Welcome to Puma 5: Spoony Bard.

Puma 5 brings new experimental performance features, a few quality-of-life features and loads of bugfixes.

What's New

Better throughput and lower latency via new experimental option
Better memory usage via new experimental option
Loads of bugfixes
Faster phased restarts and worker timeouts
GC Compact/friendly fork
pumactl now has a thread-backtraces command to print thread backtraces, bringing thread backtrace printing to all platforms, not just *BSD and Mac. (#2053)
Added incrementing requests_count to Puma.stats. (#2106)
Faster phased restart and worker timeout (#2220)
Added state_permission to config DSL to set state file permissions (#2238)
Ruby 2.2 support will be dropped in Puma 6. This is the final major release series for Ruby 2.2.

If you did not explicitly set environment before, we now check RAILS_ENV and use that, if available. This may effect which config file Puma attempts to load.
If you have been using the --control CLI option, update your scripts to use --control-url.
If you are using worker_directory in your config file, change it to directory.
If you are running MRI, default thread count on Puma is now 5, not 16. This may change the amount of threads running in production. We believe 5 is a better default for most Ruby web applications on MRI. Higher settings increase latency by causing GVL contention.
If you are using a worker count of more than 1 and you are not using phased_restart, Puma will now preload by default. We believe this is a better default, but may cause issues in non-Rails applications if you do not have the proper before and after fork hooks configured.
tcp mode and daemonization have been removed without replacement.
connected_port was renamed to connected_ports and now returns an Array, not an Integer.

Then, update your Gemfile:

gem 'puma', '< 6'