Asynchronous Observer Pattern

Question

I wanted to find out other ways to do Asynchronous Observer Pattern without using Message Queue. Ideas and examples are mostly welcomed. :-) (Think of this as a brainstorming session).

PS Language preference is up to you.

Answer 1

In Java, you could submit notification tasks to an Executor which manages a single thread of group of threads that are all responsible just for handling observable events. You still effectively have a queue, but instead of a "message queue" in the traditional sense it's a queue of notification tasks.