The publish and subscribe model can be used in several different ways. It can be used as a process integration tool where a Ruby object publishes to external application using something Redis or NATS. It can also be used as means for Ruby object to exchange data using subject or channel based addressing. It can also be used to push data to external listeners over a WebSocket or SSE connection.
If there is a desire to merge all three into one API then a glue component can be introduced that distributes messages across all three (or more) systems. The glue component could also be used to bridge publish servers so that a message received on the Extern.publish (represents a server) could then be bridge to both the Local.publish and Rack::Push.publish servers.
The use of a glue component allows each of the three servers to be written completely independently. It also allows different implemenations of each to be used if the developer wish to change any of the servers.
The diagram shows publishing from a Ruby object to a listener through a server. In each case the reverse is also possible except for an SSE connection.
