Akira Matsuda gave a really good talk on some of the problems that CookPad has run into with their quite sizable Rails app:
300 app servers, deploys are slow (~20min) => replaced capistrano with a handmade deploy program (mamiya) which deploys in 1 min
1000+ models, 30 databases, ActiveRecord is choking => read / write splitting, handmade ActiveRecord adapter for this (switch_point)
ActiveRecord migrations take forever to execute => ridgepole
Big rails app with big team results in code conflicts => created a rapid prototyping framework for rails (chanko)
Upgrading rails: created a response verification tool (kage)
Conclusion: Monolithic apps can scale, you don’t have to split it into many apps, eventho' microservices are really hot these days. Microservices bring their own problems and are not a silver bullet.