1、Actor-Oriented Design: A focus on domain-specific languages for embedded systems,Edward A. Lee Professor, UC Berkeley Director, Center for Hybrid and Embedded Software Systems (CHESS),Formal Methods and Models for Codesign (MEMOCODE2004) June 22-25, 2004 San Diego, California,Abstract,Concurrent, do
2、main-specific languages such as Simulink, LabVIEW, Modelica, VHDL, SystemC, and OPNET provide modularization mechanisms that are significantly different from those in prevailing object-oriented languages such as C+ and Java. In these languages, components are concurrent objects that communicate via
3、messaging, rather than abstract data structures that interact via procedure calls. Although the concurrency and communication semantics differ considerably between languages, they share enough common features that we consider them to be a family. We call them actor-oriented languages. Actor-oriented
4、 languages, like object-oriented languages, are about modularity of software. I will argue that we can adapt for actor-oriented languages many (if not all) of the innovations of OO design, including concepts such as the separation of interface from implementation, strong typing of interfaces, subtyp
5、ing, classes, inheritance, and aspects. I will show some preliminary implementations of these mechanisms in a Berkeley system called Ptolemy II.,The Questions,Is this a good way to do design? How does it relate to prevailing SW engineering? Does it support abstraction and modularity? Will it scale?
6、Can it become mainstream?,Platforms,A platform is a set of designs.Relations between platforms represent design processes.,big gap,Progress,Many useful technical developments amount to creation of new platforms.microarchitecturesoperating systemsvirtual machinesprocessor coresconfigurable ISAs,Recen
7、t Action,Giving the red platforms useful modeling properties (e.g. verification, SystemC, UML, MDA)Getting from red platforms to blue platforms (e.g. correctness, efficiency, synthesis of tools),Better Platforms,Platforms with modeling properties that reflect requirements of the application, not acc
8、idental properties of the implementation.,How to View This Design,From above: Signal flow graph with linear, time-invariant components.,From below: Synchronous concurrent composition of components,Actor-Oriented Platforms,Actor oriented models compose concurrent components according to a model of co
9、mputation.Time and concurrency become key parts of the programming model.,Actor-Oriented Design,Actor Orientation vs. Object Orientation,Identified limitations of object orientation: Says little or nothing about concurrency and time Concurrency typically expressed with threads, monitors, semaphores
10、Components tend to implement low-level communication protocols Re-use potential is disappointing,OO interface definition gives procedures that have to be invoked in an order not specified as part of the interface definition.,actor-oriented interface definition says “Give me text and Ill give you spe
11、ech”,Actor oriented,Object oriented,The First (?) Actor-Oriented Programming Language The On-Line Graphical Specification of Computer Procedures W. R. Sutherland, Ph.D. Thesis, MIT, 1966,MIT Lincoln Labs TX-2 Computer,Bert Sutherland with a light pen,Partially constructed actor-oriented model with a
12、 class definition (top) and instance (below).,Bert Sutherland used the first acknowledged object-oriented framework (Sketchpad, created by his brother, Ivan Sutherland) to create the first actor-oriented programming framework.,Your Speaker in 1966,Modern Examples of Actor-Oriented Component Framewor
13、ks,Simulink (The MathWorks) Labview (National Instruments) Modelica (Linkoping) OPNET (Opnet Technologies) Polis & Metropolis (UC Berkeley) Gabriel, Ptolemy, and Ptolemy II (UC Berkeley) OCP, open control platform (Boeing) GME, actor-oriented meta-modeling (Vanderbilt) SPW, signal processing worksys
14、tem (Cadence) System studio (Synopsys) ROOM, real-time object-oriented modeling (Rational) Easy5 (Boeing) Port-based objects (U of Maryland) I/O automata (MIT) VHDL, Verilog, SystemC (Various) ,Except Ptolemy, all of these define a fixed model of computation.,Ptolemy II Framework for Experimenting w
15、ith AO Design,Basic Ptolemy II infrastructure:,Actors in 2004: “Capsules” (UML-RT) and “Composite Structures” (UML-2),UML-RT borrowed from Selics ROOM the notion of “capsules,” which structurally look like actors.UML-2 is introducing the notion of “composite structures,” which also look like actors.
16、UML capsules and composite structures specify abstract syntax (and a concrete syntax), but no semantics.What this says is that there is huge potential for actor-oriented design to be done wrong,Why Use the Term “Actors”,The term “actors” was introduced in the 1970s by Carl Hewitt of MIT to describe
17、autonomous reasoning agents.The term evolved through the work of Gul Agha and others to refer to a family of concurrent models of computation, irrespective of whether they were being used to realize autonomous reasoning agents.The term “actor” has also been used since 1974 in the dataflow community
18、in the same way, to represent a concurrent model of computation.But UML uses the term “actor” in its use cases.,Does Actor-Oriented Design Offer Best-Of-Class SW Engineering Methods?,Abstraction procedures/methods classes Modularity subclasses inheritance interfaces polymorphism aspects Correctness
19、type systems,Example of an Actor-Oriented Framework: Simulink,Observation,By itself, hierarchy is a very weak abstraction mechanism.,Tree Structured Hierarchy,Does not represent common class definitions. Only instances.Multiple instances of the same hierarchical component are copies.,hierarchical co
20、mponent,leaf components: instances of an OO class,Alternative Hierarchy: Roles and Instances,class,role hierarchy (“design-time” view),one definition, multiple containers,Role Hierarchy,Multiple instances of the same hierarchical component are represented by classes with multiple containers.This mak
21、es hierarchical components more like leaf components.,hierarchical class,A Motivating Application: Modeling Sensor Networks,These 49 sensor nodes are actors that are instances of the same class, defined as:,Making these objects instances of a class rather than copies reduced the XML representation o
22、f the model from 1.1 Mbytes to 87 kBytes, and offered a number of other advantages.,Model of Massimo Franceschettis “small world” phenomenon with 49 sensor nodes.,Subclasses, Inheritance? Interfaces, Subtypes? Aspects?,Now that we have classes, can we bring in more of the modern programming world? s
23、ubclasses? inheritance? interfaces? subtypes? aspects?,Example Using AO Classes,instance,instance,subclass,inherited actors,override actors,local class definition,execution,Inner Classes,Local class definitions are important to achieving modularity.,Ordering Relations,Mathematically, this structure
24、is a doubly-nested diposet, the formal properties of which help to define a clean inheritance semantics. The principle we follow is that local changes override global changes.,Defining Actor Interfaces: Ports and Parameters,input ports,output port,p1,p2,p3,parameters:,a1 = value,a2 = value,input/out
25、put port,port,Example:,Actor Subtypes,a1: Int = value,p3: Double,p1: Int,Example of a simple type lattice:,Covariant,Contravariant,Actor Subtypes (cont),a1: Int = value,p3: Double,p1: Int,p3: Int,Remove (ignore) or add parameters,subtype relation,p4: Double,Remove (ignore) input ports,Add output por
26、ts,Subtypes can have:Fewer input portsMore output portsOf course, the types of these can have co/contravariant relationships with the supertype.,Observations,Subtypes can remove (or ignore) parameters and also add new parameters because parameters always have a default value (unlike inputs, which a
27、subtype cannot add)Subtypes cannot modify the types of parameters (unlike ports). Co/contravariant at the same time.PortParameters are ports with default values. They can be removed or added just like parameters because they provide default values.Are there similar exceptions to co/contravariance in
28、 OO languages?,Composing Actors,A connection implies a type constraint. Can:,Source,in: Int,Sink,out: Int,in: Double,out: Int,in: Unknown,out: Int,check compatibility,perform conversions,infer types,The Ptolemy II type system does all three.,What Happens to Type Constraints When a Subclass Adds Conn
29、ections?,Type resolution results may be different in different subclasses of the same base class (connection with let-bound variables in a Hindley-Milner type system?),Source,Sink,t1 = t2,BaseClass,t1,t2,Abstract Actors?,Suppose one of the contained actors is an interface only. Such a class definiti
30、on cannot be instantiated (it is abstract). Concrete subclasses would have to provide implementations for the interface.Is this useful?,Implementing Multiple Interfaces An Example,energy: Double,EnergyConsumer interface has a single output port that produces a Double representing the energy consumed
31、 by a firing.,in: Event,Event is a peculiar type that can yield a token of any type. It is the bottom of the type lattice.,A Model Using Such an Actor,out: Double,out: Double,power: Double,in: Double,EnergyConsumingFilter,Source,in: Double,in: Double,Sink,EnergyTabulator,Heterarchy? Multi-View Model
32、ing? Aspects?,This is multi-view modeling, similar to what GME (Vanderbilt) can do.,Is this an actor-oriented version of aspect-oriented programming?,Recursive Containment Can Hierarchical Classes Contain Instances of Themselves?,class,role hierarchy,instance hierarchy,class,instance,instance,instan
33、ce,Note that in this case, unrolling cannot occur at “compile time”.,Early Realization of this in Ptolemy Classic,FFT implementation in Ptolemy Classic (1995) used a partial evaluation strategy on higher-order components.,recursive reference,Conclusion,Actor-oriented design remains a relatively immature area, but one that is progressing rapidly.It has huge potential.Many questions remain,
