What are engines?

[adamakhtar]

@codereading/readers

Seeing as this is the first project to use Rails Engines it be a good start to understand exactly what an engine is.

How does an independent engine hook into a rails app? How does it get access to the rails apps db, params etc.

What other ways are there to create a project for rails use and how do they differ? Why would you choose one way over another?

I.e. why an engine and not just a simple gem. When should you use a vendored plugin?

Just some questions swimming in my mind.

[chamnap]

As fas as I know, a rails engine is a way to share app controllers, models, and views to other apps. In rails 3, every rails app is also engine and can be bundled as a gem as well.

There are two types of rails engines: mountable and full engine. Mountable engine is designed to be namespaced controllers, models,... For example, a forem engine, add forum to any existing rails applications. Some engines use the existing application layout (forem) or use its own layout (spree). A full engine is to augment something to an existing rails app.

Vendored plugins are deprecated in Rails 4.0, and I think people will stop using that.

[agis]

http://edgeapi.rubyonrails.org/classes/Rails/Engine.html
http://railscasts.com/episodes/277-mountable-engines
http://www.stubbleblog.com/index.php/2011/04/writing-rails-engines-getting-started/

So Engines are a way to DRY up your apps by sharing common functionality without having to mess with the main app's code.

[samnang]

Engines can be considered miniature applications that provide functionality to their host applications. A Rails application is actually just a “supercharged” engine, with the Rails::Application class inheriting a lot of its behaviour from Rails::Engine.

I found few interesting links:
http://edgeguides.rubyonrails.org/engines.html (I'm not sure why this page doesn't list in RailsGuides Index)
http://gregmoreno.wordpress.com/2012/05/29/create-your-own-rails-3-engine/
http://stackoverflow.com/questions/6118905/rails-3-1-engine-vs-mountable-app
http://www.astjohn.ca/2011/08/06/rails-31-engines-mountable-or-full-part-1
http://www.astjohn.ca/2011/08/07/rails-31-engines-mountable-or-full-part-2

I'm still not so clear what are the benefit to create full engines instead mountable engines? Because I think most of the time we need mountable engines.

[samnang]

Just found Ryan Bigg's talk about Engines https://vimeo.com/41952172

[orlando]

@samnang 👍 thanks for the video

[adamakhtar]

Excellent resources everyone +1. These will be of a great help to newcomers.

[agis]

http://confreaks.com/videos/1109-gogaruco2012-writing-a-rails-engine