Yes, this is similar to Plug, but we only have a "connection". There is no distinction b/w request/response. A series of Plugs form a transformation layer on the connection where plug middleware can transform the connection, send a response etc. "Yielding to the next plug" is just a matter of returning the connection. There is no explicit yield. You either halt the connection (don't invoke further plugs) or continue the stack by returning the conn unhalted. Where your "upstream" concept fits is a connection can have callbacks bound that are invoked just before the response is sent, this is where a cache plug would cache the response. Plug avoids the issue of separating request/response because for things like streaming, sending a response does not mean that the stack is done with the request. Rack has this issue with streaming and José put a great deal of thought into Plug around the lessoned learned from Rack style APIs. You can read about his thoughts here: http://blog.plataformatec.com.br/2012/06/why-your-web-framew...

> From what I understand, once a Plug chooses to reply, the response ends at that Plug.

In Phoenix, we halt the connection when you use most of our functions that send a response, because there is little you can do afterwards. The Plug response API itself never halts when a response is sent.