Rather than putting all domain logic into a single application monolith, modern software architectures tend to split functionality into multiple applications and services.

Here is a list of 12 points we consider when building services, including using our open sourced gem stiches to encapsulate the commonalities and rspec_api_documention to document our internal APIs.

http://multithreaded.stitchfix.com/blog/2016/03/01/services-checklist/