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Slide 2 Actor Languages Maher Motivation Actor Actor system Bill Communication mechanism Structure of Actor languages Acts syntax and examples Actor-based languages Jack SALSA Mobile Agent Conclusion Slide 3 Actor Languages Maher Shinouda mshinoud@uwaterloo.ca Slide 4 Objectives Motivation Brief history What are actors Characteristics of actor-based languages Actor communication Events in actor languages Actor system Actors behavior Simple Example Actors VS Linda Slide 5 Motivation Develop language for concurrency Parallel Execution of actions. Concurrency in distributed systems exhibits: Divergence Deadlock Mutual Exclusion Slide 6 Brief History A number of individuals have contributed to the development of the actor model. Actor model was first described by Carl Hewitt (70s) The model was object-oriented: every object was a computationally active entity capable of receiving and reacting to messages. The Objects were called Actors. Gul Agha, later develop the actor model and develop a mathematical theory of actors. Actor languages can all trace its ancestor either directly or indirectly to Lisp. Slide 7 What are Actors Actors are independent concurrent objects that interact by sending asynchronous messages; each actor has its mail address and a behavior. An actor can do the following: Send communication to other actors. Create new actors. Define a new behaviour for itself, which may be the same or different to its previous behaviour. Slide 8 Three actions an actor may perform Slide 9 What are Actors Anatomy of Actor Slide 10 Characteristics of Actor-based language Encapsulation All procedures and declarative information are encapsulated into a single entity Actor. Actors share the main characteristics of objects in Simula and Smalltalk. Actors considered to be Autonomous objects. Slide 11 Characteristics of Actor-based language (Cont.) Inheritance In OOP, Each object is related to a class and further more the class may be an object belonging to meta- class. The notion of class is not integral to the actor model. Inheritance in actors provide a conceptual organization of the system which is dynamically reconfigurable. Slide 12 Characteristics of Actor-based language (Cont.) Delegation Sub-computation can be passed on by an actor to another actor which continue the processing. Delegation promote modularity of the code. Concurrency Actor language allow actors to specify a replacement which has 2 implications: Capture history-sensitive information. Allow for concurrent execution of expressions that dont involve data dependency. Slide 13 Actor Communication Actor communicate using message passing only. All communication is asynchronous. Each Actor has mail address with mail queue. An actor may know the mail address because: It has always know the address. It received the address within a communication from another actor. It created the address as part of creating another actor. Slide 14 Actor communication (cont.) A message (Task) is represented as 3 tuples: 1- A tag which distinguish it from other tasks in the system. 2- A target which is the mail address to which the communication is to be delivered. 3- A communication which contain information which made available to the actor at the target. Slide 15 Actor Actor = Component An actor is a special type of object that: Communicates with other actors by sending and receiving data via the mail address. No shared state between actors. An actor may be described by specifying: Its mail address with sufficiently large mail queue. Behavior, which is a function of the communication accepted. Slide 16 Event What is event? An event cause a communication to be sent; or represent the processing of the communication. Arrival order of events is nondeterministic. Actor computation may be represented by event diagram. Slide 17 Event Diagram Slide 18 Actor System Group of actors within it and the set of tasks to be carried out. Two types of special actors needed: A receptionist: An actor which may receive communication from outside the system. An external actor: Is one which is not in the system but its address is known to one or more actors within the system, allowing them to send communication. A C B Slide 19 The basic constructs A program in an actor language consists of: behavior definitions: which simply associate a behavior schema with an identifier. new expressions: which create actors. Send commands: which create tasks. Slide 20 The basic constructs (cont.) A receptionist declaration: which lists actors that may receive communications from the outside. An external declaration: which lists actors that are not part of the system but to whom communications may be sent from the system. Slide 21 Actors behavior The behavior of actor consists of three kind of actions: Create actors with specified behavior. ::= new ({expr{, expr}*}) Send message asynchronously to specified actor. ::= send to Become a new actor, assuming a new behavior to respond to the next message. become Slide 22 Actor System (cont.) Two important facts about mail system in the actor system: Mail arrives in random, non deterministic order (asynchronous). Ensures concurrent execution. The mail delivery is guaranteed. The system guarantee to execute all tasks eventually. Slide 23 Simple Example actor AdderAndMutlipy (Double k) Double A, Double B ==> Double C: action [a], [b] ==> [c] with Double c: c:=k*(a+b): endaction endactor Slide 24 Another Example actor AdderAndMutlipy2[T] (T k) T A, T B ==> T C: action [a], [b] ==> [k*(a+b)]: endaction action [a], [] ==> [k*a]: endaction action [], [b] ==> [k*b]: endaction endactor Slide 25 Actors VS Linda Provide point-to-point communication and object- style encapsulation. The locality property in actor: there is no way for an actor to contact other actors whose name hasnt received in a previous communication. Actors are components. Provide pattern directed invocation Decoupled in both space and time: Information may be available so that any one can potentially access it. Linda doesnt include the notion of component directly; nevertheless, the Tuple Space that it defines can be viewed as a generic connection component. Slide 26 Manifold Inter-process communication in MANIFOLD is asynchronous, using broadcast of events and a dynamic data-flow. Slide 27 Actor Languages Communication mechanisms Structure of Actor Languages Kernel language Act Actor-based languages Lehui (Bill) Nie @ UW Slide 28 Communication mechanisms Actors are active objects which communicate by message passing Asynchronous buffered communication It's hard to imagine a reasonable system built on asynchronous, unbuffered communication. Too much would get lost. Slide 29 Communication mechanisms Communication is asynchronous, without any guaranteed arrival order. Message order: if actor A sends a sequence of communications to actor B, then B may not receive them in the same order that A sent them. It is possible for A to tag each message with a sequence number, so that B may rearrange messages into the correct order. Slide 30 Communication mechanisms Shared variables are not allowed in the actor model. Shared variables means that they provide data transfer without any coordination mechanism or concurrency control. Variables sharing further complicates distribution and synchronization. Latency and timing issues make it awkward to use shared variables in an asynchronous environment. Slide 31 The Structure of Actor Languages Slide 32 Control structure: recursive factorial computation When actor factorial receives 3 and mail address c, it creates a new actor m, and sends itself the request to evaluate the factorial of 2 When actor m receives a result, m multiplies 3 and the result and send to c There is nothing inherently concurrent in the recursive algorithm to evaluate a factorial. Slide 33 Comparison Sequential language Using a stack of activations. No mechanism for distributing the work of computing a factorial or concurrently processing more than one request. Actor-based language Creating actors which waits for the appropriate communications. They are free to concurrently process the next communication. Delegating most of the processing to a large number of actors. Given a network of processors, an actor-based language could process a large number of requests much faster by simply distributing the actors it creates among these processors. Slide 34 Join continuations: tree product Divide and conquer concurrency can often be naturally expressed by using a functional form which evaluates its arguments concurrently. Join continuation is used to synchronize the evaluation of the different arguments. Slide 35 Tree product event diagram Slide 36 Language Act Act is a sufficient kernel for Act3 Syntax ::= * ( *) ::= (define (id {(with identifier )}*) *) ::= (Is-communication do *) Slide 37 Language Act ::= | | | ::= (let ( *) do *) ::= (if (then do *) (else do *)) ::= (send ) ::= (become ) Slide 38 Example: Factorial c Slide 39 Actor-based languages nameoriginsnotes PLASMA (1975) -The first actor language. Planner-like System Modeled on Actors. Originally called Planner-73, and implemented in MacLisp. Lisp-like syntax, but with several kinds of parentheses and brackets. "A PLASMA Primer", B. Smith et al, AI Lab Working Paper 92, MIT Oct 1975. "Viewing Control Structures as Patterns of Passing Messages", C. Hewitt, AI Lab Memo 410, MIT 1976. Act 1 (1981) LispDescendant of PLASMA. "Concurrent Object Oriented Programming in Act1", H. Lieberman in Object Oriented Concurrent Programming, A. Yonezawa et al eds, MIT Press 1987. Written in LISP by Henry Liberman Act 3 (1985) (1987) LispProvides support for automatic generation of customers and for delegation and inheritance. "Linguistic Support of Receptionists for Shared Resources", Carl Hewitt et al in Seminar on Concurrency, S.D. Brookes et al eds, LNCS 197, Springer 1985, pp