunicorn --help when installed on any-old-system through pip which defaults to 1 worker if the -w flag is not provided:

-w INT, --workers INT
The number of worker processes for handling requests. [1]

However when executed via the Heroku console, notice that it defaults to 2:

-w INT, --workers INT
The number of worker processes for handling requests. [2]

Heroku appear to have customised their gunicorn build for some reason (edit: figured out how), so the following Procfile launches with 2 workers:

web: gunicorn some:app

Where-as on a non-Heroku system this would launch with a single worker.

You'll probably find the following Procfile will solve your issue:

web: gunicorn --workers 1 some:app

This is, of course, suitable if it's a small project which doesn't need to scale to several workers. To mitigate this issue and scale the application, you may need to investigate making code changes to implement a separate storage backend (eg. Redis) within your app.