Seldon is made up of many components that work together to deliver the best recommendations. Roughly, all user actions are captured via the REST API and streamed to logs. Those logs are processed in batch and new user models are delivered to the API Server. Then recommendations are delivered via the REST API.
http://docs.seldon.io/tech.html
The code for Seldom is on Github at https://github.com/SeldonIO/seldon-server.