I am currently working on modifying an existing large rails app.  I am customizing some of the look and feel and extending functionality.

The app is under current development and I wanted to be able to take advantage of bug fixes or improvements, without impacting my customizations.  Or at least minimizing that impact.

Being fairly new to Rails, I surveyed the landscape and thought that building my customizations as an engine would be a good way to go.  (I was wrong, because engines have a hard time reaching out and modifying the application that they are part of.  At least that seemed to be a non standard use of engines from what I can find.) The author of Modular Rails has some good blog posts about engines and modularity, so I bought his book.

Pluses:

• Good overview of how to extend three major components of rails app, models, views and controllers
• Easy reading style
• Leverages existing gems like deface
• Mentions testing
• Starts from first principles and then later gives you a gem to speed up development
• Not too long
• Information on setting up your own gems server

Minuses:

• Focus on ‘greenfield’ apps.  No mention of integration with existing monoliths.
• Uses nested modules, unlike every other engine article out there
• Assumes relatively advanced knowledge of rails
• Fair bit of fluff–lots of ‘mv’ commands
• Extra charge for source code

All in all I am glad I read this book.  It didn’t fit my needs, but it didn’t promise that either.  I found it a good overview of the engine concept, even if he did do some things in a non standard manner and was a bit verbose about unix commands.

If you have done more Rails development, it will be more useful, and it is a great way to think about building new freestanding applications.  I haven’t surveyed the entire rails book landscape but I haven’t found anything out there focusing on Rails engines that is better.