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Publications of year 2001
Thesis
  1. C. Varela. Worldwide Computing with Universal Actors: Linguistic Abstractions for Naming, Migration, and Coordination. PhD thesis, U. of Illinois at Urbana-Champaign, 2001. Keyword(s): distributed computing, concurrent programming, grid computing, internet programming languages, coordination models, middleware, software agents.
    Abstract:
    The enormous growth of the World-Wide Web has created the opportunity to use the combined computing and communication resources of millions of computers and devices connected to the Internet. The goal of the World-Wide Computer (WWC) project is to effectively turn the Web into a unified, dependable, distributed computing infrastructure. The WWC harnesses under-utilized computing resources by providing application programmers with the potential to globally distribute computations. Furthermore, the WWC provides mobile users and remote collaborators with a unified interface to their data and programs. Several applications in multiple domains -- as diverse as massively parallel computing, remote collaboration, coordinated computing, and Internet agents -- motivate the WWC. To realize this vision, we develop mechanisms for naming, migration, and coordination of software components and applications running on top of the Web. We represent software components as collections of actors. Actors provide autonomy, simplicity of communication and computation, and a well-developed formal semantics. Therefore, the WWC project uses the actor model of concurrent computation as a basis for studying and implementing different strategies for distributed software component naming, migration, and coordination. Universal naming is a critical aspect of accomplishing worldwide computing. The WWC''s naming strategy, based on Uniform Resource Identifier:s (URI), enables transparent migration and interconnection of actors. While a Universal Actor Name (UAN) persists over the life-time of an actor, Universal Actor Locators (UAL) change according to the actor''s current location, and prescribe a protocol for communication with the actor. Data and code migration enable scalability, more efficient network usage, improved graphical user interfaces, and mobile users and resources. Much work has been done on providing transparent access to remote objects, hiding their location information from application programmers, albeit with the unfortunate consequence of hindering efficient network programming. This thesis describes a model for both fine-grained and coarse-grained migration, which enables the development of applications with transparent actor mobility, yet allows programming control on the locality of the participating actors. Coordination of concurrent computations in the WWC is difficult. Traditional coordination mechanisms rely on a shared space, and therefore do not scale to the World-Wide Web. This thesis develops a scalable hierarchical model for coordination. Actors are grouped into casts which contain a special actor, designated as the cast director. Directors may themselves belong to other casts, creating a coordination forest. Message passing is constrained by the directors of a recipient. All directors of a target actor, up to the first common ancestor with the message sender, need to approve a message before it is delivered: directors above the first

    @PhdThesis{varela-phd-2001,
    author = {C. Varela},
    title = {{Worldwide Computing with Universal Actors: Linguistic Abstractions for Naming, Migration, and Coordination}},
    school = {U. of Illinois at Urbana-Champaign},
    year = 2001,
    pdf = {http://web.archive.org/web/20010404065326/http://osl.cs.uiuc.edu/Theses/varela-phd.pdf},
    keywords = {distributed computing, concurrent programming, grid computing, internet programming languages, coordination models, middleware, software agents},
    abstract = {The enormous growth of the World-Wide Web has created the opportunity to use the combined computing and communication resources of millions of computers and devices connected to the Internet. The goal of the World-Wide Computer (WWC) project is to effectively turn the Web into a unified, dependable, distributed computing infrastructure. The WWC harnesses under-utilized computing resources by providing application programmers with the potential to globally distribute computations. Furthermore, the WWC provides mobile users and remote collaborators with a unified interface to their data and programs. Several applications in multiple domains -- as diverse as massively parallel computing, remote collaboration, coordinated computing, and Internet agents -- motivate the WWC. To realize this vision, we develop mechanisms for naming, migration, and coordination of software components and applications running on top of the Web. We represent software components as collections of actors. Actors provide autonomy, simplicity of communication and computation, and a well-developed formal semantics. Therefore, the WWC project uses the actor model of concurrent computation as a basis for studying and implementing different strategies for distributed software component naming, migration, and coordination. Universal naming is a critical aspect of accomplishing worldwide computing. The WWC''s naming strategy, based on Uniform Resource Identifier:s (URI), enables transparent migration and interconnection of actors. While a Universal Actor Name (UAN) persists over the life-time of an actor, Universal Actor Locators (UAL) change according to the actor''s current location, and prescribe a protocol for communication with the actor. Data and code migration enable scalability, more efficient network usage, improved graphical user interfaces, and mobile users and resources. Much work has been done on providing transparent access to remote objects, hiding their location information from application programmers, albeit with the unfortunate consequence of hindering efficient network programming. This thesis describes a model for both fine-grained and coarse-grained migration, which enables the development of applications with transparent actor mobility, yet allows programming control on the locality of the participating actors. Coordination of concurrent computations in the WWC is difficult. Traditional coordination mechanisms rely on a shared space, and therefore do not scale to the World-Wide Web. This thesis develops a scalable hierarchical model for coordination. Actors are grouped into casts which contain a special actor, designated as the cast director. Directors may themselves belong to other casts, creating a coordination forest. Message passing is constrained by the directors of a recipient. All directors of a target actor, up to the first common ancestor with the message sender, need to approve a message before it is delivered: directors above the first} 
    }
    


Articles in journal, book chapters
  1. Carlos A. Varela and Gul Agha. Programming Dynamically Reconfigurable Open Systems with SALSA. ACM SIGPLAN Notices. OOPSLA'2001 Intriguing Technology Track Proceedings, 36(12):20-34, December 2001. Keyword(s): concurrent programming, internet programming languages.
    Abstract:
    Applications running on the Internet, or on limited-resource devices, need to be able to adapt to changes in their execution environment at run-time. Current languages and systems fall short of enabling developers to migrate and reconfigure application sub-components at program-execution time.In this paper, we describe essential aspects of the design and implementation of SALSA, an actor-based language for mobile and Internet computing. SALSA simplifies programming dynamically reconfigurable, open applications by providing universal names, active objects, and migration. Moreover, SALSA introduces three language mechanisms to help programmers coordinate asynchronous, mobile computations: token-passing continuations, join continuations and first-class continuations.We provide some examples which illustrate how SALSA programs are not only dynamically reconfigurable and open, but also much more concise and easier to follow than comparable Java code. Furthermore, we provide empirical results which show SALSA's performance to be better than Java code using an actor library, and which illustrate the difference between local, local area, and wide area communication and migration. Finally, we discuss the implementation of our preprocessor which translates SALSA code into Java.

    @Article{varela-agha-salsa-oopsla-2001,
    author = "Carlos A. Varela and Gul Agha",
    title = "Programming Dynamically Reconfigurable Open Systems with {SALSA}",
    journal = "ACM SIG{\-}PLAN Notices. OOPSLA'2001 Intriguing Technology Track Proceedings",
    volume = "36",
    number = "12",
    pages = "20--34",
    month = dec,
    year = "2001",
    pdf = "http://wcl.cs.rpi.edu/papers/b3.pdf",
    keywords = "concurrent programming, internet programming languages",
    abstract = {Applications running on the Internet, or on limited-resource devices, need to be able to adapt to changes in their execution environment at run-time. Current languages and systems fall short of enabling developers to migrate and reconfigure application sub-components at program-execution time.In this paper, we describe essential aspects of the design and implementation of SALSA, an actor-based language for mobile and Internet computing. SALSA simplifies programming dynamically reconfigurable, open applications by providing universal names, active objects, and migration. Moreover, SALSA introduces three language mechanisms to help programmers coordinate asynchronous, mobile computations: token-passing continuations, join continuations and first-class continuations.We provide some examples which illustrate how SALSA programs are not only dynamically reconfigurable and open, but also much more concise and easier to follow than comparable Java code. Furthermore, we provide empirical results which show SALSA's performance to be better than Java code using an actor library, and which illustrate the difference between local, local area, and wide area communication and migration. Finally, we discuss the implementation of our preprocessor which translates SALSA code into Java.} 
    }
    


  2. G. Agha, N. Jamali, and C. A. Varela. Agent Naming and Coordination: Actor Based Models and Infrastructures. In A. Ominici, F. Zambonelli, M. Klusch, and R. Tolksdorf, editors, Coordination of Internet Agents, chapter 9, pages 225-248. Springer-Verlag, 2001. Note: Invited book chapter. Keyword(s): distributed computing, concurrent programming, coordination models, software agents.
    Abstract:
    Flexible and efficient naming, migration and coordination schemes are critical components of concurrent and distributed systems. This chapter describes actor naming and coordination models and infrastructures, which enable the development of mobile agent systems. A travel agent example is used to motivate the requirements and proposed solutions for naming, migration and coordination. Universal Actor Names provide location and migration transparency, while ActorSpaces enable the unanticipated connection of users, agents and services in the open, dynamic nature of today’s networks. An actor-based architecture, the World Wide Computer, is presented as a basis for implementing higher-level naming and coordination models for Internet-based agent systems. Finally, multiagent coordination is accomplished with cyborgs, an abstraction which provides a unit for group migration and resource consumption through the use of e-cash.

    @InCollection{agha-coordination-2001,
    author = {G. Agha and N. Jamali and C. A. Varela},
    editor = {A. Ominici and F. Zambonelli and M. Klusch and R. Tolksdorf},
    booktitle = {Coordination of Internet Agents},
    title = {{Agent Naming and Coordination: Actor Based Models and Infrastructures}},
    publisher = {Springer-Verlag},
    chapter = 9,
    year = 2001,
    pages = "225-248",
    note = {invited book chapter},
    pdf = {http://web.archive.org/web/20010404065326/http://osl.cs.uiuc.edu/Papers/coord01.pdf},
    keywords = {distributed computing, concurrent programming, coordination models, software agents},
    abstract = {Flexible and efficient naming, migration and coordination schemes are critical components of concurrent and distributed systems. This chapter describes actor naming and coordination models and infrastructures, which enable the development of mobile agent systems. A travel agent example is used to motivate the requirements and proposed solutions for naming, migration and coordination. Universal Actor Names provide location and migration transparency, while ActorSpaces enable the unanticipated connection of users, agents and services in the open, dynamic nature of today’s networks. An actor-based architecture, the World Wide Computer, is presented as a basis for implementing higher-level naming and coordination models for Internet-based agent systems. Finally, multiagent coordination is accomplished with cyborgs, an abstraction which provides a unit for group migration and resource consumption through the use of e-cash.} 
    }
    



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