Asynchronous method invocation
Encyclopedia
In object-oriented programming
, asynchronous method invocation (AMI), also known as asynchronous method calls or asynchronous pattern is a design pattern for asynchronous
invocation of potentially long-running methods
of an object
.
It is equivalent to the IOU pattern described in 1996 by Allan Vermeulen.
The event-based asynchronous pattern in .NET Framework
and the java.util.concurrent.FutureTask class in Java
use events
to solve the same problem. This pattern is a variant of AMI whose implementation carries more overhead, but it is useful for objects representing software components
.
In most programming languages a called method is executed synchronously, i.e. in the thread of execution
from which it is invoked. If the method needs a long time to completion, e.g. because it is loading data over the internet, the calling thread is blocked until the method has finished. When this is not desired, it is possible to start a "worker thread" and invoke the method from there. In most programming environments this requires many lines of code, especially if care is taken to avoid the overhead that may be caused by creating many threads. AMI solves this problem in that it augments a potentially long-running ("synchronous") object method with an "asynchronous" variant that returns immediately, along with additional methods that make it easy to receive notification of completion, or to wait for completion at a later time.
One common use of AMI is in the active object
design pattern. Alternatives are synchronous method invocation and future objects
.
An example for an application that may make use of AMI is a web browser that needs to display a web page even before all images are loaded.
.
Given a method
Upon calling
One can also pass a callback method to
In the .NET Framework documentation, the term event-based asynchronous pattern refers to an alternative API style (available since .NET 2.0) using a method named
A superficial difference is that in this style the return value of the long-running method is passed directly to the callback method. Much more importantly, the API uses a special mechanism to run the callback method (which resides in an event object of type
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,...
, asynchronous method invocation (AMI), also known as asynchronous method calls or asynchronous pattern is a design pattern for asynchronous
Asynchronous I/O
Asynchronous I/O, or non-blocking I/O, is a form of input/output processing that permits other processing to continue before the transmission has finished....
invocation of potentially long-running methods
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
of an object
Object (computer science)
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
.
It is equivalent to the IOU pattern described in 1996 by Allan Vermeulen.
The event-based asynchronous pattern in .NET Framework
.NET Framework
The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
and the java.util.concurrent.FutureTask class 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...
use events
Event (synchronization primitive)
In computer science, an event is a type of synchronization mechanism that is used to indicate to waiting processes when a particular condition has become true....
to solve the same problem. This pattern is a variant of AMI whose implementation carries more overhead, but it is useful for objects representing software components
Component-based software engineering
Component-based software engineering is a branch of software engineering that emphasizes the separation of concerns in respect of the wide-ranging functionality available throughout a given software system...
.
In most programming languages a called method is executed synchronously, i.e. in the thread 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...
from which it is invoked. If the method needs a long time to completion, e.g. because it is loading data over the internet, the calling thread is blocked until the method has finished. When this is not desired, it is possible to start a "worker thread" and invoke the method from there. In most programming environments this requires many lines of code, especially if care is taken to avoid the overhead that may be caused by creating many threads. AMI solves this problem in that it augments a potentially long-running ("synchronous") object method with an "asynchronous" variant that returns immediately, along with additional methods that make it easy to receive notification of completion, or to wait for completion at a later time.
One common use of AMI is in 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....
design pattern. Alternatives are synchronous method invocation and future objects
Futures and promises
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...
.
An example for an application that may make use of AMI is a web browser that needs to display a web page even before all images are loaded.
Example
The following example is loosely based on a standard AMI style used in the .NET Framework.NET Framework
The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
.
Given a method
Accomplish, one adds two new methods BeginAccomplish and EndAccomplish:
Class Example {
Result Accomplish(args …)
IAsyncResult BeginAccomplish(args …)
Result EndAccomplish(IAsyncResult a)
…
}Upon calling
BeginAccomplish, the client immediately receives an object of type AsyncResult (which implements the IAsyncResult interface), so it can continue the calling thread with unrelated work. In the simplest case, eventually there is no more such work, and the client calls EndAccomplish (passing the previously received object), which blocks until the method has completed and the result is available. The AsyncResult object normally provides at least a method that allows the client to query whether the long-running method has already completed:
Interface IAsyncResult {
bool HasCompleted
…
}One can also pass a callback method to
BeginAccomplish, to be invoked when the long-running method completes. It typically calls EndAccomplish to obtain the return value of the long-running method. A problem with the callback mechanism is that the callback function is naturally executed in the worker thread (rather than in the original calling thread), which may cause race conditions.In the .NET Framework documentation, the term event-based asynchronous pattern refers to an alternative API style (available since .NET 2.0) using a method named
AccomplishAsync instead of BeginAccomplish.A superficial difference is that in this style the return value of the long-running method is passed directly to the callback method. Much more importantly, the API uses a special mechanism to run the callback method (which resides in an event object of type
AccomplishCompleted) in the same thread in which BeginAccomplish was called. This eliminates the danger of race conditions, making the API easier to use and suitable for software components; on the other hand this implementation of the pattern comes with additional object creation and synchronization overhead.