I’ll introduce how to write a Procfile to use Ruby on Rails 5 applications on Heroku’s free tier.

This mainly covers the options specified when starting the puma server.

How to Write a Procfile for Using Rails Apps on Heroku's Free Tier

The Procfile configuration for using Ruby on Rails applications on Heroku’s free tier is described in the following official documentation:

• Webserver - Getting Started with Rails 5.x on Heroku | Heroku Dev Center

Procfile

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

The puma server is started with the bundle exec command using the following options:

• -t 5:5
• -p ${PORT:-3000}
• -e ${RACK_ENV:-development}

Let’s check the content of each option.

puma -t 5:5 Option

The -t (or —threads) option specifies the minimum and maximum number of threads for the puma server to handle parallel request processing.

puma -h
puma  
    -t, --threads INT                min:max threads to use (default 0:16)

In the Heroku official documentation, both minimum and maximum are set to 5 threads, but since only one web dyno is running, it fits within Heroku’s free tier.

puma -p ${PORT:-3000} Option

The -p (or —port) option uses the PORT environment variable value as the port number if the PORT environment variable is defined.

If there’s no PORT value, it specifies port 3000 for startup.

puma -h
puma  
    -p, --port PORT                  Define the TCP port to bind to

puma -e ${RACK_ENV:-development} Option

The -environment ${RACK_ENV:-development} option starts the puma server in the RACK_ENV mode if the RACK_ENV environment variable is defined.

If there’s no RACK_ENV value, it starts in development mode. By the way, puma’s default behavior is also development mode.

puma -h
puma  
    -e, --environment ENVIRONMENT    The environment to run the Rack app on (default development)

The key point of this article was that 5 threads are running but only one Dyno is started, so it fits within Heroku’s free tier.

Reference Information

• Process Types and the Procfile | Heroku Dev Center
• Deploying Rails Applications with the Puma Web Server | Heroku Dev Center

That’s all from the Gemba.