When I first looked up Passenger, a Gumdam-like robot icon appears. From the site itself, “Battle-hardened by some of the most demanding web apps for over a decade, Passenger is considered one of the most performant, secure and mature app servers currently available.” (1)
OK so what is it? Passenger is an open source web application server which handles HTTP requests, manages processes and resources, and enables administration, monitoring and problem diagnosis. But then question is, if I type “Rails S” why would I need passenger? Passenger doesn’t replace “rails server” in the sense of it being a test platform only. What “rails server” does is it uses an application server – such as Passenger – directly to server ruby files.
The default web application server activated when you use “rails server” is Puma. Looking at the Gemfile, you can see “gem ‘puma’, ‘~> 3.7’”. I think this is a great thing to realize, a lot of new Rails developers need to not only know about how to write code in Ruby and Rails, but also about the ecosystem that runs a web application, instead if using Heroku as a crutch. Standing up a server on a standalone box can be a great learning experience. With Linux you would use Nginx/Apache to host pages, as they are web servers. They provide the HTTP transaction handling and serve static files. However, they are not Ruby application servers and don’t run the Ruby applications by themselves directly. Nginx and Apache are used with an application server, such as Puma or Passenger. What application servers do is make it possible for Ruby apps to speak HTTP. Ruby apps (and frameworks like Rails) can’t do that by themselves. So we need the application servers so the ruby apps can talk to http. But application servers aren’t as good as Nginx and Apache at handling HTTP requests because Nginx and Apache are better at handling I/O security, HTTP concurrency management, connection timeouts, etc. This is why a good Rails Engineer will still need to know about Linux, Apache, and Nginx.
Why choose Passenger over Puma? Passenger’s strength is that is also can host Python, Node.js, and Meteor apps as well. Passenger is the best choice when you have apps developed in several languages that Passenger supports and you want to consolidate your app server stack. Puma’s strength is that it’s easy to configure and mostly “just works” out-of-the-box. I think that after getting familiar with what Puma does, its better to look at Passenger for growth cases