$30.2 THE RISE OF CONCURRENCY 955 Object request brokers Another important recent development has been the emergence of the CORBA proposal from the Object Management Group,and the OLE 2/ActiveX architecture from Microsoft. Although the precise goals,details and markets differ,both efforts promise substantial progress towards distributed computing. The general purpose is to allow applications to access each other's objects and services as conveniently as possible,either locally or across a network.The CORBA effort (more precisely its CORBA 2 stage,clearly the interesting one)has also placed particular emphasis on interoperability: CORBA-aware applications can cooperate even if they are based on"object request brokers"from different vendors. Interoperability also applies to the language level:an application written in one of the supported languages can access objects from an application written in another.The interaction goes through an intermediate language called IDL (Interface Definition Language);supported languages have an official IDL binding,which maps the constructs of the language to those of IDL. IDL is a common-denominator O-O language centered on the notion ofinterface.An IDL interface for a class is similar in spirit to a short form,although more rudimentary (IDL in particular does not support assertions);it describes the set of features available on a certain abstraction.From a class written in an O-O language such as the notation of this book,tools will derive an IDL interface,making the class and its instances of interest to client software.A client written in the same language or another can,through an IDL interface,access across a network the features provided by such a supplier. Remote execution Another development of the late nineties is the mechanism for remote execution through the World-Wide Web. The first Web browsers made it not just possible but also convenient to explore information stored on remote computers anywhere in the world,and to follow logical connections,or hyperlinks,at the click of a button.But this was a passive mechanism: someone prepared some information,and everyone else accessed it read-only The next step was to move to an active setup where clicking on a link actually triggers execution of an operation.This assumes the presence,within the Web browser,of an execution engine which can recognize the downloaded information as executable code, and execute it.The execution engine can be a built-in part of the browser,or it may be dynamically attached to it in response to the downloading of information of the corresponding type.This latter solution is known as a plug-in mechanism and assumes that users interested in a particular execution mechanism can download the execution engine,usually free,from the Internet.§30.2 THE RISE OF CONCURRENCY 955 Object request brokers Another important recent development has been the emergence of the CORBA proposal from the Object Management Group, and the OLE 2/ActiveX architecture from Microsoft. Although the precise goals, details and markets differ, both efforts promise substantial progress towards distributed computing. The general purpose is to allow applications to access each other’s objects and services as conveniently as possible, either locally or across a network. The CORBA effort (more precisely its CORBA 2 stage, clearly the interesting one) has also placed particular emphasis on interoperability: • CORBA-aware applications can coöperate even if they are based on “object request brokers” from different vendors. • Interoperability also applies to the language level: an application written in one of the supported languages can access objects from an application written in another. The interaction goes through an intermediate language called IDL (Interface Definition Language); supported languages have an official IDL binding, which maps the constructs of the language to those of IDL. IDL is a common-denominator O-O language centered on the notion of interface. An IDL interface for a class is similar in spirit to a short form, although more rudimentary (IDL in particular does not support assertions); it describes the set of features available on a certain abstraction. From a class written in an O-O language such as the notation of this book, tools will derive an IDL interface, making the class and its instances of interest to client software. A client written in the same language or another can, through an IDL interface, access across a network the features provided by such a supplier. Remote execution Another development of the late nineties is the mechanism for remote execution through the World-Wide Web. The first Web browsers made it not just possible but also convenient to explore information stored on remote computers anywhere in the world, and to follow logical connections, or hyperlinks, at the click of a button. But this was a passive mechanism: someone prepared some information, and everyone else accessed it read-only. The next step was to move to an active setup where clicking on a link actually triggers execution of an operation. This assumes the presence, within the Web browser, of an execution engine which can recognize the downloaded information as executable code, and execute it. The execution engine can be a built-in part of the browser, or it may be dynamically attached to it in response to the downloading of information of the corresponding type. This latter solution is known as a plug-in mechanism and assumes that users interested in a particular execution mechanism can download the execution engine, usually free, from the Internet