Saturday, October 4, 2008

COSA: A New Kind of Programming, Part IV

Part I, II, III, IV, V, VI

In Part III, I introduced the concept of the control effector, a high-level behavior control mechanism that makes it possible to control a COSA component as if it were a low-level effector. In this post, I describe the overall control architecture used in the COSA programming model and I explain what happens internally to a component under control.

Control Architecture

None of this is chiseled in stone but the way I envisioned it, every high-level component should contain a single master supervisor in charge of one or more slave components. The only components that do not contain a supervisor are low-level components, i.e., components that are composed of cells. It goes without saying that a supervisor is a low-level component. (See Two Types of Components). In the figure below, a component is shown with its supervisor and five slave components. Note that only the control connections are shown for clarity. Normally, the slave components will make data connections with one another and some of the connectors may be visible outside the component. Every one of the slave components may have internal supervisors of their own, if necessary.

Only a supervisor can access the control connector (small red circle) of a slave. The latter cannot access the control connector of its supervisor or that of another slave. The control connector of a supervisor component can only be accessed by an external supervisor component. When a supervisor receives a start or stop signal, it may pass it to the slaves concurrently or in a given sequential order dictated by the design. In an actual development environment, the order in which the components are activated can be shown in slow motion by visually highlighting them in some fashion.

Control Effector

A control effector is a special COSA cell that is used to activate and/or deactivate a low-level component. In a previous illustration (reproduced below) I showed a control effector connected to its 3-input multi-connector. That is the default configuration. This is not a requirement, however. It is up to the designer to decide what to do with the start and stop signals. For example, the component may need to initialize or reset certain variables after starting and before stopping. Or it may do nothing other than outputting a ‘Done’ signal when it receives a ‘Stop’ signal (by routing the ‘Stop’ signal to the ‘Done’ terminal). It also has the option of stopping itself for whatever reason by sending a 'Stop' signal to its control effector.

Stopping a component means to deactivate it so that it can no longer process signals. Deactivating a component is a simple matter of clearing a special activation flag in every cell of the component. This causes the processor to ignore them. A component is fully deactivated only if its control effector receives a 'Stop' signal.


Activating or starting a component is a little more complicated than just resetting the activation flags. Recall that, unlike conventional programming models, COSA is a change-based or reactive model that uses reactive sensing. That is to say, in a COSA program, a comparison operation (i.e., sensor) is not explicitly executed by the program but is invoked automatically whenever there is a change in a data variable that may potentially affect the comparison (See Effector-Sensor Associations). Being a change detector, a sensor must compare the current state of its assigned variable with its previous state. It fires only when there is a correct change. For example, a non-zero sensor fires only if its variable changes from zero to non-zero. The problem is that, when a component is activated, its sensors have no idea what the previous states were. The solution is to set all sensors to their default states upon activation and invoke them immediately afterwards. This way, when a component is activated, all of its sensors perform their assigned tests or comparisons on their assigned data and fire if necessary. A component is fully activated when its control effector receives a 'Start' signal.

In Part V, I will describe reactive data connectors and explain why they are preferable to active message passing.

Related Articles:

How to Solve the Parallel Programming Crisis
Parallel Computing: Why the Future Is Compositional

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