From https://once.com/writebook

config/puma.rb

@@ -0,0 +1,18 @@
+threads_count = ENV.fetch("RAILS_MAX_THREADS", 3)
+threads threads_count, threads_count
+
+rails_env = ENV.fetch("RAILS_ENV", "development")
+environment rails_env
+
+case rails_env
+when "production"
+  workers_count = Integer(ENV.fetch("WEB_CONCURRENCY") { (Concurrent.processor_count * 0.666).ceil })
+  workers workers_count if workers_count > 1
+
+  preload_app!
+when "development"
+  worker_timeout 3600 # Don't let worker die during debugger session
+end
+
+port ENV.fetch("PORT", 3000)
+plugin :tmp_restart