Software Engineering-Control Hierarchy

Control hierarchy, also called program structure, represents the organization of program components (modules) and implies a hierarchy of control. It does not represent procedural aspects of software such as sequence of processes, occurrence or order of decisions, or repetition of operations; nor is it necessarily applicable to all architectural styles.

Different notations are used to represent control hierarchy for those architectural styles that are amenable to this representation. The most common is the treelike diagram  that represents hierarchical control for call and return architectures. However, other notations, such as Warnier-Orr  and Jackson diagrams may also be used with equal effectiveness. In order to facilitate later discussions of structure, we define a few simple measures and terms. Referring to figure , depth and width provide 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 by another module. Fan-in indicates how many modules directly control a given module.

The control relationship among modules is expressed in the following way: A module that controls another module is said to be superordinate to it, and conversely, a module controlled by another is said to be subordinate to the controller . For example, referring to figure, module M is superordinate to modules a, b, and c. Module h is subordinate to module e and is ultimately subordinate to module M. Width-oriented relationships (e.g., between modules d and e) although possible to express in practice, need not be defined with explicit terminology.

The control hierarchy also represents two subtly different characteristics of the software architecture: visibility and connectivity. Visibility indicates the set of program components that may be invoked or used as data by a given component, even when this is accomplished indirectly. For example, a module in an object-oriented system may have access to a wide array of data objects that it has inherited, but makes use of only a small number of these data objects. All of the objects are visible to the module. Connectivity indicates the set of components that are directly invoked or used as data by a given component. For example, a module that directly causes another module to begin execution is connected to it.

Structural Partitioning

If the architectural style of a system is hierarchical, the program structure can be partitioned both horizontally and vertically.Reffering to figure(a) horizontal partitioning defines separate branches of the modular hierarchy for each major program function. Control modules, represented in a darker shade are used to coordinate communication between and execution of the functions. The simplest approach to horizontal partitioning defines three partitions—input, data transformation (often called processing) and output. Partitioning the architecture horizontally provides a number of distinct benefits:
software that is easier to test
software that is easier to maintain
propagation of fewer side effects
software that is easier to extend

Because major functions are decoupled from one another, change tends to be less complex and extensions to the system (a common occurrence) tend to be easier to accomplish without side effects. On the negative side, horizontal partitioning often causes more data to be passed across module interfaces and can complicate the overall control of program flow (if processing requires rapid movement from one function to another).

Vertical partitioning (Figure (b)), often called factoring, suggests that control (decision making) and work should be distributed top-down in the program structure. Toplevel modules should perform control functions and do little actual processing work. Modules that reside low in the structure should be the workers, performing all input, computation, and output tasks.

The nature of change in program structures justifies the need for vertical partitioning. Referring to figure(b), it can be seen that a change in a control module (high in the structure) will have a higher probability of propagating side effects to modules that are subordinate to it. A change to a worker module, given its low level in the structure, is less likely to cause the propagation of side effects. In general, changes to computer programs revolve around changes to input, computation or transformation, and output. The overall control structure of the program (i.e., its basic behavior is far less likely to change). For this reason vertically partitioned structures are less likely to be susceptible to side effects when changes are made and will therefore be more maintainable—a key quality factor.
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