What is the actor model and how does it apply to real-time systems?

The actor model is a mathematical model of concurrent computation where the fundamental unit is an actor — an independent entity with its own state that communicates exclusively via asynchronous message passing. Actors don't share memory, eliminating concurrency bugs like race conditions and deadlocks. Application to real-time WebSocket systems: each WebSocket connection can be modeled as an actor that processes messages sequentially in its own mailbox. Frameworks like Akka (JVM) and Elixir/Phoenix use the actor model natively. Phoenix Channels (Elixir) run each channel on a lightweight process (BEAM actor), enabling millions of concurrent WebSocket connections per server — the "2 million websocket connections" demo used this architecture. The actor model also naturally handles fault isolation: one crashed actor doesn't affect others, and supervisors can restart failed actors automatically. Orleans (.NET) and Proto.Actor bring this model to other ecosystems.