Travis CI – what's inside?

This document appears to be notes from a presentation about the evolution of Travis CI from a monolithic application to a distributed, service-oriented architecture. It describes how Travis CI was originally a single Ruby on Rails application, but was broken out into multiple services for improved scalability, reliability and maintainability. Key points discussed include introducing background workers, switching to VirtualBox for isolated builds, adding a message queue and splitting functionality like the API into separate services/domains. Design philosophies around incremental changes, metrics, feature flags and database migrations are also covered.