Puma: A Ruby Web Server Built For Concurrency

Description

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.

Quick Start

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

Advanced Setup

Sinatra

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).

Rails

First, make sure Puma is in your Gemfile:

gem 'puma'

Then start your server with the rails command:

$ rails s Puma

Rackup

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

Configuration

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

Thread Pool

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).

Binding TCP / Sockets

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=0777'

Need a bit of security? Use SSL sockets!

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

Control/Status Server

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 https://github.com/puma/puma/blob/master/lib/puma/app/status.rb to see what the app has available.

Configuration file

You can also provide a configuration file which puma will use:

$ puma --config /path/to/config

$ puma -C /path/to/config

Take the following sample configuration as inspiration or check out configuration.rb to see all available options.

Restart

Puma includes the ability to restart itself, allowing for new versions to be easily upgraded to. When available (currently anywhere but 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.

To perform a restart, there are 2 builtin mechanism:

Send the puma process the SIGUSR2 signal
Use the status server and issue /restart

No code is shared between the current and restarted process, so it should be safe to issue a restart any place where you would manually stop puma and start it again.

If the new process is unable to load, it will simply exit. You should therefore run puma under a supervisor when using it in production.

Cleanup Code

Puma isn't able to understand all the resources that your app may use, so it provides a hook in the configuration file you pass to -C call on_restart. The block passed to on_restart will be called, unsurprisingly, just before puma restarts itself.

You should place code to close global log files, redis connections, etc in this block so that their file descriptors don't leak into the restarted process. Failure to do so will result in slowly running out of descriptors and eventually obscure crashes as the server is restart many times.

pumactl

If you start puma with -S some/path then you can pass that same path to the pumactl program to control your server. For instance:

$ pumactl -S some/path command

$ pumactl -C url -T token command

will cause the server to perform a restart. pumactl is a simple CLI frontend to the control/status app described above.

Allowed commands: status, restart, halt, stop

Managing multiple Pumas / init.d / upstart scripts

If you want an easy way to manage multiple scripts at once check tools/jungle for init.d and upstart scripts.

Capistrano deployment

Puma has included Capistrano deploy script, you just need require that:

config/deploy.rb

require 'puma/capistrano'

and then

$ bunde exec cap puma:start
$ bunde exec cap puma:restart 
$ bunde exec cap puma:stop

glebm/puma