Feature: Draft actor system.

[Zhou-London]

Outlines v1

• Core entities: Stage, Interface, Dispatcher, Actor.
• Stage is the tier 0 instance. Data won't run out of an Stage.
• Users interact with Stage using Interface. interface only passes these "subscriptions and publishes" to Stage and Dispathcer, and never really define them.

auto actor = stage.introduce([&](Interface& iface){
    // Define subscription
    iface.subscribe<GreetingMessage>(func);
}, [&](){
    // Define the function to be called when being added into the Stage
    iface.publish<GreetingMessage>(otherActor, Args...)
}
);

• The publish & subscribe is one-way by default, based on Stream (a SPSC ring buffer).
• Dispatcher defines how Stage manages and execute its actors, using single-thread dispatching, or one-thread-per-actor dispathcing.
• Thread pool dispatching will be taken into consideration, but not for now since it's a bit complex in this case. Feel free to reserve a dummy interface for it.

[Zhou-London]

Questions

1. Message Routing
   In your example, iface.publish(otherActor, Args...) targets a specific actor. Does this mean:

• Direct addressing only - sender must know the recipient?
• Or do you also want topic-based pub/sub - "publish to anyone subscribed to GreetingMessage"?

2. Stream Ownership
   With SPSC, each Stream has exactly one producer and one consumer. So if Actor A wants to send to Actor B and
   Actor C:

• Does A get two separate Streams (A→B and A→C)?
• Or does the Stage multiplex through a central router?

3. Actor Handle Semantics
   When stage.introduce() returns an actor handle:

• Is it just an ID for addressing?
• Can you remove() an actor? What happens to in-flight messages?
• Can actors be moved between Stages?

4. Dispatcher Triggering
   How does the Dispatcher know an actor has messages to process?

• Polling each actor's incoming Streams?
• Event notification when a Stream becomes non-empty?