The Actor pattern (also known as the Actor architecture or the Actor model) provides a powerful tool to abstract complex distributed systems in which a large number of individual agents work together without necessarily needing any central governance. Agents are self-sufficient to make local decisions based on the messages they receive and their own states, making them adaptable to a variety of heterogeneous environments. Because agents can be evolved, migrated, replaced, or destroyed without breaking other agents, such systems often demonstrate impressive robustness, adaptiveness, and concurrency.
The easiest way to visualize the Actor pattern at work is to look at human society. Each human (agent) is an independent entity. We all hold our own attributes, and we make decisions based on our knowledge (state and behavior) and in reaction to the surrounding stipulates (messages). We communicate with (send messages to) each other. And collectively we form a vibrant, dynamic society.
In human society, there are no central entities that dictate our actions. There are governments and laws, but no one tells you your course of the action, move by move, during a day. Instead, we formulate our behaviors ourselves based on our learnings and observations. And what’s more fascinating is that although we make decisions locally, some social phenomena emerge without centralized coordination.
Actor
An actor is an independent single-threaded component that encapsulates certain states and behaviors. You can understand an actor type as a class in OOP. Out of each actor type you can create any number of actor instances, which is analog to instantiating a class in OOP. Each actor instance is identified by a unique actor id, which is used for clients or other actors to address the actor.
An actor can have state. An actor with state is called a stateful actor, and an actor without state is called a stateless actor. The state is managed by an actor state provider, which can save the state in memory, on disk, or in external storage.
Actor lifetime
Actors are virtual, which means logically they always exist. You don’t need to create or destroy an actor. When you need to talk to an actor, you just use the actor proxy to send a message to it.
The Actor Pattern offers several benefits, particularly in the context of concurrent and distributed systems:
- Concurrency: Actors operate independently and concurrently. This makes them well-suited for handling parallel and distributed tasks.
- Asynchronous Messaging: Communication between actors is achieved through asynchronous message passing. This allows actors to exchange information and coordinate their activities without blocking.
- Isolation: Each actor encapsulates its own state and behavior. This isolation ensures that actors do not interfere with each other’s state, removing the need for lock-based synchronization.
- Location Transparency: Actors can be distributed across different machines, providing a level of transparency regarding their physical location. This makes it easier to design distributed systems.
- Scalability: The Actor Pattern allows for the creation of systems that are highly scalable. As the system grows, more actors can be added without changing the underlying model.
- Fault Tolerance: Actors can fail independently without affecting the entire system. This makes the system more resilient to failures.
- Efficiency: By using actors to represent individual units of computation and message passing to facilitate communication between actors, we can create systems that are highly concurrent, efficient, and scalable.