Abstract

Actor Model of computation is a mathematical theory of computations that exist to solve the problem of concurrency in distributed systems. The actor serves as a process and communicates with other actors via asynchronous message passing. The actor models can be incorporated into any programing language either sequential or functional using CPS transform. Erlang is the most popular actor model and it has a separate toolkit(AKKA) for the actor model. Overtime, industry’s and technologies such as Twitter, Facebook and Microsoft have leverage on the actor model. Trending technologies like IOT and cloud computing have included the actor model to their technologies. With all these growths, the actor model has its own downside such as lack of inheritance and polymorphism. ESL (actor-based language) is in a growth stage to solve these challenges. System development methodology will be used to advance the ESL technology. 

1)   Introduction: Actor Model

“One actor is no actor”. The actor model comes as a system where everything is an actor. The actors communicate by sending an asynchronous message to each other. 

The actor model is a mathematical theory of computation that treats “Actors” as the universal primitives of concurrent digital computation (Hewitt,2010). The actor model was invented by Carl Hewitt in 1973.

Dissimilar to preceding models of computation, Actor model was motivated by physical law. Programming languages like Lisp, Smalltalk, ideas for Petri Nets, capability-based systems and packet switching also motivated the invention of the model.

The actor model had been used as a tool to aid the understanding of concurrency and conceptual implications of concurrent systems.  

The emergence of massive concurrency through client-cloud computing and many-core computer architectures has galvanized interest in the Actor model (Hewitt, 2009).

Before the invention of the actor model, Threads were used for concurrency. Threads faced challenges such as lost update; where different process tries to change the value of a variable at the same time and the use of a lock to solve this problem also created another problem called deadlock; happens when two process tries to obtain two locks at the same time. An algorithm representation is shown below.

The actor model provides a solution to the above challenges.

2) The Actor model

2.1) Fundamental concept

The actor model is a light-weight entity that has a state, thread of control, mailbox and a supervisor strategy. It is inherently concurrent.

2.1.1) The actor model of computation involves

• Autonomous concurrent actor: They are inherently concurrent. Programming languages like Java that uses threads for currency, tends to separate the model of concurrency with the model of the object (Agha, 2013). Actor model unifies these two notion.
• No shared state: To avoid the issue of a data race, race condition and sharing of a variable, the actor model does not implement the sharing of state.
• Asynchronous message passing: primitively, actors communicate asynchronously. They exchange and share information and it aids in synchronization.

2.1.2) Standard actor semantics

• Encapsulation:  The state of the actor model is isolated. The actor is not directly exposed to the environment. Actors do not modify the state of another actor directly. This can only be done indirectly through message passing (Waldron and Nash, 2016)
• Fairness: If an actor sends a message, and the actor receiving the message is not permanently disabled then eventually the message will be received (Agha, 2013).
• Location Transparency: You don’t have to know which memory is processing a certain command. The systems decide which commands needs to be executed by a certain core.
• Mobility: Enhancement of Location transparency.  Actors can move around and the underlying system can take care of where actors are.

2.1.3) Actor anatomy

As shown in the figure below, an actor consists of a mailbox where messages are stored/queue. A thread of control that gets messages from the mailbox for processing. And a state that changes when processing a message.

Actor = Encapsulated state + behaviour + thread of control + mailbox

2.1.4) Messaging