Problems and Exercises Solutions 1. Using the example of a retail clothing store in a mall, list relevant data flows, data stores, stores, processes, and sources/sinks. Observe several sales transactions. Draw a context diagram and a level-0 diagram that represent the selling system at the store. Explain why you chose certain elements as processes versus sources/sinks. A suggested context diagram and level-0 diagram are provided below.
Problem and Exercise 1 Context Diagram
C u st o m e r
Payment Receipt
0 Point of Sale System
Management Report
S t o r e M a n ag er
Problem and Exercise 1 Level-0 Diagram
Receipt Customer Payment
Goods Sold
2 Update Goods Sold File
Formatted Goods Sold Amount
Goods Sold File
1 Transform Customer Purchase
Inventory Data
3 Update Inventory File
4 Update Sales Total File
Formatted Inventory Amount
Formatted Sales Total Amount
Inventory File
Sales Total File
A I n m v o e n u t n o t s y r
Goods Sold Amounts
Sales Data
5 Produce Management Reports
Sales Totals
Management Report Store Manager
2.
Choose a transaction that you are likely to encounter, perhaps ordering a cap and gown for graduation, and develop a high-level DFD, or context diagram. Decompose this to a level-0 diagram. A suggested context diagram and a level-0 diagram are provided below.
Problem and Exercise 2 Context Diagram Problem and Exercise 2 Level-0 Diagram Receipt Student
Cap & Gown Purchase Request
0 Order Entry System
Cap and Gown Order
Cap & Gown Company
Receipt Cap and Gown Order
Student
Cap & Gown Company
Cap & Gown Purchase Request
1 Validate Order
Valid Order Information
2 Finalize Order
Inventory Data
3 Update Inventory File
Inventory Status
Inventory File
Formatted Inventory Data
4. Choose an example like that in Problem and Exercise 2, and draw a context diagram. Decompose this diagram until it doesn’t make sense to continue. Be sure that your diagrams are balanced, as discussed in this chapter. Students may choose a variety of situations to use for the n th level data flow diagrams for this answer. Basically, students should continue the process of decomposition until they have reached the point where no sub-process can logically be broken down further (i.e., each process meets the definition of a primitive process). See the level-1 data flow diagram for this exercise, which shows a sample decomposition of the process titled Finalize Order from the level-0 data flow diagram provided for Problem and Exercise 3. The (italicized) labels for processes and sources/sinks without borders represent the origin or destination of flows that pass between this subsystem and other system components. Note that the Goods Sold File is a potential black hole, or should possibly be treated as a sink.
Problem and Exercise #4 Level-1 Diagram 5. Cap & Gown Company
Validate Order
Cap and Gown Order
2.1 Generate Receipt
Valid Order Information
Receipt
2.2 Log Goods Sold Data
Goods Sold Data
Goods Sold File
Receipt
2.3 Generate Information For Shipping
Inventory Data
Update Inventory File
Validate Order
Refer to Figure 5-22, which contains drafts of a context and a level-0 DFD for a university class registration system. Identify and explain potential violations of rules and guidelines on these diagrams. Some errors and peculiarities in these diagrams include:
In the level –0 diagram, the data store, Class Roster, does not have the data flow, Scheduled Classes, flowing into it, rather this data flow connects processes 2 and 3, thus these DFDs are not balanced. Process 1 appears to accomplish nothing since its inflow and outflow are identical; such processes are uninteresting and probably unnecessary; it is possible that this process will become interesting when it is decomposed, where validation and error handling processes might appear. Process 2 does not appear to need Course Request as input in order to perform its function, as implied by its name. Some students may also wonder if process 3 has input sufficient to produce its output; for example, where are prior class registrations kept so that process 3 can determine when a course is full?
6. Why should you develop both logical and physical DFDs for systems? What advantage is there for drawing a logical DFD before a physical DFD for a new information system? Physical data flow diagrams help you better understand the people and/or computer systems that are used in the overall system’s processing. Logical data flow diagrams help you better understand the essence of the system, the data and the processes that transform them, regardless of actual physical form. Further, the new logical data flow diagrams can then show any additional functionality necessary in the new system, to indicate which, if any, obsolete components have been eliminated, and any changes in the logical flow of data between system components, including different data stores. The data flow diagrams for the new physical system can then be constructed with the data flow diagrams for the new logical system as a guide.
7. This chapter has shown you how to model, or structure, just one aspect, or view, of an information system, namely the process view. Why do you think analysts have different types of diagrams and other documentation to depict different views (e.g. process, logic, and data) of an information system? The various views (e.g., process, logic, data) of an information system each have their own unique characteristics and provide the most relevant information to different information system specialists. This variety is best understood, expressed, and managed by using diagrams and documentation that are specifically tailored for each view of the system. For example, data flow diagrams are useful for capturing the flow of data through business processes, but they are not useful for describing the forms and relationships among data. As information systems become larger and more complex, it becomes even more important to use the right tool and technique to develop each component of an information system. One technique that captured all aspects of an information system model on one diagram or in one notation would likely be too complex for systems professionals to handle. 8. Consider the DFD in Figure 5-23. List three errors (rule violations) on this DFD. Three major errors in Figure 5-23 are:
Process 1.0 (P2) has only inputs, making it a “black hole.” Data flow DF5 should not move directly from source E1 to data store DS1 without first going through a process. Data flow DF1 should not move directly from source E1 to sink E2 without first going through a process. Other peculiarities (such as Process 1.0 has label P2 and the data store has only a label, not a number) are only that, not errors.
9. Consider the three DFDs in Figure 5-24. List three errors (rule violations) on these DFDs. These diagrams show the decomposition of process P1 on the level-0 diagram. Three particular logical errors in Figure 5-24 are:
The data store DS1, not DS2, should be represented on the level-1 diagram. Data flow DF3 should be an outflow on the level-1 diagram, and data flow DF6 should not be on the level-1 diagram. Process P1.4.2 has no inputs and is thus a “miracle.”
10. Starting with a context diagram, draw as many nested DFDs as you consider necessary to represent all the details of the employee hiring system described in the following narrative. You must draw at least a context and a level-0 diagram. In drawing these diagrams, if you discover that the narrative is incomplete, make up reasonable explanations to complete the story. Supply these extra explanations along with the diagrams. (The Projects, Inc. narrative is provided in the textbook.) A suggested context diagram and level-0 diagram are provided below.
