Rails + Sidekiq + Puma + Heroku

database.yml

production:
  adapter: postgresql
  url:  <%= ENV["DATABASE_URL"] %>
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>

Environment variables in Heroku

WEB_PROCESSES=2
WEB_THREADS=5
WORKER_THREADS=15

Procfile

web: RAILS_MAX_THREADS=$WEB_THREADS bundle exec puma -C config/puma.rb
worker: RAILS_MAX_THREADS=$WORKER_THREADS bundle exec sidekiq -C config/sidekiq.yml

puma.rb

threads_count = ENV.fetch("WEB_THREADS") { 5 }.to_i
threads threads_count, threads_count

port ENV.fetch("PORT") { 3000 }
environment ENV.fetch("RAILS_ENV") { "development" }

workers ENV.fetch("WEB_PROCESSES") { 3 }.to_i

preload_app!

before_fork do
  ActiveRecord::Base.connection_pool.disconnect! if defined?(ActiveRecord)
end

on_worker_boot do
  ActiveRecord::Base.establish_connection if defined?(ActiveRecord)
  Rails.logger.info("Database connection pool size for this web process is: #{ActiveRecord::Base.connection.pool.instance_variable_get('@size')}")
end

sidekiq.yml

---
:concurrency: <%= ENV['WORKER_THREADS'] %>