review articles

Doi: 10.1145/2209249.2209270

A novel paradigm for programming reactive
systems centered on naturally specified
modular behavior.
BY DaViD haReL, assaF maRRon, anD GeRa Weiss
Behavioral
Programming

sPeLLing oUT The requirements for a software system under development is not an easy task, and translating captured requirements into correct operational software can be even harder. Many technologies (languages, modeling tools, programming paradigms) and methodologies (agile, test-driven, model-driven) were designed, among other things, to help address these challenges. One widely accepted practice is to formalize requirements in the form of use cases and scenarios. Our work extends this approach into using scenarios for actual programming. Specifically, we propose scenario-coding techniques and design approaches for constructing reactive systems28 incrementally from their expected behaviors.

The work on behavioral programming began with scenario-based programming, a way to create executable specifications of reactive systems, introduced through the language of live sequence charts (LSC) and its Play-Engine implementation. 11, 21 The initial purpose was to enable testing and refining specifications and prototypes, and it was later extended toward building actual systems. To this end, the underlying behavioral principles have also been implemented in Java via the BPJ package25 and in additional environments, 26, 34, 42, 43 adding a programming point of view to that of requirement specification.

To illustrate the naturalness of constructing systems by composing behaviors, consider how children may be taught, step-by-step, to play strategy games (See Gordon et al. 14). For example, in teaching the game of Tic-Tac-Toe, we first describe rules of the game, such as:

Enforce Turns: To play, one player marks a square in a 3 by 3 grid with X, then the other player marks a square with O, then X plays again, and so on;

SquareTaken: Once a square is marked, it cannot be marked again;

DetectXWin/DetectOWin: When a player places three of his or her marks in a horizontal, vertical, or diagonal line, the player wins;

Now we may already start playing. Later, the child may infer, or the teacher may suggest, some tactics:

Add ThirdO: After placing two Os in a line, the O player should try to mark the third square (to win the game);

Prevent ThirdX: After the X player marks two squares in a line, the O player should try to mark the third square (to foil the attack); and

DefaultOMoves: When other tactics are not applicable, player O should prefer the center square, then the cor-

 

key insights

Behavioral programming is a novel, language-independent paradigm for programming reactive systems, centered on natural and incremental specification of behavior, and implemented in the visual formalism of live sequence charts (LsC), and in the BPJ Java package.

the approach allows coding applications as multi-modal scenarios, each corresponding to an individual requirement, specifying what can, must, or may not happen following certain sequences of events.

to facilitate full behavioral modularity via the independent coding of separate facets of behavior, all scenarios run simultaneously, and all are consulted at every decision point during execution.

the paradigm is supported by tools
for debugging, execution planning,
learning-based adaptivity, and
model-checking for early detection of
conflicting and incomplete requirements.