Puma: A Ruby Web Server Built For Parallelism

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Puma is a simple, fast, multi-threaded, and highly parallel HTTP 1.1 server for Ruby/Rack applications.

Built For Speed & Parallelism

Puma is a server for Rack-powered HTTP applications written in Ruby. It is:

  • Multi-threaded. Each request is served in a separate thread. This helps you serve more requests per second with less memory use.
  • Multi-process. "Pre-forks" in cluster mode, using less memory per-process thanks to copy-on-write memory.
  • Standalone. With SSL support, zero-downtime rolling restarts and a built-in request bufferer, you can deploy Puma without any reverse proxy.
  • Battle-tested. Our HTTP parser is inherited from Mongrel and has over 15 years of production use. Puma is currently the most popular Ruby webserver, and is the default server for Ruby on Rails.

Originally designed as a server for Rubinius, Puma also works well with Ruby (MRI) and JRuby.

On MRI, there is a Global VM Lock (GVL) that ensures only one thread can run Ruby code 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 IO waiting to be done in parallel. Truly parallel Ruby implementations (TruffleRuby, JRuby) don't have this limitation.

Quick Start

$ gem install puma
$ puma

Without arguments, puma will look for a rackup (.ru) file in working directory called config.ru.

SSL Connection Support

Puma will install/compile with support for ssl sockets, assuming OpenSSL development files are installed on the system.

If the system does not have OpenSSL development files installed, Puma will install/compile, but it will not allow ssl connections.



Puma is the default server for Rails, included in the generated Gemfile.

Start your server with the rails command:

$ rails server

Many configuration options and Puma features are not available when using rails server. It is recommended that you use Puma's executable instead:

$ bundle exec puma


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

$ ruby app.rb -s Puma

In order to actually configure Puma using a config file, like puma.rb, however, you need to use the puma executable. To do this, you must add a rackup file to your Sinatra app:

# config.ru
require './app'
run Sinatra::Application

You can then start your application using:

$ bundle exec puma


Puma provides numerous options. Consult puma -h (or puma --help) for a full list of CLI options, or see Puma::DSL or dsl.rb.

You can also find several configuration examples as part of the test suite.

For debugging purposes, you can set the environment variable PUMA_LOG_CONFIG with a value and the loaded configuration will be printed as part of the boot process.

Thread Pool

Puma uses a thread pool. 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, from the minimum until it caps out at the maximum, based on how much traffic is present. The current default is 0:16 and on MRI is 0:5. Feel free to experiment, but be careful not to set the number of maximum threads to a large number, as you may exhaust resources on the system (or cause contention for the Global VM Lock, when using MRI).

Be aware that additionally Puma creates threads on its own for internal purposes (e.g. handling slow clients). So, even if you specify -t 1:1, expect around 7 threads created in your application.

Clustered mode

Puma also offers "clustered mode". Clustered mode forks workers from a master process. Each child process still has its own thread pool. You can tune the number of workers with the -w (or --workers) flag:

$ puma -t 8:32 -w 3

Or with the WEB_CONCURRENCY environment variable:

$ WEB_CONCURRENCY=3 puma -t 8:32

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 spawn 32 threads in total, with 16 in each worker process.

For an in-depth discussion of the tradeoffs of thread and process count settings, see our docs.

In clustered mode, Puma can "preload" your application. This loads all the application code prior to forking. Preloading reduces total memory usage of your application via an operating system feature called copy-on-write.

If the WEB_CONCURRENCY environment variable is set to a value > 1 (and --prune-bundler has not been specified), preloading will be enabled by default. Otherwise, you can use the --preload flag from the command line:

$ puma -w 3 --preload

Or, if you're using a configuration file, you can use the preload_app! method:

# config/puma.rb
workers 3

Preloading can’t be used with phased restart, since phased restart kills and restarts workers one-by-one, and preloading copies the code of master into the workers.

When using clustered mode, 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

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.

Constants loaded by your application (such as Rails) will not be available in on_worker_boot unless preloading is enabled.

You can also specify a block to be run before workers are forked, using before_fork:

# config/puma.rb
before_fork do
  # configuration here

You can also specify a block to be run after puma is booted using on_booted:

# config/puma.rb
on_booted do
  # configuration here

Error handling

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 Puma::Server#lowlevel_error or server.rb). 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|
  [500, {}, ["An error has occurred, and engineers have been informed. Please reload the page. If you continue to have problems, contact [email protected]\n"]]

Binding TCP / Sockets

Bind Puma to a socket with the -b (or --bind) flag:

$ puma -b tcp://

To use a UNIX Socket instead of TCP:

$ 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://'

Self-signed SSL certificates (via the localhost gem, for development use):

Puma supports the localhost gem for self-signed certificates. This is particularly useful if you want to use Puma with SSL locally, and self-signed certificates will work for your use-case. Currently, the integration can only be used in MRI.

Puma automatically configures SSL when the localhost gem is loaded in a development environment:

Add the gem to your Gemfile:

group(:development) do
  gem 'localhost'

And require it implicitly using bundler:

require "bundler"
Bundler.require(:default, ENV["RACK_ENV"].to_sym)

Alternatively, you can require the gem in your configuration file, either config/puma/development.rb, config/puma.rb, or set via the -C cli option:

require 'localhost'
# configuration methods (from Puma::DSL) as needed

Additionally, Puma must be listening to an SSL socket:

$ puma -b 'ssl://localhost:9292' -C config/use_local_host.rb

# The following options allow you to reach Puma over HTTP as well:
$ puma -b ssl://localhost:9292 -b tcp://localhost:9393 -C config/use_local_host.rb

Controlling SSL Cipher Suites

To use or avoid specific SSL cipher suites, use ssl_cipher_filter or ssl_cipher_list options.

$ puma -b 'ssl://!aNULL:AES+SHA'
$ puma -b 'ssl://,TLS_RSA_WITH_AES_256_CBC_SHA'

See https://www.openssl.org/docs/man1.1.1/man1/ciphers.html for cipher filter format and full list of cipher suites.

Disable TLS v1 with the no_tlsv1 option:

$ puma -b 'ssl://'

Controlling OpenSSL Verification Flags

To enable verification flags offered by OpenSSL, use verification_flags (not available for JRuby):

$ puma -b 'ssl://'

You can also set multiple verification flags (by separating them with coma):

$ puma -b 'ssl://,CRL_CHECK'


Controlling OpenSSL Password Decryption

To enable runtime decryption of an encrypted SSL key (not available for JRuby), use key_password_command:

$ puma -b 'ssl://'

key_password_command must:

  1. Be executable by Puma.
  2. Print the decryption password to stdout.

For example:


echo "this is my password"

key_password_command can be used with key or key_pem. If the key is not encrypted, the executable will not be called.

Control/Status Server

Puma has a built-in status and control app that can be used to query and control Puma.

$ puma --control-url tcp:// --control-token foo

Puma will start the control server on localhost port 9293. All requests to the control server will need to include control token (in this case, token=foo) as a query parameter. This allows for simple authentication. Check out Puma::App::Status or status.rb to see what the status app has available.

You can also interact with the control server via pumactl. This command will restart Puma:

$ pumactl --control-url 'tcp://' --control-token foo restart

To see a list of pumactl options, use pumactl --help.

Configuration File

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

$ puma -C /path/to/config

If no configuration file is specified, Puma will look for a configuration file at config/puma.rb. If an environment is specified (via the --environment flag or through the APP_ENV, RACK_ENV, or RAILS_ENV environment variables) Puma looks for a configuration file at config/puma/<environment_name>.rb and then falls back to config/puma.rb.

If you want to prevent Puma from looking for a configuration file in those locations, include the --no-config flag:

$ puma --no-config

# or

$ puma -C "-"

The other side-effects of setting the environment are whether to show stack traces (in development or test), and setting RACK_ENV may potentially affect middleware looking for this value to change their behavior. The default puma RACK_ENV value is development. You can see all config default values in Puma::Configuration#puma_default_options or configuration.rb.

Check out Puma::DSL or dsl.rb to see all available options.


Puma includes the ability to restart itself. When available (MRI, Rubinius, JRuby), Puma performs a "hot restart". This is the same functionality available in Unicorn and NGINX which keep the server sockets open between restarts. This makes sure that no pending requests are dropped while the restart is taking place.

For more, see the Restart documentation.


Puma responds to several signals. A detailed guide to using UNIX signals with Puma can be found in the Signals documentation.

Platform Constraints

Some platforms do not support all Puma features.

  • JRuby, Windows: server sockets are not seamless on restart, they must be closed and reopened. These platforms have no way to pass descriptors into a new process that is exposed to Ruby. Also, cluster mode is not supported due to a lack of fork(2).
  • Windows: Cluster mode is not supported due to a lack of fork(2).
  • Kubernetes: The way Kubernetes handles pod shutdowns interacts poorly with server processes implementing graceful shutdown, like Puma. See the kubernetes section of the documentation for more details.

Known Bugs

For MRI versions 2.2.7, 2.2.8, 2.2.9, 2.2.10, 2.3.4 and 2.4.1, you may see stream closed in another thread (IOError). It may be caused by a Ruby bug. It can be fixed with the gem https://rubygems.org/gems/stopgap_13632:

if %w(2.2.7 2.2.8 2.2.9 2.2.10 2.3.4 2.4.1).include? RUBY_VERSION
    require 'stopgap_13632'
  rescue LoadError


  • Puma has support for Capistrano with an external gem.

  • Additionally, Puma has support for built-in daemonization via the puma-daemon ruby gem. The gem restores the daemonize option that was removed from Puma starting version 5, but only for MRI Ruby.

It is common to use process monitors with Puma. Modern process monitors like systemd or rc.d provide continuous monitoring and restarts for increased reliability in production environments:

Community guides:

Community Extensions


  • puma-metrics — export Puma metrics to Prometheus
  • puma-plugin-statsd — send Puma metrics to statsd
  • puma-plugin-systemd — deeper integration with systemd for notify, status and watchdog. Puma 5.1.0 integrated notify and watchdog, which probably conflicts with this plugin. Puma 6.1.0 added status support which obsoletes the plugin entirely.
  • puma-plugin-telemetry - telemetry plugin for Puma offering various targets to publish
  • puma-acme - automatic SSL/HTTPS certificate provisioning and setup


  • puma-status — Monitor CPU/Mem/Load of running puma instances from the CLI


Find details for contributing in the contribution guide.


Puma is copyright Evan Phoenix and contributors, licensed under the BSD 3-Clause license. See the included LICENSE file for details.