This is the application server. It's a Ruby/Sinatra application that exposes a number of APIs that are consumed by the UI (or via curl if you fancy that). 

Originally this application was included in a single file (`yelb-appserver.rb`). This has been since refactored by extracting the single API definition in their separate adapters and modules. This made the transition to Lambda/Serverless easier (one lambda per API definition). This hasn't changed the other deployment models (containers and instances) because those models still launch the `yelb-appserver.rb` main application which imports the modules instead of having everything in a single file (as it was conceived originally).