Live distributed object (also abbreviated as live object) refers to a running instance of a distributed multi-party (or peer-to-peer) protocol, viewed from the object-oriented perspective, as an entity that has a distinct identity, may encapsulate internal state and threads of execution, and that exhibits a well-defined externally visible behavior.
The key programming language concepts, as applied to live distributed objects, are defined as follows:
The semantics and behavior of live distributed objects can be characterized in terms of distributed data flows; the set of messages or events that appear on the instances of a live object's endpoint forms a distributed data flow [1] . [2]
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 developed in the early 1970s; a comprehensive discussion of the relevant prior work can be found in Krzysztof Ostrowski's Ph.D. dissertation. [3]
The term live distributed object was first used informally in a series of presentations given in the fall of 2006 at an ICWS conference, [4] STC conference, [5] and at the MSR labs in Redmond, WA , [6] and then formally defined in 2007, in an IEEE Internet Computing article. [7] Originally, the term was used to refer to the types of dynamic, interactive Web content that is not hosted on servers in data centers, but rather stored on the end-user's client computers, and internally powered by instances of reliable multicast protocols. The word live expressed the fact that the displayed information is dynamic, interactive, and represents current, fresh, live content that reflects recent updates made by the users (as opposed to static, read-only, and archival 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 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. [3]
The more general definition presented above has been first proposed in 2008, in a paper published at the ECOOP conference. [8] 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, which pioneered the uniform perspective that everything is an object, and Jini, 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, [8] was the Live Distributed Objects [9] 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 pages, and containing XML-serialized live object references. Visual content such as chat windows, shared desktops, and various sorts of mashups 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 documents, [10] and to support various types of hosted content such as Google Maps. [11] As of March 2009, the platform is being actively developed by its creators. [12] [13] [14] [15] [16] [17] [18] [19]
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