Why Data Flow Diagrams?

Transcription

1 Flow Diagrams A structured analysis technique that employs a set of visual representations of the data that moves through the organization, the paths through which the data moves, and the processes that produce, use, and transform data.

2 Why Flow Diagrams? Can diagram the organization or the system Can diagram the current or proposed situation Can facilitate analysis or design Provides a good bridge from analysis to design Facilitates communication with the user at all stages 2

3 Types of DFDs Current - how data flows now Proposed - how we d like it to flow Logical - the essence of a process Physical - the implementation of a process Partitioned physical - system architecture or high-level design

4 Levels of Detail Context level diagram - shows just the inputs and outputs of the system Level 0 diagram - decomposes the process into the major subprocesses and identifies what data flows between them Child diagrams - increasing levels of detail Primitive diagrams - lowest level of decomposition

5 Recommended Progression Current logical diagrams start with context level decompose as needed for understanding Proposed logical diagrams start at level where change takes place decompose as far as possible Current physical diagrams at level of change Proposed physical diagrams same levels as proposed logical lower levels become design

6 Four Basic Symbols Source/ Sink Flow # Process # Store

7 Context Level Diagram Just one process All sources and sinks that provide data to or receive data from the process Major data flows between the process and all sources/sinks No data stores

8 Running Example Course Registration: Context level Diagram Class roster Professor Student Class Request Payment Receipt Student Schedule 0 Course Registration System Enrollment statistics Registrar

9 Level 0 Diagram Process is exploded Sources, sinks, and data flows repeated from context diagram Process broken down into subprocesses, numbered sequentially Lower-level data flows and data stores added

10 Running Example Course Registration: Current Logical Level 0 Diagram Class Request Student Payment Receipt 1.0 Register Student for Course 3.0 Produce Student Schedule Student and Course Student Class Student Schedule D1 Student Class s 4.0 Produce Class Roster Student Class 2.0 Collect Student Fee Payment Student Class Class Roster Payment D2 Student Payments 5.0 Student Class Produce Enrollment Report Enrollment Report Student Professor Registrar

11 Child Diagrams Explode one process in level 0 diagram Break down into lower-level processes, using numbering scheme Must include all data flow into and out of parent process in level 0 diagram Don t include sources and sinks May add lower-level data flows and data stores

12 Running Example Course Registration: Current Logical Child Diagram D3 Semester Schedule Available Seats Available Seats Class Request Error Check Prerequisites Met Valid Class Request Check for Availability Feasible Class Request Enroll Student in Class Student Course Error Student and Course D4 Student Transcripts D5 Course Catalogue D1 Student Class s

13 Physical DFDs Model the implementation of the system Start with a set of child diagrams or with level 0 diagram Add implementation details indicate manual vs. automated processes describe form of data stores and data flows extra processes for maintaining data

14 Running Example Course Registration: Current Physical Child Diagram D3 Semester Schedule DB Available Seats Available Seats Class Request Student Notified (verbally) 1.1 Check Prerequisites Met (manual) Student File Advisement Authorization Course Description 1.2 Check for Availability (myumbc) Feasible Class Request Unavailability Message 1.3 Enroll Student in Class (STARS) Student and Course D4 Department Student File D5 Course Catalogue (text) D1 Semester Enrollment DB

15 Running Example Course Registration: Proposed Physical Child Diagram D3 Semester Schedule DB Available Seats Available Seats Class Request Student Notified ( ) 1.1 Check Prerequisites Met (automated) Student Authorized Class Request Course 1.2 Check for Availability (automated) Valid Class Request Student ed 1.3 Enroll Student in Class (automated) Student and Course D4 Registrar s Student DB D5 Course Catalogue DB D1 Semester Enrollment DB

16 Partitioning a physical DFD Part of system design System architecture high-level design overall shape of system some standard architectures Decide what processes should be grouped together in the system components

17 Running Example Course Registration: Physical diagram (partitioned) D3 Semester Schedule DB Available Seats Available Seats Class Request Student Notified ( ) 1.1 Check Prerequisites Met (automated) Student Authorized Class Request Course 1.2 Check for Availability (automated) Valid Class Request Student ed 1.3 Enroll Student in Class (automated) Student and Course D4 Registrar s Student DB D5 Course Catalogue DB D1 Semester Enrollment DB

18 Another Example Perfect Pizza: Context Level Diagram Phone Number Info 0 System Weekly Report Management Delivery Person Delivery Cook Cook

19 Another Example Perfect Pizza: Current Logical Level 0 Diagram Phone Number 1.0 Find 2.0 Take 3.0 Print Delivery Delivery Delivery Person Info History Discount Info D1 Master 5.0 Add Management D3 D2 History Sales s Sales Info Weekly Report 7.0 Print Weekly Totals 6.0 Send to Cook Cook Cook

20 Another Example Perfect Pizza: Current Logical Child Diagram History D2 History 3.1 Determine Discount 3.3 Discount Amount 3.2 Discount Print Delivery Instructions Discount Delivery D3 Sales s

21 Another Example Perfect Pizza: Current Logical Child Diagram Raw Store D1 Master

22 Another Example Perfect Pizza: Physical Child Diagram Phoned Phone Number 5.1 Clerk Types ed Syntax Errors 5.2 System Validates Valid Cancelled Transaction 5.3 Clerk Visually Confirms Cust. Info. D1 DB 5.4 Format New

23 Another Example Perfect Pizza: Current Physical Level 0 Diagram Phoned Phone Number 1.0 Clerk Finds Row Phoned Info Phone # D1 Spreadsheet 5.0 Clerk Adds Row Cust. Info. 2.0 Clerk Takes (by phone) Copy of Slip & Info History 3.0 System Prints Delivery D2 History DB D3 Sales s File & Del. Printouts Copies of Slips Weekly Report Management 7.0 Mgr Prints Weekly Totals (batch) History Copies of Slips Delivery Printout 8.0 Mgr Updates History (nightly) Cook Copy of order slip Delivery Person Phoned 6.0 Clerk Sends to Cook (paper)

24 Another Example Perfect Pizza: Proposed Physical Level 0 Diagram Phone Number 1.0 System Finds Phoned Info D1 DB Phoned Cust. Info. 2.0 Clerk Enters (by phone) Info Info D3 Info Sales DB Discount Info 3.0 System Prints Delivery D2 History DB History Delivery Printout Delivery Person Phone # 5.0 Clerk Adds Cook D3 Sales s Sales DB Weekly Report Management 7.0 System Prints Weekly Totals (batch)

25 Another Example Perfect Pizza: Partitioned Physical Level 0 Diagram Phone Number 1.0 System Finds Phoned Info D1 DB Phoned Cust. Info. 2.0 Clerk Enters (by phone) Info Info D3 Info Sales DB Discount Info 3.0 System Prints Delivery D2 History DB History Delivery Printout Delivery Person Phone # 5.0 Clerk Adds Cook D3 Sales s Sales DB Weekly Report Management 7.0 System Prints Weekly Totals (batch)

26 Flow Diagramming Rules Processes a process must have at least one input a process must have at least one output a process name (except for the context level process) should be a verb phrase usually three words: verb, modifier, noun on a physical DFD, could be a complete sentence

27 1.0 Gather Demographic 2.0 Compile Statistics Survey Responses 3.0 Analyze Responses Final Report

28 2.0 Visa Authorization BETTER 2.0 Check Credit 2.0 Total s BETTER 2.0 Total Sales s 2.0 QA Process BETTER 2.0 Inspect Finished Products

29 Flow Diagramming Rules stores and sources/sinks no data flows between two data stores; must be a process in between no data flows between a data store and a source or sink; must be a process in between no data flows between two sources/sinks such a data flow is not of interest, or there is a process that moves that data

30 2.1 Store 2.1 Store Preferences D1 D1 Preferences D2 Preferences D2 Preferences

31 2.1 Store D1 2.1 Store D1 Preferences Preferences 2.2 Extract Preferences D2 Preferences D2 Preferences

32 D1 2.0 Store D1

33 Doctor Service 0 Diagnosis Medical Billing System Patient Bill

34 Flow Diagramming Rules flows data flows are unidirectional a data flow may fork, delivering exactly the same data to two different destinations two data flows may join to form one only if the original two are exactly the same no recursive data flows data flows (and data stores and sources/sinks) are labelled with noun phrases

35 Take Take Total Total Total Daily Sales Print Delivery Instructions Total Daily Sales Print Delivery Instructions

36 Take Lookup Take Lookup Address Address 3.0 Print Delivery Instructions 3.0 Print Delivery Instructions

37 Daily Sales 1.0 Calculate Weekly Sales Cumulative To-Date Sales

38 Flow Diagramming Guidelines The inputs to a process are different from the outputs Every object in a DFD has a unique name

39 1.0 Validate 1.0 Validate Valid

40 1.0 Get 2.0 Take 3.0 Process Get Take Process

41 1.0 Get 2.0 Take 3.0 Validate Only if these are exactly the same

42 Flow Diagramming Guidelines A data flow at one level may be decomposed at a lower level All data coming into and out of a process must be accounted for On low-level DFDs, new data flows can be added to represent exceptional situations

43 1.0 Get Address Address Phone 1.1 Get Phone 1.3 Phone 1.2 Lookup Address Address Address Request Address

44 1.0 Get Address Address Phone Invalid Phone Number Message 1.1 Get Phone 1.3 Phone 1.2 Lookup Address Address Address Request Address

45 Elements Indivisible pieces of data flows and data stores are made up of data elements Like attributes on an ER diagram The data elements of a data flow flowing in or out of a data store must be a subset of the data elements in that data store

46 Employee D1 Employee Master Hours Worked D2 Employee Time File Employee Time Employee 1.0 Calculate Gross Pay Gross Pay 2.0 Calculate Withholding Amount Withholding D1 Employee Master Check Reconciliation Employee 4.0 Print Employee Paycheck Employee Paycheck Net Pay 3.0 Calculate Net Pay D3 Check Reconciliation Employee

47 Employee 5.0 Hours Worked Create Time Employee Time D2 Employee Time File D1 Employee Master 6.0 Reconcile Pay Check Check Reconciliation D3 Check Reconciliation D1 Employee Master Employee Time Employee Paycheck 1.0 Calculate Gross Pay Gross Pay 4.0 Print Employee Paycheck Employee Employee Employee Paycheck Number of Dependents Gross Pay Net Pay D4Withholding Tables 2.0 Calculate Withholding Amount 3.0 Calculate Net Pay Withholding Rates Withholding Amount

48 DFDs and ERDs DFDs and ERDs are both used to model systems, but they show two very different perspectives on the system A DFD shows what the system does as well as the data that the system manipulates An ERD shows only the data that the system manipulates.

49 DFDs and ERDs (cont.) Entities on an ERD often (but not always) correspond to data stores on a DFD Attributes on an ERD usually correspond to data elements (listed in the data dictionary) that make up the data store and data flows on a DFD Relationships on an ERD do not correspond to processes on a DFD. Sources and sinks on a DFD usually do not show up as entities on an ERD

50 Example DFD and ERD DFD Cook Places 1.0 Take Name Hours Name Inventory Address Convert to Cooking Instructions Processed Convert to Ingredient List Item Quantity Cooking Instructions Ingredients Cook D1 Log Inventory Processing Incorrect ERD

51 Example DFD and ERD Id DFD Date Time 1.0 Contains ItemQuantity Take Includes Ingredient Item Ingredient Quantity Description Convert to Cooking Instructions Processed Convert to Ingredient List ItemId ItemName Requires Cooking Instructions Cooking Instructions Ingredients Index StepId Description Cook D1 Log Inventory Processing Correct ERD