Context Models

Context Models

Context Models

At an early stage in the requirements elicitation and analysis process, you should decide on the boundaries of the system. This involves working with system stakeholders to distinguish what is the system and what is the system’s environment. You should make these decisions early in the process to limit the system costs and the time needed for analysis.

In some cases, the boundary between a system and its environment is relatively clear. For example, where an automated system is replacing an existing manual or computerized system, the environment of the new system is usually the same as the existing system’s environment. In other cases, there is more flexibility, and you decide what constitutes the boundary between the system and its environment during the requirements engineering process.

For example, say you are developing the specification for the library system LIBSYS. Recall that this system is intended to deliver electronic versions of copyrighted material to users’ computers. The users may then print personal copies of the material. developing the specification for this system, you have to decide whether other library database systems such as are within the system boundary they are, then you may have to allow access to the system the catalog user interface, if they are not, then users may be inconvenienced by having moved from one system to another.

The definition of a boundary is not a value-free judgment Social and organizational concerns mean that the position of a system boundary may be determined by non-technical factors. For example, a system boundary may be portioned so that the analysis process can all be carried out on one site; it may be chosen so that a particularly difficult manager need not consulted; it may be positioned so that the system cost is increased, and the system development division must therefore expand to design and implement the system.

Once some decisions on the boundaries of the system have been made, part of the analysis activity is the definition of that context and the dependencies that a system has on its environment. Normally, producing a simple architectural model is the first step in this activity. An architectural model that illustrates the structure of the information system that includes a bank auto-teller network. High-level architectural models are usually expressed in simple block diagrams where each sub-system is represented by

a named rectangle, and lines indicate associations between subsystems. ATM is connected to an account database, a local branch accounting system, a security system, and a system to support machine maintenance. The system is also connected to a user database that monitors how the network of ATMs is used and to a local branch counter system, This-counter system provides services such as backup and printing. These, therefore, need not be included in the ATM system itself. Architectural models describe the environment of a system. However, they do not show the relationships between the other systems in the environment and the system that is being specified, Extemal systems might produce data for or consume data from the system.

They might share data with the system, or they might be connected directly, through a network, or not at all. They might be physically co-located or located in separate buildings. All of these relations might affect the requirements of the being defined and must be taken into account.

Therefore, simple architectural models are normally supplemented by other models, such as process models, that show the process activities supported by the system. Data-flow models (described in the following section) may also be used to show the data that is transferred between the system and other systems in its environment.

A process model for the process of procuring equipment in an organization. This involves specifying the equipment required, finding and choosing suppliers, ordering the equipment, taking delivery of the equipment, and testing it after delivery. When specifying computer support for this process, you have to decide which of these activities will actually be supported. The other activities are outside the boundary of the system.  The dotted line encloses the activities that are within the system boundary.

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