message(pub, sub)

Called before delivering a message to a subscriber. Receives pub and sub, the tables returned from the publish and subscribe functions. Modifications to these tables affect only this subscriber, not others receiving the same message. Typically used for routing and subscriber-specific adjustments.

NOTE: Changes to the inner req tables will not be preserved.

function message(pub, sub)
  -- Subscriber-specific logic such as routing, message modifications, etc.
  
  -- Returning `nil` (or just nothing at all) will prevent this
  -- subscriber from receiving the message.
  -- Returning the `pub` table to the server will continue with
  -- delivery of the (possibly modified) SSE message to the subscriber.
  return pub
end