I’m testing server-side events with Plug’s send_chunked/2 and chunk/2 but I also receive data from a stream. Due to the latter detail I decided to implement the receiving of the streamed data in a GenServer, the request is made with HTTPoison and it is similar to what is mentioned in this StackOverflow post, although I receive the chunk data in a slightly different format (still a non-JSON string with a data key), and had to also implement a handle_info callback for messages with a %HTTPoison.AsyncEnd{} struct.

Other than that it’s working and the GenServer is started from my controller. So essentially I have a controller that receives a request, then spins up a GenServer that makes another request, receives streamed data and has to send the data parsed back to the original connection.

The issue comes when trying to send the data back from the GenServer. I tried passing the conn struct to the genserver and calling Plug.Conn.chunk/2 from it but it seems that Cowboy recognized that a separate process sends a messages, raises a warning and then the GenServer stops processing. The Cowboy warning raised is here in the Cowboy code: [warning] Received message #{message inspected} for unknown stream 1. (or another stream ID, but generally integers 1 to 3).

As Cowboy spawns a process for each request I assume it expects that responses should be sent from this process which is what is causing this issue. I’ll probably implement a receive loop without spawning a different process to handle this, but I was wondering if I’m not missing or misunderstanding something and if there isn’t a way to send a response with Plug (Cowboy) from a separate process, in this case a GenServer.