Lecture 09
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This document discusses data flow diagrams and modeling processes at different levels of abstraction. It provides examples of typical processes like computations, decisions, data manipulation, and triggering other processes. It then gives an example data flow diagram of a patient monitoring system to illustrate modeling a system at three different levels - showing more detail at each lower level. Finally, it discusses some common mistakes in data flow diagrams like missing inputs/outputs and irrelevant inputs not corresponding to the outputs.
Lecture 09
- 1. Lecture 09
- 2. Typical Processes Perform Computations Example – Process for calculating commission Make Decision Example – Order Management System Change Information Summarize Data Filter Data Example – ‘Where’ clause in SQL statements Sort Data Example – Issue logs
- 3. Typical Processes Triggering Processes Processes that trigger other processes. Process execution on a certain event. Example – Monthly billing cycle. Generate monthly bills against all connections. Actions on stored data (CRUD) Create Create data and store it in database. Read Read stored data and take action. Update Modify and store data. Delete Delete the stored data.
- 4. In data flow modeling only those processes can be expressed that perform certain processing or transformation of information. Now the question arises how far these processes need to be expressed? Moreover, requirement analysis is an ongoing activity in which knowledge expands as you dig out details of processes. Therefore, it may not be possible for an analyst to know each bit of all the processes of the system from the very beginning. Keeping the complexity of systems in view, data flow modeling technique has suggested disseminating information of a system in more then just one levels of abstraction. Levels of Abstraction to Data Flow Modeling
- 5. An Example - PMS
- 6. Following is the 0-level or the context level data flow diagram of the Patient Monitoring System. In this data flow diagram, three external entities (users) are interacting with the centralized system. Point to note here is that in this context level diagram, only one process or transform takes place that is the Patient Monitoring System itself. A patient’s vital signs are transmitted to this system which may invoke a warning message to the nurse if these signs fall into the critical range. Nurse may request for a report, which the patient monitoring system retrieves from the patient log, and return it to the nurse again. In this manner the 0-level data flow diagram describes the context of this system. An Example - PMS
- 8. Level 1 data flow diagram is the refinement of the context (0-level) data flow diagram. All the external entities are the same (Nurse, and Patient), however, the process of ‘Patient Monitoring System’ is further elaborated by the three processes Local Monitoring, Report Generator, and Central Monitoring. The Local Monitoring process transforms vital signs that it receives from Patient entity into Patient data and passes this information to Central Monitoring process. Central Monitoring process retrieves vital signs bounds and compares Patient data and it may generate Warning message if the Patient data goes out of normal Vital signs bounds. A nurse may request for a report, in response the Report Generator process retrieves Log data from Patient Log, generates the report and displays it back to the nurse. It should be noted here that this level 1 diagram is a further refinement of level 0 diagram such that the underlying system is the same but processes which were hidden in level 0 are represented in this diagram. Refining the Model
- 9. Information Flow in “Central Monitoring”
- 10. In the above level 2 data flow diagram, Patient’s data is sent to the Unpack Signs process which unpacks it and send Pulse, Temperature, and Blood pressure to the Evaluate Bounds Violation process. This process retrieves Vital signs bounds information, compares it with unpacked patient data and sends a violation sign to the Produce Warning Message process upon an out of bound result of the comparison. The patient data is sent to Format Patient Data process that generates the formatted patient data to be maintained against patient profile. In this manner we elaborated the patient monitoring system up to three levels describing different details at each level. In a similar manner, other two processes in level 1 DFD could also be expanded in their respective level two diagrams in order to describe their functionality in more detail. Information Flow in “Central Monitoring”
- 11. Common Mistakes in DFD
- 12. Common Mistakes in DFD There is no input for the process Freeze Member Account. The Freeze Member Account process that does not have any input is an example of a requirement whose source is not known. In a similar manner, the process Create a New Member Account does not produce any output. Similarly, the Create a New Member Account process that does not produce any output is an example of a requirement whose sink has not been specified. Similarly, Generate Employee Bank Statement process is having two inputs and an output but the question really is, do these inputs correspond to the output? Lastly, the Generate an Employee Bank Account process though have two inputs and produces an output but in order to generate a bank statement, all that is needed is an account number and the time period for which the statement is required. If we analyze the inputs given to this process, we can observe that both of these inputs cannot help in generating the account statement. Therefore, these inputs are irrelevant to the output being generated by this process.