Message-passing

In computer science, message passing sends a message to a process (which may be an actor or object) and relies on the process and the supporting infrastructure to select and invoke the actual code to run. Message passing differs from conventional programming where a process, subroutine, or function is directly invoked by name. Message passing is key to some models of concurrency and object-oriented programming.

Message passing is used ubiquitously in modern computer software. It is used as a way for the objects that make up a program to work with each other and as a means for objects and systems running on different computers (e.g., the Internet) to interact. Message passing may be implemented by various mechanisms, including channels.

Message passing is a technique for invoking behavior (i.e., running a program) on a computer. In contrast to the traditional technique of calling a program by name, message passing uses an object model to distinguish the general function from the specific implementations. The invoking program sends a message and relies on the object to select and execute the appropriate code. The justifications for using an intermediate layer essentially falls into two categories: encapsulation and distribution.

Encapsulation is the idea that software objects should be able to invoke services on other objects without knowing or caring about how those services are implemented. Encapsulation can reduce the amount of coding logic and make systems more maintainable. E.g., rather than having IF-THEN statements that determine which subroutine or function to call a developer can just send a message to the object and the object will select the appropriate code based on its type.

One of the first examples of how this can be used was in the domain of computer graphics. There are all sorts of complexities involved in manipulating graphic objects. For example, simply using the right formula to compute the area of an enclosed shape will vary depending on if the shape is a triangle, rectangle, elipse, or circle. In traditional computer programming this would result in long IF-THEN statements testing what sort of object the shape was and calling the appropriate code. The object-oriented way to handle this is to define a class called Shape with subclasses such as Rectangle and Ellipse (which in turn have subclasses Square and Circle) and then to simply send a message to any Shape asking it to compute its area. Each Shape object will then invoke the way code with the formula appropriate for that kind of object.

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