How to build server sent events with Phoenix?

yes, it is doable. Check out the source code for the official phoenix.js client library for reference, here’s direct link to the class/object/whatever responsible for establishing and handling websocket connection:

https://github.com/phoenixframework/phoenix/blob/master/priv/static/phoenix.js#L649

Basically you connect to a websocket by URL and listen to onMessage, where you get a JSON payload to deserialize and interpret. For 1-way connection that could be all you need.