Communications - July 2012

choices may depend on some priority order, as in standard BPJ execution; the mechanism may use look-ahead subject to desired properties of the resulting event sequence, as in smart play-out19, 29 in LSC; it may vary over time, based on learning; 13 or, as in Kugler et al., 34 the entire execution may diverge into multiple concurrent paths.

The programming idioms of request, wait for, block thus express multi-modality. Reminiscent of modal verbs in a natural language (such as shall, can or mustn’t), they state not only what must be done (and how) as in standard programming, but also what may be done, and, more uniquely to behavioral programming, what is forbidden and therefore must not be done.

Behavioral programming principles can be readily implemented as part of different languages and programming approaches, with possible variations of idioms. In addition to Java with the BPJ package25 ( discussed later in more detail) we have implemented them in the functional language Erlang26, 43 and Shimony et al. 42 applied them in the PicOS environment using C. Implementations in visual contexts beyond the original Play-Engine include PlayGo23 and SBT by Kugler et al. 34

In behavioral programming, all one must do in order to start developing and experimenting with scenarios that will later constitute the final system, is to determine the common set of events that are relevant to these scenarios. While this still requires contemplation, it is often easier to answer the question “what are the events?” than “which are the objects/functions, etc.?” By default, events are opaque entities carrying nothing but their name, but they may be extended with rich data and functionality.

to specify its requested, waited-for and blocked events as follows:

bSync(requested Events, waitedForEvents, blockedEvents);

By calling bSync the b-thread suspends itself until all other b-threads are at a synchronization point and is resumed when an event that it requested or waited for is selected, as described below.

To enforce predictable and repeatable execution, we require that the event selected at each synchronization point be uniquely defined. To this end, the programmer assigns a unique priority to each b-thread, and places the requested events of each b-thread in an ordered set. The event selection mechanism in BPJ then uses this ordering to choose the first event that is requested and not blocked.

The source code package of BPJ is available online at http://www.b-prog. org with examples and video demonstrations.

Example: Water flow control. To il-
lustrate how these constructs can be
used to allow new behaviors to non-
intrusively affect existing ones, con-
sider scenarios that are part of a system
that controls hot and cold water taps,
whose output flows are mixed.

Figure 3. Visualizing an execution of the water-tap application with traceVis. selected events are marked with a green star; blocked events are marked with a red square; cells marked R/W/B show requested, waited for, and blocked events.

AddHotThreeTimes

LeaderReset

Leader
AddColdThreeTimes

Active
Interleave

1 AddHot
R AddHot
W
B
R  Add Hot
W
B
R
AddHot W
B AddCold

2 AddCold
R AddHot
W
B
R AddCold
W
B
R
AddCold W
B AddHot

Programming Behaviors in Java Our implementation of behavioral programming in Java uses the BPJ package. 25 With BPJ, each behavior thread is an instance of the class BThread. Events are instances of the class Event or classes that extend it (mainly for adding data to events). The logic of each behavior is coded as a method supplied by the programmer, which in turn invokes the method bSync to synchronize with other behaviors, and