Difference between revisions of "Actor Model"

Revision as of 10:29, 23 January 2014 (view source) Miranda ricardo (talk | contribs) ← Older edit		Revision as of 10:30, 23 January 2014 (view source) Miranda ricardo (talk | contribs) (→‎Notes on the Actor Model) Newer edit →
Line 41:		Line 41:
	<ul>		<ul>
	<li>each message is sent only once;</li>		<li>each message is sent only once;</li>
−	<li>the sender does not verify if message reaches destination;</li>	+	<li>the sender does not verify if the message reaches destination;</li>
	<li>the order of arrival is independent of the order of sending;</li>		<li>the order of arrival is independent of the order of sending;</li>
	<li>there is no central point of control; and</li>		<li>there is no central point of control; and</li>
	<li>messages in mailbox are not processed in the order of arrival, there is a loop over the queue checking which messages it can react upon.</li>		<li>messages in mailbox are not processed in the order of arrival, there is a loop over the queue checking which messages it can react upon.</li>
	</ul>		</ul>

---

Revision as of 10:30, 23 January 2014

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

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.

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).

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.

Now actor MW2 has terminated time-step computations. It now sends 2 different messages to MW1 and MW3 respectively. MW2 is checking its message box's queue but it has not enough information to start a new time-step so it stays idle.

MW1 received a message form MW2 and will react accordingly.

MW1 terminated the time-step. Meanwhile MW3 processed message M3 and is already computing the next time-step.

MW1 sends a message to MW2 and both actors have enough information to proceed (reacting to messages).

All actors are computing a new timestep

Notes on the Actor Model

There are some remarks to be done:

• each message is sent only once;
• the sender does not verify if the message reaches destination;
• the order of arrival is independent of the order of sending;
• there is no central point of control; and
• messages in mailbox are not processed in the order of arrival, there is a loop over the queue checking which messages it can react upon.