Software Design Methodology

258 Chapter 9. Analysis and Evaluation of Modifiability: The SAAM Method

10.2.3 Classification of scenarios

There is an important distinction between two types of scenarios. Recall that a

scenario is a brief description of some anticipated or desired use of a system. The

system may directly support that scenario, meaning that anticipated use requires no

modification to the system for the scenario to be performed. This would usually be

determined by demonstrating how the existing architecture would behave in

performing the scenario (rather like a walk-through simulation of the system

conducted in terms of the architectural constructs). If a scenario is not directly

supported, there must be some changes to the system that we could represent

architecturally. This change could be a change to how one or more components

perform an assigned activity, the addition of a component to perform some activity,

the addition of a connection between existing components, or a combination of

these. The first type of scenario is referred to as direct scenario, and the second as

indirect scenario. It is worth noting that a scenario is direct or indirect only with

respect to a particular architecture. A candidate architecture may execute a scenario

directly; another may require modification before it can be executed. It is such

differences that enable us to apply scenario analysis to assess architectural design.

For the KWIC example, among the eight scenarios given above, only scenario

1 is direct for both of two candidates. All others are indirect with respect to the

candidates.

10.2.4 Scenario evaluation

For each indirect scenario, the changes to the architecture that are necessary for it

to support the scenario must be listed, and the cost of performing each change must

be estimated. A modification to an architecture means that either a new component

or connection is introduced or an existing component or connection requires a

change in its specification.

For example, for the scenario 2 of the KWIC example, i.e. the scenario of

operation in an incremental fashion, the following modifications on the shared data

architecture must be made to support it.

(1) Modification of the Input component: The Input component must be modified

so that after reading each line from the input device, it must pass the control

back to the master control component.

(2) Modification of the Master Control component: The control component must

be modified so that it repetitively calls the subroutines Input, Circular Shift

and Alphabetise rather than just once.

(3) Modification of the Alphabetiser component: The Alphabetiser must now use

an incremental sorting algorithm so that circular shifted lines can be added

into sorted lines incrementally.