Puma is a simple, fast, threaded, and highly concurrent HTTP 1.1 server for Ruby/Rack applications. Puma is intended for use in both development and production environments. In order to get the best throughput, it is highly recommended that you use a Ruby implementation with real threads like Rubinius or JRuby.

Puma is a simple, fast, and highly concurrent HTTP 1.1 server for Ruby web applications. It can be used with any application that supports Rack, and is considered the replacement for Webrick and Mongrel. It was designed to be the go-to server for Rubinius, but also works well with JRuby and MRI. Puma is intended for use in both development and production environments.

Under the hood, Puma processes requests using a C-optimized Ragel extension (inherited from Mongrel) that provides fast, accurate HTTP 1.1 protocol parsing in a portable way. Puma then serves the request in a thread from an internal thread pool (which you can control). This allows Puma to provide real concurrency for your web application!

With Rubinius 2.0, Puma will utilize all cores on your CPU with real threads, meaning you won't have to spawn multiple processes to increase throughput. You can expect to see a similar benefit from JRuby.

On MRI, there is a Global Interpreter Lock (GIL) that ensures only one thread can be run at a time. But if you're doing a lot of blocking IO (such as HTTP calls to external APIs like Twitter), Puma still improves MRI's throughput by allowing blocking IO to be run concurrently (EventMachine-based servers such as Thin turn off this ability, requiring you to use special libraries). Your mileage may vary. In order to get the best throughput, it is highly recommended that you use a Ruby implementation with real threads like Rubinius or JRuby.

The easiest way to get started with Puma is to install it via RubyGems. You can do this easily:

$ gem install puma

Now you should have the puma command available in your PATH, so just do the following in the root folder of your Rack application:

$ puma app.ru

You can run your Sinatra application with Puma from the command line like this:

$ ruby app.rb -s Puma

Or you can configure your application to always use Puma:

require 'sinatra'
configure { set :server, :puma }

If you use Bundler, make sure you add Puma to your Gemfile (see below).

First, make sure Puma is in your Gemfile:

gem 'puma'

Then start your server with the rails command:

$ rails s Puma

You can pass it as an option to rackup:

$ rackup -s Puma

Alternatively, you can modify your config.ru to choose Puma by default, by adding the following as the first line:

#\ -s puma

Puma provides numerous options for controlling the operation of the server. Consult puma -h (or puma --help) for a full list.

Puma utilizes a dynamic thread pool which you can modify. You can set the minimum and maximum number of threads that are available in the pool with the -t (or --threads) flag:

$ puma -t 8:32

Puma will automatically scale the number of threads based on how much traffic is present. The current default is 0:16. Feel free to experiment, but be careful not to set the number of maximum threads to a very large number, as you may exhaust resources on the system (or hit resource limits).

Puma 2 offers clustered mode, allowing you to use forked processes to handle multiple incoming requests concurrently, in addition to threads already provided. You can tune the number of workers with the -w (or --workers) flag:

$ puma -t 8:32 -w 3

On a ruby implementation that offers native threads, you should tune this number to match the number of cores available. Note that threads are still used in clustered mode, and the -t thread flag setting is per worker, so -w 2 -t 16:16 will be 32 threads.

If you're running in Clustered Mode you can optionally choose to preload your application before starting up the workers. This is necessary in order to take advantage of the Copy on Write feature introduced in MRI Ruby 2.0. To do this simply specify the --preload flag in invocation:

# CLI invocation
$ puma -t 8:32 -w 3 --preload

If you're using a configuration file, use the preload_app! method, and be sure to specify your config file's location with the -C flag:

$ puma -C config/puma.rb

# config/puma.rb
threads 8,32
workers 3
preload_app!

Additionally, you can specify a block in your configuration file that will be run on boot of each worker:

# config/puma.rb
on_worker_boot do
  # configuration here
end

This code can be used to setup the process before booting the application, allowing you to do some Puma-specific things that you don't want to embed in your application. For instance, you could fire a log notification that a worker booted or send something to statsd. This can be called multiple times to add hooks.

If you're preloading your application and using ActiveRecord, it's recommend you setup your connection pool here:

# config/puma.rb
on_worker_boot do
  ActiveSupport.on_load(:active_record) do
    ActiveRecord::Base.establish_connection
  end
end

When you use preload_app, your new code goes all in the master process, and is then copied in the workers (meaning it’s only compatible with cluster mode). General rule is to use preload_app when your workers die often and need fast starts. If you don’t have many workers, you probably should not use preload_app.

Note that preload_app can’t be used with phased restart, since phased restart kills and restarts workers one-by-one, and preload_app is all about copying the code of master into the workers.

If puma encounters an error outside of the context of your application, it will respond with a 500 and a simple textual error message (see lowlevel_error in this file). You can specify custom behavior for this scenario. For example, you can report the error to your third-party error-tracking service (in this example, rollbar):

lowlevel_error_handler do |e|
  Rollbar.critical(e)
  [500, {}, ["An error has occurred, and engineers have been informed. Please reload the page. If you continue to have problems, contact support@example.com\n"]]
end

In contrast to many other server configs which require multiple flags, Puma simply uses one URI parameter with the -b (or --bind) flag:

$ puma -b tcp://127.0.0.1:9292

Want to use UNIX Sockets instead of TCP (which can provide a 5-10% performance boost)? No problem!

$ puma -b unix:///var/run/puma.sock

If you need to change the permissions of the UNIX socket, just add a umask parameter:

$ puma -b 'unix:///var/run/puma.sock?umask=0111'

Need a bit of security? Use SSL sockets!

$ puma -b 'ssl://127.0.0.1:9292?key=path_to_key&cert=path_to_cert'

Puma comes with a builtin status/control app that can be used query and control Puma itself. Here is an example of starting Puma with the control server:

$ puma --control tcp://127.0.0.1:9293 --control-token foo

This directs Puma to start the control server on localhost port 9293. Additionally, all requests to the control server will need to include token=foo as a query parameter. This allows for simple authentication. Check out status.rb to see what the app has available.

You can also provide a configuration file which Puma will use with the -C (or --config) flag:

$ puma -C /path/to/config