In Puma, how do I calculate DB connections?
Actually I found the answer explained well here... https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server#database-connections
As you add more concurrency to your application, it will need more connections to your database. A good formula for determining the number of connections each application will require is to multiply the RAILS_MAX_THREADS by the WEB_CONCURRENCY. This combination will determine the number of connections each dyno will consume.
Rails maintains its database connection pool, with a new pool created for each worker process. Threads within a worker will operate on the same pool. Make sure there are enough connections inside of your Rails database connection pool so that RAILS_MAX_THREADS number of connections can be used. If you see this error:
ActiveRecord::ConnectionTimeoutError - could not obtain a database connection within 5 secondsThis error is an indication that your Rails connection pool is too low. For an in-depth look at these topics, please read the Dev Center article Concurrency and Database Connections.