Control Hierarchy

The Control hierarchy also called program structure represents the companies of program parts and implies a hierarchy of control. The Control hierarchy does not represent procedural aspects of software such as sequence of processes occurrence or order of decisions or repetition of operations.

Various different notations are used to represent control hierarchy. The most general is the tree-like diagram that represents the hierarchy. Moreover, other notations like as Warnier-Orr and Jackson diagrams may also be used with equal efficiency. In order to facilitate later discussions of structure as we describe a few simple terms and measures. In the Figure 18.3 depth and width give an indication of the number of levels of control and overall span of control respectively. Fan out is a measure of the number of modules that are directly controlled through another module the Fan-in indicates how many modules directly control a given module.

The control relationship between modules is expressed in the following way that are A module which controls another module is said to be super ordinate to it; conversely a module controlled through another is said to be subordinate to the controller. Example for, M is  super ordinate to modules a, b, c. Module h is subordinate to module e and is ultimately subordinate to module M. Width-oriented relationships example for among modules d and e although possible to express in practice need not be defined with explicit terminology.

The control hierarchy also presents 2 subtly various characteristics of the software architecture that are: connectivity and visibility. The Visibility indicates the group of programs components which may be invoked or used as data by a given component even when this is accomplished indirectly. Example for, a module in an object-oriented, but only makes use of a small number of these data attributes. All of the attributes are visible to the module. The Connectivity indicates the group of components which are directly invoked or used as data through a given component. Example for a module which directly    causes another module to starts execution in connected to it.