Software Engineering - Old Questions

8. Discuss the use of control models. Differentiate between centralized control and event based control.

Control models are models deployed in software engineering that are concerned with the control flow between the subsystems. They are distinct from the system decomposition model.

Centralized Control Model

Centralized model is a formulation of centralized control in which one subsystem has overall responsibility for control and starts and stops other subsystems. It is a control subsystem that takes responsibility for managing the execution of other subsystems. Centralized models are classified into call-return and manager model.

1. Call-return Model: In call-return model, it is a model which has top-down subroutine architecture where control starts at the top of a subroutine hierarchy and moves downwards.

The call-return model is illustrated in above figure. The main program can call Routines 1, 2 and 3; Routine 1 can call Routines 1.2 or 1.2; Routine 3 can call Routines 3.1 or 3.2; and so on.

2.  Manager Model: Manager model is applicable to concurrent systems. One system component controls the stopping, starting and coordination of other system processes.

Event-based Control Model

Event-based models are those in which each sub-system can respond to externally generated events from other subsystems or the system’s environment. It is a system driven by externally generated events where the timing of the events is out with the control of the subsystems which process the event.

Event-based models are classified into broadcast and interrupt-driven models.

1. Broadcast models: In these models, an event is, in principle, broadcast to all sub-systems. Any sub-system, which is designed to handle that event, responds to it.

2. Interrupt-driven models: These are exclusively used in real-time systems where an interrupt handler detects external interrupts. They are then passed to some other component for processing.