Establishing the Product Vision and Project Scope

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90 Part II Software Requirements Development considerable design and implementation effort. Ask the Stakeholders questions such as the following: • Are the users widely distributed geographically or located close to each other? How many time zones are they in? • When do the users in various locations need to access the system? • Where is the data generated and used? How far apart are these locations? Does data from multiple locations need to be combined? • Are specific maximum response times known for accessing data that might be stored remotely? • Can the userstolerate service interruptions or is continuous access to the system critical for the operation of their businesses? • What access security controls and data protection requirements are needed? The Context Diagram The scope description establishes the boundary and connections between the system we are developing and everything else in the universe. The context diagram graphically illustrates this boundary. It identifies terminators outside the system that interface to it in some way, as well as data, control, and material jlows between the terminators and the system. The context diagram is the top level of abstraction in a data flow diagram developed according to the principles of structured analysis (Robertson and Robertson 1994), but it's a useful model for projects that follow any development methodology. You can include the context diagram in the vision and scope document, in or as an appendix to the SRS, or as part of a data flow model for the system. Figure S-3 illustrates a portion of the context diagram for the Chemical Tracking System. The entire system is depicted as a single circle; the context diagram deliberately provides no visibility into the system's internal objects, processes, and data. The "system" inside the circle could encompass any combination of software, hardware, and human components. The terminators in the rectangles can represent user classes ("Chemist" or "Buyer"), organizations ("Health and Safety Department"), other systems ("Training Database"), or hardware devices ("Bar Code Reader"). The arrows on the diagram represent the flow of data ("request for chemical") or physical items ("chemical container") between the system and the terminators.
Chapter 5 Establishing the Product Vision and Project Scope 91 Chemlsts Figure 5-3 Gontext diagram for the Chemical Tracking System. You might expect to see chemical vendors shown as a terminator in this diagram. After all, the company will route orders to vendors for fulfillment, the vendors will send chemical containers and invoices to Contoso Pharmaceuticals, and purchasing will send checks to the vendors. However, those processes take place outside the scope of the Chemical Tracking System, as part of the operations of the purchasing and receiving departments. The context diagram makes it clear that this system is not directly involved in placing orders with the vendors, receiving the products, or paying the bills. The purpose of tools such as the context diagram is to foster clear and accurate communication among the project stakeholders. That clarity is more important than dogmatically adhering to the mies for a ··correct" context diagram. I strongly recommend, though, that you adopt the notation illu.strated in Figure 5-3 as a standard for drawing context diagrams. Suppose you were to use a triangle for the system instead of a cirde and ovals rather than rectangles for terminators. Your colleagues would have difficulty reading a diagram that follows your personal preferences rather than a team standard. Keeping the Scope in Focus The business requirements and an understanding of how customers willuse the product provide valuable tools for dealing with scope creep. Scope change isn't
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Establishing the Product Vision and Project Scope

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