Grid computing is a term referring to the combination of computer resources from multiple administrative domains to reach a common goal. The grid can be thought of as a distributed system with non-interactive workloads that involve a large number of files...
Middleware is computer software that connects software components or people and their applications. The software consists of a set of services that allows multiple processes running on one or more machines to interact...
for parallel, distributed, and multi-threaded computing. It is developed by the OW2 Consortium, including INRIA, CNRS, University of Nice Sophia Antipolis
University of Nice Sophia Antipolis
The University of Nice Sophia Antipolis is a university located in Nice, France and neighboring areas. It was founded in 1965 and is organized in 8 faculties, 2 autonomous institutes and an engineering school....
Open-source software is computer software that is available in source code form: the source code and certain other rights normally reserved for copyright holders are provided under a software license that permits users to study, change, improve and at times also to distribute the software.Open...
A parallel programming model is a concept that enables the expression of parallel programs which can be compiled and executed. The value of a programming model is usually judged on its generality: how well a range of different problems can be expressed and how well they execute on a range of...
to simplify the programming and execution of parallel applications running on multi-core processors, distributed on Local Area Network (LAN
Län
Län and lääni refer to the administrative divisions used in Sweden and previously in Finland. The provinces of Finland were abolished on January 1, 2010....
An intranet is a computer network that uses Internet Protocol technology to securely share any part of an organization's information or network operating system within that organization. The term is used in contrast to internet, a network between organizations, and instead refers to a network...
Grid computing is a term referring to the combination of computer resources from multiple administrative domains to reach a common goal. The grid can be thought of as a distributed system with non-interactive workloads that involve a large number of files...
.
The ProActive programming model combines the active object
Active Object
The active object design pattern decouples method execution from method invocation that reside in their own thread of control. The goal is to introduce concurrency, by using asynchronous method invocation and a scheduler for handling requests....
In computer science, future, promise, and delay refer to constructs used for synchronization in some concurrent programming languages. They describe an object that acts as a proxy for a result that is initially not known, usually because the computation of its value has not yet completed.The term...
The University of Nice Sophia Antipolis is a university located in Nice, France and neighboring areas. It was founded in 1965 and is organized in 8 faculties, 2 autonomous institutes and an engineering school....
.
Several extensions of the model were made later on by members of the OASIS team at INRIA.
The book A Theory of Distributed Objects presents the ASP calculus that formalizes ProActive features, and provides formal semantics
Formal semantics of programming languages
In programming language theory, semantics is the field concerned with the rigorous mathematical study of the meaning of programming languages and models of computation...
to the calculus, together with properties of ProActive program execution.
Active objects
Active objects are the basic units of activity and distribution used for building concurrent
Concurrency (computer science)
In computer science, concurrency is a property of systems in which several computations are executing simultaneously, and potentially interacting with each other...
applications using ProActive. An active object runs with its own thread
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...
. This thread only executes the methods invoked on this active object by other active objects, and those of the passive objects of the subsystem that belongs to this active object. With ProActive, the programmer does not have to explicitly manipulate Thread objects, unlike in standard Java.
Active objects can be created on any of the hosts involved in the computation. Once an active object is created, its activity (the fact that it runs with its own thread) and its location (local or remote) are perfectly transparent. Any active object can be manipulated as if it were a passive instance of the same class.
An active object is composed of two objects: a body, and a standard Java object. The body is not visible from the outside of the active object.
The body is responsible for receiving calls (or requests) on the active object and storing them in a queue of pending calls. It executes these calls in an order specified by a synchronization policy. If a synchronization policy is not specified, calls are managed in a "First in, first out" (FIFO) manner.
The thread of an active object then chooses a method in the queue of pending requests and executes it. No parallelism is provided inside an active object; this is an important decision in ProActive's design, enabling the use of "pre-post" conditions and class invariant
Class invariant
In computer programming, specifically object-oriented programming, a class invariant is an invariant used to constrain objects of a class. Methods of the class should preserve the invariant. The class invariant constrains the state stored in the object....
s.
On the side of the subsystem that sends a call to an active object, the active object is represented by a proxy. The proxy generates future objects for representing future values, transforms calls into Request objects (in terms of metaobject, this is a reification
Reification (computer science)
Reification is the process by which an abstract idea about a computer program is turned into an explicit data model or other object created in a programming language. A computable/addressable object — a resource — is created in a system as a proxy for a non computable/addressable object...
) and performs deep copies of passive objects passed as parameters.
Active object basis
ProActive is a library designed for developing applications in the model introduced by Eiffel//, a parallel extension of the Eiffel programming language
Eiffel (programming language)
Eiffel is an ISO-standardized, object-oriented programming language designed by Bertrand Meyer and Eiffel Software. The design of the language is closely connected with the Eiffel programming method...
.
In this model, the application is structured in subsystems. There is one active object (and therefore one thread) for each subsystem, and one subsystem for each active object (or thread). Each subsystem is thus composed of one active object and any number of passive objects—possibly no passive objects. The thread of one subsystem only executes methods in the objects of this subsystem. There are no "shared passive objects" between subsystems.
These features impact the application's topology. Of all the objects that make up a subsystem—the active object and the passive objects—only the active object is known to objects outside of the subsystem. All objects, both active and passive, may have references onto active objects. If an object o1 has a reference onto a passive object o2, then o1 and o2 are part of the same subsystem.
This has also consequences on the semantics of message-passing between subsystems. When an object in a subsystem calls a method on an active object, the parameters of the call may be references on passive objects of the subsystem, which would lead to shared passive objects. This is why passive objects passed as parameters of calls on active objects are always passed by deep-copy. Active objects, on the other hand, are always passed by 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...
. Symmetrically, this also applies to objects returned from methods called on active objects.
Parallel computing is a form of computation in which many calculations are carried out simultaneously, operating on the principle that large problems can often be divided into smaller ones, which are then solved concurrently . There are several different forms of parallel computing: bit-level,...
, futures, and no data sharing, an application written with ProActive doesn't need any structural change—actually, hardly any change at all—whether it runs in a sequential, multi-threaded, or 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...
environment.
Asynchronous calls and futures
Whenever possible, a method call on an active object is reified
Reification (computer science)
Reification is the process by which an abstract idea about a computer program is turned into an explicit data model or other object created in a programming language. A computable/addressable object — a resource — is created in a system as a proxy for a non computable/addressable object...
as an asynchronous request. If not possible, the call is synchronous, and blocks
Blocking (computing)
Blocking occurs when a subroutine does not return until it either completes its task or fails with an error or exception. A process that is blocked is one that waits for some event, such as a resource becoming available or the completion of an I/O operation.In a multitasking computer system,...
until the reply is received. If the request is asynchronous, it immediately returns a future object.
The future object acts as a placeholder for the result of the not-yet-performed method invocation. As a consequence, the calling thread can go on with executing its code, as long as it doesn't need to invoke methods on the returned object. If the need arises, the calling thread is automatically blocked if the result of the method invocation is not yet available. Although a future object has structure similar to that of an active object, a future object is not active. It only has a Stub and a Proxy.
A simple example
The code excerpt below highlights the notion of future objects. Suppose a user calls a method foo and a method bar from an active object a; the foo method returns void and the bar method returns an object of class V:
// a one way typed asynchronous communication towards the (remote) AO a
// a request is sent to a
a.foo (param);
// a typed asynchronous communication with result.
// v is first an awaited Future, to be transparently filled up after
// service of the request, and reply
V v = a.bar (param);
...
// use of the result of an asynchronous call.
// if v is still an awaited future, it triggers an automatic
// wait: Wait-by-necessity
v.gee (param);
When foo is called on an active object a, it returns immediately (as the current thread cannot execute methods in the other subsystem). Similarly, when bar is called on a, it returns immediately but the result v can't be computed yet. A future object, which is a placeholder for the result of the method invocation, is returned. From the point of view of the caller subsystem, there is no difference between the future object and the object that would have been returned if the same call had been issued onto a passive object.
After both methods have returned, the calling thread continues executing its code as if the call had been effectively performed. The role of the future mechanism is to block the caller thread when the gee method is called on v and the result has not yet been set : this inter-object synchronization policy is known as wait-by-necessity.
These features impact the application's topology. Of all the objects that make up a subsystem—the active object and the passive objects—only the active object is known to objects outside of the subsystem. All objects, both active and passive, may have references onto active objects. If an object o1 has a reference onto a passive object o2, then o1 and o2 are part of the same subsystem.
This has also consequences on the semantics of message-passing between subsystems. When an object in a subsystem calls a method on an active object, the parameters of the call may be references on passive objects of the subsystem, which would lead to shared passive objects. This is why passive objects passed as parameters of calls on active objects are always passed by deep-copy. Active objects, on the other hand, are always passed by reference
The future object acts as a placeholder for the result of the not-yet-performed method invocation. As a consequence, the calling thread can go on with executing its code, as long as it doesn't need to invoke methods on the returned object. If the need arises, the calling thread is automatically blocked if the result of the method invocation is not yet available. Although a future object has structure similar to that of an active object, a future object is not active. It only has a Stub and a Proxy.