Class Actor notes

Inventor of actor model: Carl Hewitt

-Actor is a primitive unit that embodies all fundamental elements of computation needs.

• Processing: Get something done
• Storage: Be able to remember things
• Communication:

-You cannot have just one actors. A system of actors that can communicate with each other make up actors.

-Fundamental properties of actor

• Everything is an actor
• Then you are recursed to no end
• But the actor end in axioms
• What can an actor do:
  • Create another actor,
  • Send message to other actors
  • Designate how its going to handle the next message
• Process one message at a time:
  • Implement a pipeline to take message queue
  • If messages in pipeline are all the same, process it all at once.
• Idea of a futures contract:
  • While its processing something you can buy a future to process in it
  • If it breaks, the futures contract breaks
  • So you can send this future to yourself and you don’t get a deadlock for sending message to yourself.
  • So you can get the future immediately and pass it around and store it
• Is address equivalent to identity?
  • No because you can have one address for a bunch of actors replicated behind the scenes
  • It could be a message box that forwards to a bunch of other actors
• On Addresses
  • All you can do with an address is send a message
  • CLR enforces address integrity
  • Between the machines, you use encryption
  • Cannot take an integer and cast it to an object
  • It is too expensive on the system to maintain sequential time messaging arrival. Just like internet packets
  • Addresses are like capabilities
  • Messages are sent via best effort
  • There are no channels, if you want it, it’ll be another actor
• Messaging is indeterministic: they sort themselves out
  • Start mesage
  • Go message, stop message
  • Consider the case of sending myself a message
  • If I get a go message, I increment the counter by one and send a go message
  • Synchronization is built into the fact that one message is handled at a time. In the order they arrived. Like a checking account
  • This is where the arbitor comes in
  • If I get a stop message, then I report what the count is
• Arbitors
  • Arbitors cannot be made out of AND gates and OR gates
  • 2 inputs i1 and i2.
  • 2 outputs o1 and o2
  • Allowed to put in i1 and i2 at the same time, but only 1 can come out of outputs
  • An arbitor may take however long it needs to decide, but it must decide
  • It is used to decide which inputs came in first.
• Assignment:
  • In the checking account example: Actor returns the balance and also, the balance+ deposit
  • So the answer and the assignment as well.
• You can’t enforce consistency. There’s no way to compute truth in absolute term
• You can not get a global picture, so only a local situation.