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START YOUR (RAILS) ENGINES! UNDERSTANDING, BUILDING, AND INTEGRATING RAILS ENGINES

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Ariel Caplan @amcaplan amcaplan.ninja QUALITY COST $ $ ACCESS

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OUR ITINERARY

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OUR ITINERARY 1. What are Rails Engines?

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OUR ITINERARY 1. What are Rails Engines? 2. Live Code an Engine!

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OUR ITINERARY 1. What are Rails Engines? 2. Live Code an Engine! 3. A Brief History
 of Rails Engines

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OUR ITINERARY 1. What are Rails Engines? 2. Live Code an Engine! 3. A Brief History
 of Rails Engines 4. The When and Why
 of Rails Engines

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WHAT ARE ENGINES?

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- RailsGuides: Getting Started with Engines
 (http://guides.rubyonrails.org/engines.html) Engines can be considered miniature applications that provide functionality to their host applications.

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- me Engines let you build gems with access to autoloading magic, and the full Rails MVC architecture.

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OK, BUT$ ARE THEY USEFUL?

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ENGINES YOU ALREADY USE • resque-web$ • rollout-ui$ • PaperTrail$ • RailsAdmin

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ENGINES YOU ALREADY USE • bootstrap-sass

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ENGINES YOU ALREADY USE • bootstrap-sass • coffee-rails

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ENGINES YOU ALREADY USE • bootstrap-sass • jquery-rails • coffee-rails

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ENGINES YOU ALREADY USE • bootstrap-sass • jquery-rails • turbolinks • coffee-rails

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ENGINES YOU ALREADY USE All Your Rails Apps!

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- RailsGuides: Getting Started with Engines
 (http://guides.rubyonrails.org/engines.html) A Rails application is actually just a "supercharged" engine, with the Rails::Application class inheriting a lot of its behavior from Rails::Engine.

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https://github.com/rails/rails/blob/master/railties/lib/rails/ application.rb#L79 module Rails # ... class Application < Engine # ... end end

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LET’S BUILD$ AN ENGINE!

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USER STORY

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COMMON ENDPOINT TO CLEAR THE CACHE

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THERE WILL BE CONFUSION

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THERE WILL BE HURDLES

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IT’S A GREAT HAMMER

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IT’S A GREAT HAMMER … BUT NOT EVERYTHING IS A NAIL

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ANATOMY OF AN ENGINE

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ENGINE BUILDING 101

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LET’S WRITE SOME CODE!

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module MyRailsEngine class Engine < ::Rails::Engine isolate_namespace MyRailsEngine ! initializer :append_migrations do |app| unless app.root.to_s.match root.to_s + File::SEPARATOR config.paths["db/migrate"].expanded.each do |path| app.config.paths["db/migrate"] << path end end end end end KEEPING MIGRATIONS IN YOUR ENGINE

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Copy Migrations Add Migrations to the Load Path Simplified Upgrades ❌ ✅ Migrations Stored in Host App ✅ ❌

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A HISTORY LESSON

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IN THE BEGINNING…

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IN THE BEGINNING… There was Adam.

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IN THE BEGINNING… There was Adam. James Adam.

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- DHH, “Why engines and components are not evil but distracting”
 (http://weblog.rubyonrails.org/2005/11/11/why-engines-and- components-are-not-evil-but-distracting/) I think these developments are basically another name for high-level components. And you all know how I feel about those. $ ! The short summary is that high-level components are a mirage: By the time they become interesting, their fitting will require more work than creating something from scratch.

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Engines have not recieved (sic) the blessing of the RoR core team, and I$ wouldn't expect any different, because it would be madness to include them in the core Rails. It's a mechanism far too easily [ab]used for things it's not actually suitable for. - James Adam, from the Ruby on Rails mailing list
 (http://article.gmane.org/gmane.comp.lang.ruby.rails/29166)

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SO WHY WERE ENGINES CREATED?

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- James Adam, from the Ruby on Rails mailing list
 (http://article.gmane.org/gmane.comp.lang.ruby.rails/29166) Engines fill a fairly specific need that we
 (my team) have, which is the development of multiple distinct Rails applications which share common components (authentication, reporting, importing data from hellish Excel), and ensuring that a means to apply patches & bug fixes across each of these applications, including models, controllers and views with the *minimum* of developer time spend (sic) on managing those updates.

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FAST-FORWARD…

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COMMON USE CASES FOR ENGINES - Trent Albright “Open Sourcing with Rails Engines”, LivingSocial tech blog
 (https://techblog.livingsocial.com/blog/2014/01/24/open-sourcing-with-rails-engines/)

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COMMON USE CASES FOR ENGINES • Mini-applications (resque-web) - Trent Albright “Open Sourcing with Rails Engines”, LivingSocial tech blog
 (https://techblog.livingsocial.com/blog/2014/01/24/open-sourcing-with-rails-engines/)

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COMMON USE CASES FOR ENGINES • Mini-applications (resque-web) • Plug-in for other Rails applications (Devise) - Trent Albright “Open Sourcing with Rails Engines”, LivingSocial tech blog
 (https://techblog.livingsocial.com/blog/2014/01/24/open-sourcing-with-rails-engines/)

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COMMON USE CASES FOR ENGINES • Mini-applications (resque-web) • Plug-in for other Rails applications (Devise) • Composition (SOA components) - Trent Albright “Open Sourcing with Rails Engines”, LivingSocial tech blog
 (https://techblog.livingsocial.com/blog/2014/01/24/open-sourcing-with-rails-engines/)

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COMMON USE CASES FOR ENGINES • Mini-applications (resque-web) • Plug-in for other Rails applications (Devise) • Composition (SOA components) • Stand-Alone Application (Rearview) - Trent Albright “Open Sourcing with Rails Engines”, LivingSocial tech blog
 (https://techblog.livingsocial.com/blog/2014/01/24/open-sourcing-with-rails-engines/)

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WHY THE SKEPTICISM?

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PAPERTRAIL::VERSION

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DEFINING CHARACTERISTICS OF ENGINES

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DEFINING CHARACTERISTICS OF ENGINES • Which parts of MVC are required?

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DEFINING CHARACTERISTICS OF ENGINES • Which parts of MVC are required? • Database Management? Migrations?

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DEFINING CHARACTERISTICS OF ENGINES • Which parts of MVC are required? • Database Management? Migrations? • Internal Tool (full control)
 or Open Source ( ¯\_(ツ)_/¯)?

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DEFINING CHARACTERISTICS OF ENGINES What direction(s) do your dependencies run?

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DEFINING CHARACTERISTICS OF ENGINES • Independent MVC architecture What direction(s) do your dependencies run?

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DEFINING CHARACTERISTICS OF ENGINES • Independent MVC architecture • Engine depends on app (e.g., admin tool) What direction(s) do your dependencies run?

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DEFINING CHARACTERISTICS OF ENGINES • Independent MVC architecture • Engine depends on app (e.g., admin tool) • App depends on engine
 (cross-cutting concern, e.g. authentication) What direction(s) do your dependencies run?

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DEFINING CHARACTERISTICS OF ENGINES • Independent MVC architecture • Engine depends on app (e.g., admin tool) • App depends on engine
 (cross-cutting concern, e.g. authentication) • Dependencies run in both directions
 (potentially the case with components) What direction(s) do your dependencies run?

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THE LITTLE ENGINE THAT COULD

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WHAT CHANGED FROM 2005 TO TODAY?

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COMPONENTS$ ARE NOT ALWAYS INTERESTING!

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RECOMMENDED USE CASES
 FOR ENGINES

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RECOMMENDED USE CASES
 FOR ENGINES • Mini-applications or tiny utilities

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RECOMMENDED USE CASES
 FOR ENGINES • Mini-applications or tiny utilities • Distributing pieces of /M?VC/ and/or asset pipeline

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RECOMMENDED USE CASES
 FOR ENGINES • Mini-applications or tiny utilities • Distributing pieces of /M?VC/ and/or asset pipeline • Cross-cutting concerns?

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RECOMMENDED USE CASES
 FOR ENGINES • Mini-applications or tiny utilities • Distributing pieces of /M?VC/ and/or asset pipeline • Cross-cutting concerns? • PLEASE avoid migrations if possible

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RECOMMENDED USE CASES
 FOR ENGINES • Mini-applications or tiny utilities • Distributing pieces of /M?VC/ and/or asset pipeline • Cross-cutting concerns? • PLEASE avoid migrations if possible • Component-based applications?

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RECOMMENDED USE CASES
 FOR ENGINES • Mini-applications or tiny utilities • Distributing pieces of /M?VC/ and/or asset pipeline • Cross-cutting concerns? • PLEASE avoid migrations if possible • Component-based applications? • Alternatively, a step towards multiple apps

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FURTHER INFORMATION • History of Rails Engines$ • GoGaRuCo 2012, Erik Michaels-Ober - “Writing a Rails Engine” - https:// www.youtube.com/watch?v=MsRPxS7Cu_Q

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FURTHER INFORMATION • Migrations$ • Ben Smith - “Leave your migrations in your Rails engines” - http://blog.pivotal.io/labs/ labs/leave-your-migrations-in-your-rails- engines

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FURTHER INFORMATION • Components$ • RailsConf 2015, Stephan Hagemann - “Get started with Component-based Rails applications!” - https://www.youtube.com/ watch?v=MsRPxS7Cu_Q

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FURTHER INFORMATION • Components$ • Brian Leonard, “Rails 4 Engines” (Blog post about splitting up an application) - http:// tech.taskrabbit.com/blog/2014/02/11/rails-4- engines/

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FURTHER INFORMATION • Components$ • Rocky Mountain Ruby 2013, Ben Smith - “How I architected my big Rails app for success!” - https://www.youtube.com/watch? v=uDaBtqEYNBo

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FURTHER INFORMATION • Components$ • Stephan Hagemann, “Migrating from a single Rails app to a suite of Rails engines” - http:// blog.pivotal.io/labs/labs/migrating-from-a- single-rails-app-to-a-suite-of-rails-engines

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FURTHER INFORMATION • Components$ • Will Read, “Experience Report: Engine Usage That Didn't Work” - http:// blog.pivotal.io/labs/labs/experience-report- engine-usage-that-didn-t-work

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IMAGE CREDITS • “The Little Engine That Could” by Cliff: https://www.flickr.com/photos/nostri-imago/ 2851664965 / CC BY 2.0$ • “Official New York City Subway Map” by Metropolitan Transportation Authority: https:// commons.wikimedia.org/wiki/File:Official_New_York_City_Subway_Map_vc.jpg / CC BY 2.0$ • “James Bond 007 13” by Themeplus: https://www.flickr.com/photos/ 85217387@N04/8445205301 / CC BY-SA 2.0$ • “Hurdles (Scenes from a Track Meet)” by Phil Roeder: https://www.flickr.com/photos/tabor- roeder/5835126514/ / CC BY 2.0$ • “Bent Screw Hole Backyard Metal Macros April 01, 20103” by Steven Depolo: https:// www.flickr.com/photos/stevendepolo/4482491047 / CC BY 2.0

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THANK YOU! @amcaplan amcaplan.ninja We’re hiring!!! Ariel Caplan