Definitions

The term live distributed object (also abbreviated as live object) refers to a running instance of a distributed

Distributed computing

Distributed computing is a field of computer science that studies distributed systems. A distributed system consists of multiple autonomous computers that communicate through a computer network. The computers interact with each other in order to achieve a common goal...

 multi-party (or peer-to-peer

Peer-to-peer

Peer-to-peer computing or networking is a distributed application architecture that partitions tasks or workloads among peers. Peers are equally privileged, equipotent participants in the application...

) protocol, viewed from the object-oriented

Object-oriented programming

Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...

 perspective, as an entity that has a distinct identity

Identity (object-oriented programming)

An identity in object-oriented programming, object-oriented design and object-oriented analysis describes the property of objects that distinguishes them from other objects. This is closely related to the philosophical concept of identity....

, may encapsulate internal state

State (computer science)

In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....

 and threads of execution

Thread (computer science)

In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...

, and that exhibits a well-defined externally visible behavior. The key programming language

Programming language

A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

 concepts, as applied to live distributed objects, are defined as follows.

• Identity. The identity of a live distributed object is determined by the same factors that differentiate between instances of the same distributed protocol. The object consists of a group of software components physically executing on some set of physical machines and engaged in mutual communication, each executing the distributed protocol code with the same set of essential parameters, such as the name of a multicast group, the identifier of a publish-subscribe topic, the identity of a membership service, etc. Thus, for example, publish-subscribe channels and multicast groups are examples of live distributed objects: for each channel or group, there exists a single instance of a distributed protocol running among all computers sending, forwarding, or receiving the data published in the channel or multicast within the group. In this case, the object's identity is determined by the identifier of the channel or group, qualified with the identity of the distributed system that provides, controls, and manages the given channel or group. In the case of multicast, the identity of the system might be determined, for example, by the address of the membership service (the entity that manages the membership of the multicast group).

• Proxies (replicas). The proxy or a replica of a live object is one of the software component instances involved in executing the live object's distributed protocol. The object can thus be alternatively defined as a group of proxies engaged in communication, jointly maintaining some distributed state, and coordinating their operations. The term proxy stresses the fact that a single software component does not in itself constitute an object; rather, it serves as a gateway through which an application can gain access to a certain functionality or behavior that spans across a set of computers. In this sense, the concept of a live distributed object proxy generalizes the notion of a RPC
  Remote procedure call
  In computer science, a remote procedure call is an inter-process communication that allows a computer program to cause a subroutine or procedure to execute in another address space without the programmer explicitly coding the details for this remote interaction...
  
  , RMI
  Java remote method invocation
  The Java Remote Method Invocation Application Programming Interface , or Java RMI, is a Java application programming interface that performs the object-oriented equivalent of remote procedure calls ....
  
  , or .NET remoting
  .NET Remoting
  .NET Remoting is a Microsoft application programming interface for interprocess communication released in 2002 with the 1.0 version of .NET Framework. It is one in a series of Microsoft technologies that began in 1990 with the first version of Object Linking and Embedding for 16-bit Windows...
  
   client-side proxy stub
  Method stub
  A method stub or simply stub in software development is a piece of code used to stand in for some other programming functionality. A stub may simulate the behavior of existing code or be a temporary substitute for yet-to-be-developed code...
  
  .

• Behavior. The behavior of a live distributed object is characterized by the set of possible patterns of external interactions that its proxies can engage in with their local runtime environments. These interactions are modeled as exchanges of explicit events (messages).

• State. The state of a live distributed object is defined as the sum of all internal, local states of its proxies. By definition, it is distributed and replicated. The different replicas of the object's state may be strongly or only weakly consistent, depending on the protocol semantics: an instance of a consensus
  Consensus (computer science)
  Consensus is a problem in distributed computing that encapsulates the task of group agreement in the presence of faults.In particular, any process in the group may fail at any time. Consensus is fundamental to core techniques in fault tolerance, such as state machine replication.- Problem...
  
   protocol will have the state of its replicas strongly consistent, whereas an instance of a leader election
  Leader election
  In distributed computing, leader election is the process of designating a single process as the organizer of some task distributed among several computers . Before the task is begun, all network nodes are unaware which node will serve as the "leader," or coordinator, of the task...
  
   protocol will have a weakly consistent state. In this sense, the term live distributed object generalizes the concept of a replicated object; the latter is a specific type of live distributed object that uses a protocol such as Paxos, virtual synchrony, or state machine replication to achieve strong consistency between the internal states of its replicas. The state of a live distributed object should be understood as a dynamic notion: as a point (or consistent cut) in a stream of values, rather than as a particular value located in a given place at a given time. For example, the externally visible state of a leader election object would be defined as the identity of the currently elected leader. The identity is not stored at any particular location; rather, it materializes as a stream of messages of the form elected(x) concurrently produced by the proxies involved in executing this protocol, and concurrently consumed by instances of the application using this protocol, on different machines distributed across the network.

• Interfaces (endpoints). The interface of a live distributed object is defined by the types of interfaces exposed by its proxies; these may include event channels and various types of graphical user interfaces. Interfaces exposed by the proxies are referred to as the live distributed object's endpoints. The term endpoint instance refers to a single specific event channel or user interface exposed by a single specific proxy. To say that a live object exposes a certain endpoint means that each of its proxies exposes an instance of this endpoint to its local environment, and each of the endpoint instances carries events of the same types (or binds to the same type of a graphical display).

• References. The reference to a live object is a complete set of serialized
  Serialization
  In computer science, in the context of data storage and transmission, serialization is the process of converting a data structure or object state into a format that can be stored and "resurrected" later in the same or another computer environment...
  
  , portable instructions for constructing its proxy. To dereference a reference means to locally parse and follow these instructions on a particular computer, to produce a running proxy of the live object. Defined this way, a live object reference plays the same role as a Java reference
  Reference (computer science)
  In computer science, a reference is a value that enables a program to indirectly access a particular data item, such as a variable or a record, in the computer's memory or in some other storage device. The reference is said to refer to the data item, and accessing those data is called...
  
  , a C/C++ pointer, or a web service
  Web service
  A Web service is a method of communication between two electronic devices over the web.The W3C defines a "Web service" as "a software system designed to support interoperable machine-to-machine interaction over a network". It has an interface described in a machine-processable format...
  
  's WSDL
  Web Services Description Language
  The Web Services Description Language is an XML-based language that is used for describing the functionality offered by a Web service. A WSDL description of a web service provides a machine-readable description of how the service can be called, what parameters it expects and what data structures...
  
   description; it contains a complete information sufficient to locate the given object and interact with it. Since live distributed objects may not reside in any particular place (but rather span across a dynamically changing set of computers), the information contained in a live distributed object's reference cannot be limited to just an address. If the object is identified by some sort of a gobally unique identifier (as might be the case for publish-subscribe topics or multicast groups), the reference must specify how this identifier is resolved, by recursively embedding a reference to the appropriate name resolution
  Name resolution
  -In computer languages:Expressions in computer languages can contain identifiers. The semantics of such expressions depend on the entities that the identifiers refer to. The algorithm that determines what an identifier in a given context refers to is part of the language definition.The complexity...
  
   object.

• Types. The type of a live distributed object determines the patterns of external interactions with the object; it is determined by the types of endpoints and graphical user interfaces exposed by the object's proxies, and the patterns of events that may occur at the endpoints. The constraints that the object's type places on event patterns may span across the network. For example, type atomic multicast might specify that if an event of the form deliver(x) is generated by one proxy, a similar event must be eventually generated by all non-faulty proxies (proxies that run on computers that never crash, and that never cease to execute or are excluded from the protocol; the precise definition might vary). Much as it is the case for types in Java
  Java (programming language)
  Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
  
  -like languages, there might exist many very different implementations of the same type. Thus, for example, behavior characteristic to atomic multicast might be exhibited by instances of distributed protocols such as virtual synchrony or Paxos.

The semantics and behavior of live distributed objects can be characterized in terms of distributed data flow

Distributed data flow

Distributed data flow refers to a set of events in a distributed application or protocol that satisfies the following informal properties:* Asynchronous, non-blocking, and one-way...

s; the set of messages or events that appear on the instances of a live object's endpoint forms a distributed data flow

.

History

Early ideas underlying the concept of a live distributed object have been influenced by a rich body of research on object-oriented environments, programming language embeddings, and protocol composition frameworks, dating back at least to the actor model

Actor model

In computer science, the Actor model is a mathematical model of concurrent computation that treats "actors" as the universal primitives of concurrent digital computation: in response to a message that it receives, an actor can make local decisions, create more actors, send more messages, and...

 developed in the early 1970s; a comprehensive discussion of the relevant prior work can be found in Krzysztof Ostrowski's Ph.D. dissertation.

The term live distributed object was first used informally in a series of presentations given in the fall of 2006 at an ICWS

International Conference on Web Services

The International Conference on Web Services or ICWS is an international forum for researchers and industry practitioners focused on Web services. ICWS is sponsored by the IEEE Computer Society Technical Committee on Services Computing....

 conference , STC conference , and at the MSR

Microsoft Research

Microsoft Research is the research division of Microsoft created in 1991 for developing various computer science ideas and integrating them into Microsoft products. It currently employs Turing Award winners C.A.R. Hoare, Butler Lampson, and Charles P...

 labs in Redmond, WA
, and then formally defined in 2007, in an IEEE Internet Computing article. Originally, the term was used to refer to the types of dynamic, interactive Web

World Wide Web

The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...

 content

Content (media and publishing)

In media production and publishing, content is information and experiences that may provide value for an end-user/audience in specific contexts. Content may be delivered via any medium such as the internet, television, and audio CDs, as well as live events such as conferences and stage performances...

 that is not hosted

Internet hosting service

An Internet hosting service is a service that runs Internet servers, allowing organizations and individuals to serve content to the Internet. There are various levels of service and various kinds of services offered....

 on servers

Server (computing)

In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...

 in data center

Data center

A data center is a facility used to house computer systems and associated components, such as telecommunications and storage systems...

s, but rather stored on the end-user's client

Client (computing)

A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....

 computers, and internally powered by instances of reliable

Reliability (computer networking)

In computer networking, a reliable protocol is one that provides reliability properties with respect to the delivery of data to the intended recipient, as opposed to an unreliable protocol, which does not provide notifications to the sender as to the delivery of transmitted data.A reliable...

 multicast

Multicast

In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...

 protocols

Communications protocol

A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...

. The word live expressed the fact that the displayed information is dynamic, interactive

Interactivity

In the fields of information science, communication, and industrial design, there is debate over the meaning of interactivity. In the "contingency view" of interactivity, there are three levels:...

, and represents current, fresh, live content that reflects recent updates made by the users (as opposed to static, read-only

Read-only

In computing, read-only can mean:* Read-only memory , a type of storage media* Read-only access to files or directories in file system permissions...

, and archival

Archive

An archive is a collection of historical records, or the physical place they are located. Archives contain primary source documents that have accumulated over the course of an individual or organization's lifetime, and are kept to show the function of an organization...

 content that has been pre-assembled). The word distributed expressed the fact that the information is not hosted, stored at a server in a data center, but rather, it is replicated among the end-user computers, and updated in a peer-to-peer

Peer-to-peer

Peer-to-peer computing or networking is a distributed application architecture that partitions tasks or workloads among peers. Peers are equally privileged, equipotent participants in the application...

 fashion through a stream of multicast messages that may be produced directly by the end-users consuming the content; a more comprehensive discussion of the live object concept in the context of Web development can be found in Krzysztof Ostrowski's Ph.D. dissertation.

The more general definition presented above has been first proposed in 2008, in a paper published at the ECOOP

ECOOP

The European Conference on Object-Oriented Programming, commonly known as ECOOP, is an annual conference covering topics on object-oriented programming systems, languages and applications. Like other conferences, ECOOP offers various tracks and many simultaneous sessions, and thus has different...

 conference. The extension of the term has been motivated by the need to model live objects as compositions of other objects; in this sense, the concept has been inspired by Smalltalk

Smalltalk

Smalltalk is an object-oriented, dynamically typed, reflective programming language. Smalltalk was created as the language to underpin the "new world" of computing exemplified by "human–computer symbiosis." It was designed and created in part for educational use, more so for constructionist...

, which pioneered the uniform perspective that everything is an object, and Jini

Jini

Jini , also called Apache River, is a network architecture for the construction of distributed systems in the form of modular co-operating services.Originally developed by Sun, Jini was released under an open source license...

, which pioneered the idea that services are objects. When applied to live distributed objects, the perspective dictates that their constituent parts, which includes instances of distributed multi-party protocols used internally to replicate state, should also be modeled as live distributed objects. The need for uniformity implies that the definition of a live distributed object must unify concepts such as live Web content, message streams, and instances of distributed multi-party protocols.

The first implementation of the live distributed object concept, as defined in the ECOOP paper, was the Live Distributed Objects platform developed by Krzysztof Ostrowski at Cornell University. The platform provided a set of visual, drag and drop tools for composing hierarchical documents resembling web page

Web page

A web page or webpage is a document or information resource that is suitable for the World Wide Web and can be accessed through a web browser and displayed on a monitor or mobile device. This information is usually in HTML or XHTML format, and may provide navigation to other web pages via hypertext...

s, and containing XML

XML

Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....

-serialized live object references. Visual content such as chat windows, shared desktops

Desktop environment

In graphical computing, a desktop environment commonly refers to a style of graphical user interface derived from the desktop metaphor that is seen on most modern personal computers. These GUIs help the user in easily accessing, configuring, and modifying many important and frequently accessed...

, and various sorts of mashups

Mashup (digital)

A digital mashup usually is in reference to:1. Digital media content containing any or all of text, graphics, audio, video and animation drawn from pre-existing sources, to create a new derivative work...

 could be composed by dragging and dropping components representing user interfaces and protocol instances onto a design form, and connecting them together. Since the moment of its creation, a number of extension have been developed to embed live distributed objects in Microsoft Office

Microsoft Office

Microsoft Office is a non-free commercial office suite of inter-related desktop applications, servers and services for the Microsoft Windows and Mac OS X operating systems, introduced by Microsoft in August 1, 1989. Initially a marketing term for a bundled set of applications, the first version of...

documents , and to support various types of hosted content such as Google Maps. As of March 2009, the platform is being actively developed by its creators.