Actor Model

There are several resources in the Internet explaining the Actor Model and Reactive Programing. Some videos with interesting interviews:

• Hewitt, Meijer and Szyperski: The Actor Model (everything you wanted to know, but were afraid to ask)[1]; and
• Francesco Cesarini and Viktor Klang on the Reactive Manifesto[2].

To read on-line:

• The Reactive Manifesto[3]; and
• Learn You Some Erlang for great good![4];

According to the Wikipedia[5] "the actor model in computer science is a mathematical model of concurrent computation that treats "actors" as the universal primitives of concurrent digital computation: in response to a message that it receives, an actor can make local decisions, create more actors, send more messages, and determine how to respond to the next message received" (changing state).

Actor Model in Mohikd

Domain in decomposed in 3 subdomains.

Suppose a domain is decomposed in 3 sub-domains according to the first image. Each sub-dmain is an actor that sends messages to other actors and reactes to messages it receives. Each model has a Main Loop that progresses in time-steps. Each model sends messages with information (for example border conditions) to actors that need it.

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Lets take a look at the system in a moment all actors are computing the next time-step. Is is non-determined which actor will terminate first (even if we know each one work load, it is impossible to know when they will terminate their computations). Even while computing they are checking their mailboxes because there are messages they can react to (for instance if they are asked if their run has ended or not). Messages they can not process stay in the queue. Messages are not processed in the order of arrival.

There is no need for a centralized controller, each actor has a state and reactes to the environment as a living organism. Inconsistencies or partial failures should be resolved in another layer, for instance with the help of supervisors (this is Erlang/OTP strategy).

(1) Actor MW3 is the first actor to terminate its time-step.

Suppose Actor MW3 is the first actor to terminate its timestep. It sends messages to other actors with border conditions. MW3 has no messages in its mailbox so it is idle. Actor MW2 has a message in its mailbox but it can not process it yet so the message stays there.

MW3 will react when a message arrives to its mailbox. When it gathers enough information to strat a new time-step it will react triggering some computations. There is no central authority ordering it to start, stay idle, etc.

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All actors are computing a new timestep.

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