It's Actors All The Way Down

Within a single address-space, the capability-security properties of actor references are guaranteed by the actor run-time. Memory-safe implementations of actor languages ensure that actor references cannot be forged. Having a reference to an actor means you have permission to send it a message. A single machine may host multiple independent actor address-spaces, each of which […]

The Actor Model explicitly avoids placing constraints on message delivery order beyond causality [1]. Messages may not be delivered before the message which caused them to be sent. In other words, time can’t flow backward. Beyond that, messages arrive in a non-deterministic order. This can sometimes have surprising consequences. Two messages sent in sequence from […]

[Update 2014-02-02: I’ve posted a video showing a visualization of this pattern at http://www.youtube.com/watch?v=WEXh1WoppMU] In a loosely-coupled system connected with asynchronous messages, a flood of client requests can exceed the capacity of a service. When this happens, we would prefer that the service respond with an indication of the overloaded condition, rather than making us […]

The Observer pattern causes temporal coupling in systems with synchronous message passing. This can lead to failure in Object-Oriented systems. Asynchronous messaging avoids the pitfalls. Actor-based implementations more accurately realize the original intent of the pattern. The intent of the Observer pattern is to “define a one-to-many dependency between objects so that when one object […]

In his article about state management in Clojure, Rich Hickey discusses his reasons for choosing not to use the Erlang-style actor model. While Erlang has made some implementation choices that can lead to problems, the problems are not intrinsic to the Actor Model. As the actor implementation with the longest history of success, Erlang naturally […]