Actor Model of Computation (2010) [pdf]
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Re: Actor Model of Computation (2010) [pdf]
#2Awesome overview! I've built actor -oriented implementations in a variety of domains, from finance to education to messaging, and find it a very useful way to think about the world. It can focus your thinking around which state (and actions) should be colocated, and orient you towards event based interfaces that focus around that data rather than specific client needs. People coming from modern programming paradigms will find many useful analogies -- e.g. to SOA/microservice decomposition, to components in a redux event loop.
Some cautions: as with any factorization scheme, granularity is important. The actor model makes it easy to think within actors, and hard to reason about interactions. Don't overfactor!
Also, orienting around event streams means forgoing synchronous requests. This can lead to insane complexity where each actor awaits specific replies to act on remote state. When adopting a framework, never ignore the useful tools it leaves behind (like the way synchronous request interfaces imply a round trip handshake being handled by another layer).
Re: Actor Model of Computation (2010) [pdf]
#3I find this resource confusing and annoying to read. When performing my thesis research I wasted a bunch of time trying to make sense of Hewitt's writing. I think a big problem is that the term "actor model" is so overloaded that it almost becomes meaningless. The actors that Hewitt is talking about is not at all the same as Erlang or Akka. As an example I consider this quote from the text nonsensical "Actor systems can perform computations that are impossible by Turing Machines" or of very little relevance to the actor models we use for programming. This has been discussed at [0].
For an overview of different actor models I prefer, see "43 years of actors: a taxonomy of actor models and their key properties" [1]. Agha's book [2] is also very good and I'd also like to recommend "Mixing Metaphors: Actors as Channels and Channels as Actors" [3].
[0]: http://lambda-the-ultimate.org/node/5208 [1]: http://soft.vub.ac.be/Publications/2016/vub-soft-tr-16-11.pd... [2]: https://dspace.mit.edu/handle/1721.1/6952 [3]: https://arxiv.org/abs/1611.06276
Re: Actor Model of Computation (2010) [pdf]
#4Hewitt is now kind of a crank. When Hofstadter came over for the Symsys thing in Stanford a few years back, Hewitt handed Hofstadter a little sheet of paper saying that mere contradiction allowed axiomatic systems to transcend Godel incompleteness theorems. Hofstadter: "Thanks, Carl." and then he threw it away when Hewitt wasn't looking. I think there's a thread in Lambda the Ultimate.
Re: Actor Model of Computation (2010) [pdf]
#5Hewitt work so relevant today, I found interesting that the Lisp Scheme was a failed attempt to implement this model of computation, ORGs, the scientific community metaphor, Planner, Inconsistency robustness, he even have a paper with David Marr from the same year of Actors "Video Ergo Scio" related today with the idea of inverse 3D graphics of capsules.
Re: Actor Model of Computation (2010) [pdf]
#6Hewitt is now kind of a crank. When Hofstadter came over for the Symsys thing in Stanford a few years back, Hewitt handed Hofstadter a little sheet of paper saying that mere contradiction allowed axiomatic systems to transcend Godel incompleteness theorems. Hofstadter: "Thanks, Carl." and then he threw it away when Hewitt wasn't looking. I think there's a thread in Lambda the Ultimate.
and yet it moves
Re: Actor Model of Computation (2010) [pdf]
#7I find this resource confusing and annoying to read. When performing my thesis research I wasted a bunch of time trying to make sense of Hewitt's writing. I think a big problem is that the term "actor model" is so overloaded that it almost becomes meaningless. The actors that Hewitt is talking about is not at all the same as Erlang or Akka. As an example I consider this quote from the text nonsensical "Actor systems…
> "Actor systems can perform computations that are impossible by Turing Machines"
As a turing machine can compute anything that is computable it is therefore impossible for an Actor to do any computation that is not possible by a turing machine. This is computer science 101 and makes me wonder about the value of anything else said in the paper.
Re: Actor Model of Computation (2010) [pdf]
#8I find this resource confusing and annoying to read. When performing my thesis research I wasted a bunch of time trying to make sense of Hewitt's writing. I think a big problem is that the term "actor model" is so overloaded that it almost becomes meaningless. The actors that Hewitt is talking about is not at all the same as Erlang or Akka. As an example I consider this quote from the text nonsensical "Actor systems…
> "Actor systems can perform computations that are impossible by Turing Machines" As a turing machine can compute anything that is computable it is therefore impossible for an Actor to do any computation that is not possible by a turing machine. This is computer science 101 and makes me wonder about the value of anything else said in the paper.
Well the interesting thing would be if that old lesson were violated.
Of course it's sealed forever for a specific definition of 'compute,' but I'm still open to the idea that we find something which resembles computation enough that we call it the same thing, yet its precise definition is different enough from the one used in connection with contemporary theory of computation that old proofs of universality won't necessarily apply to it.
That said... it seems unlikely to me that the Actor model does something non-trivially beyond what Turing machines are capable of, since it would likely be better known, and we would see Chomsky's hierarchy expanded to hold another class, etc. (Unless it goes beyond Turing Machines by being non-linguistic in some way, which could certainly be interesting. Maybe dealing with non-sequential symbols or something?)
Re: Actor Model of Computation (2010) [pdf]
#9I find this resource confusing and annoying to read. When performing my thesis research I wasted a bunch of time trying to make sense of Hewitt's writing. I think a big problem is that the term "actor model" is so overloaded that it almost becomes meaningless. The actors that Hewitt is talking about is not at all the same as Erlang or Akka. As an example I consider this quote from the text nonsensical "Actor systems…
Only that Carl took part in research into the model a few decades ago [1] when there was no Erlang, according to wikipedia.
[1] Carl Hewitt; Peter Bishop & Richard Steiger (1973). "A Universal Modular Actor Formalism for Artificial Intelligence". IJCAI.
Re: Actor Model of Computation (2010) [pdf]
#10According to the abstract at https://arxiv.org/abs/1008.1459 , this document has been revised many times (the current version, from 2015, is v38). Is there a particular reason that v8 was the version selected for submission?