Set Rails API in Heroku to development and impact of the Procfile?

I'm new to Heroku and despite all the documentation, I'm a little unsure what the profile is for. I can set a port and the environment as follows, but Heroku always starts in production mode (which makes sense) and not with the specified port.

I suppose that the port cannot be set because it is determined by Heroku?

Is the Procfile only for the command "heroku local" to test?

Because when I run "heroku ps" I get info about the procfile, but the API runs without the procfile port in production mode.

Thank you for any explanation!

Procfile:

web: bundle exec puma -t 5:5 -p ${PORT:-3000} -e ${RACK_ENV:-development}

Output of heroku ps after deploying:

=== web (Hobby): bundle exec puma -t 5:5 -p ${PORT:-3000} -e ${RACK_ENV:-development} (1)
web.1: up 2020/07/27 13:50:23 +0200 (~ 1m ago)

Output of heroku logs at the same time:

Version 3.12.6 (ruby 2.5.8-p224), codename: Llamas in Pajamas
2020-07-27T11:49:32.219309+00:00 app[web.1]: * Min threads: 5, max threads: 5
2020-07-27T11:49:32.219309+00:00 app[web.1]: * Environment: production
2020-07-27T11:49:33.740321+00:00 app[web.1]: * Listening on tcp://0.0.0.0:10269

Solution

Heroku will set both $PORT and $RACK_ENV for Rails apps when they're deployed. You can confirm this by running heroku config --app <yourapp>. The construct ${PORT:-3000} means "use the PORT variable if it's present, otherwise use the value 3000.

In any case, you can't run a Heroku app on a port other than the one defined in $PORT, which is randomized for each dyno. Whatever that's set to will be forwarded to from ports 80 and 443 for HTTP/S.

If you want to override the RACK_ENV, you can run heroku config:set RACK_ENV=development.