Puma: A Ruby Web Server Built For Parallelism

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

Built For Speed & Parallelism

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 using a thread pool. Each request is served in a separate thread, so truly parallel Ruby implementations (JRuby, Rubinius) will use all available CPU cores.

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.

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.

Frameworks

Rails

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

Sinatra

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

Configuration

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

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.

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 (Ruby 2.0+ only). Use the --preload flag from the command line:

$ puma -w 3 --preload

If you're using a configuration file, use the preload_app! method:

# config/puma.rb
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.

Constants loaded by your application (such as Rails) will not be available in on_worker_boot. However, these constants will be available if preload_app! is enabled, either explicitly in your puma config or automatically if using 2 or more workers in cluster mode. If preload_app! is not enabled and 1 worker is used, then on_worker_boot will fire, but your app will not be preloaded and constants will not be available.

before_fork specifies a block to be run before workers are forked:

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

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

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|
  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

Binding TCP / Sockets

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

$ puma -b tcp://127.0.0.1:9292

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://127.0.0.1:9292?key=path_to_key&cert=path_to_cert'

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'
end

# And require it implicitly using bundler
require "bundler"
Bundler.require(:default, ENV["RACK_ENV"].to_sym)

# Alternatively, you can require the gem in config.ru:
require './app'
require 'localhost'
run Sinatra::Application

Additionally, Puma must be listening to an SSL socket:

$ puma -b 'ssl://localhost:9292' config.ru

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

Controlling SSL Cipher Suites

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

Ruby:

$ puma -b 'ssl://127.0.0.1:9292?key=path_to_key&cert=path_to_cert&ssl_cipher_filter=!aNULL:AES+SHA'

JRuby:

$ puma -b 'ssl://127.0.0.1:9292?keystore=path_to_keystore&keystore-pass=keystore_password&ssl_cipher_list=TLS_RSA_WITH_AES_128_CBC_SHA,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://127.0.0.1:9292?key=path_to_key&cert=path_to_cert&no_tlsv1=true'

Controlling OpenSSL Verification Flags

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

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

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

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

List of available flags: USE_CHECK_TIME, CRL_CHECK, CRL_CHECK_ALL, IGNORE_CRITICAL, X509_STRICT, ALLOW_PROXY_CERTS, POLICY_CHECK, EXPLICIT_POLICY, INHIBIT_ANY, INHIBIT_MAP, NOTIFY_POLICY, EXTENDED_CRL_SUPPORT, USE_DELTAS, CHECK_SS_SIGNATURE, TRUSTED_FIRST, SUITEB_128_LOS_ONLY, SUITEB_192_LOS, SUITEB_128_LOS, PARTIAL_CHAIN, NO_ALT_CHAINS, NO_CHECK_TIME (see https://www.openssl.org/docs/manmaster/man3/X509_VERIFY_PARAM_set_hostflags.html#VERIFICATION-FLAGS).

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://127.0.0.1:9293 --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://127.0.0.1:9293' --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.

Restart

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.

Signals

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
  begin
    require 'stopgap_13632'
  rescue LoadError
  end
end

Deployment

Puma has support for Capistrano with an external gem.

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:

• rc.d
• systemd

Community guides:

• Deploying Puma on OpenBSD using relayd and httpd

Community Extensions

Plugins

• 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

Monitoring

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

Contributing

Find details for contributing in the contribution guide.

License

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