SE- 565 Software System Requirements IX . Data Flow Diagrams

1. SE-565Software System RequirementsIX. Data Flow Diagrams Dr. Jiacun Wang Department of Software Engineering Monmouth University Jiacun Wang

2. Topics • Requirements engineering methods • Structured techniques for documenting and analyzing requirements • Data-Flow Diagrams • Data Dictionaries Jiacun Wang

3. Role of methods in RE • Process of requirements engineering (RE) is usually guided by a requirements method • Requirement methods are systematic ways of producing system models • System models are important bridges between the analysis and the design process Jiacun Wang

4. Necessary properties for a RE method • Suitability for agreement with the end-user (using intuitive notation) • The precision of definition of its notation • Assistance with formulating requirements • Definition of the world outside • Scope for malleability • Scope for integrating other approaches • Scope for communication • Tool support Jiacun Wang

5. No ideal RE method • There is no ideal requirement method • A number of methods use a variety of modelling techniques to formulate system requirements • System models can be enriched by modelling different aspects of using modelling techniques Jiacun Wang

6. SA and OOA • Structured analysis (SA) • focus on functionality • focus on data • Object-oriented analysis (OOA) • integrate data and functions Jiacun Wang

7. Data-Flow Modeling • One of the most popular structured methods • The Data-Flow Diagram (DFD) method (DeMarco, 1979) provides a description of a system based on modeling • the transformational processes of a system, • the collections (stores) of data that the system manipulates, and • the flows of data between the processes, stores and the outside world. • The DFD describes the functional viewpoint of the system e.g. it describes the system in terms of its operation (tasks). Jiacun Wang

8. Data-Flow Modeling (cont.) • This method works well for transaction-processing systems and other function-intensive applications. • The DFD provides a way to represent the steps involved in a business process or the operations of a proposed software system. • A DFD illustrates how the functional requirements in the SRS combine to let the user perform specific tasks. Jiacun Wang

9. The four basic elements of the DFD notation • The circle (the bubble) • It denotes an operation and is labeled with a brief description of the operation • Transformation of data into other data • Notation 3. Reserve seat Jiacun Wang

10. The box or rectangle (terminator) • It denotes external entities - source or sink of information outside the boundaries of the system • The source/sink can be a human user or other system • Notation Customer Source of information Jiacun Wang

11. User database Update user details • The paralleled bars or parallel lines • It denotes a data store or file • A place where data are held for later transformation or reference. • Notation Jiacun Wang

12. Customer’s card details Validate customer access PIN Customer Access map Network database • The arc (arrow) • It denotes the flow of information between the other three elements - data flow • Notation Jiacun Wang

13. The Data-Flow Diagram NotationSummary Data Transformation Terminator Terminator Input Output Data dictionary Jiacun Wang

14. Hierarchical/functional decomposition • The DFD can present the system at different levels of details. (Hierarchical/functional decomposition) • Context level • Level 1 • Level 2 • ………. • Level n The decomposition continues until the lowest level diagram contains only primitive process operations that can be clearly represented in narrative text, pseudocode, activity diagram, etc. details Jiacun Wang

15. Guidelines for building DFDs 1. All names should be unique. 2. Suppress logical decisions (no diamond shaped boxes!). 3. Don’t become bogged down in details. Defer error conditions and error handling until the end of the analysis (not in the case of a diagnostic system or patient monitoring system, for ex.). 4. Preserve the number of inputs and outputs between the levels. 5. As bubbles are decomposed into less abstract bubbles, the corresponding data flows may also need to be decomposed. Jiacun Wang

16. Rules of decomposition A top level process I I1 O O3 A A I2 O I1 O3 O1 O2 I1 I2 I is decomposed into some number of lower level processes I2 Jiacun Wang

17. Guidelines for building DFDs (cont.): 6. Place data stores only on the Level 1 and lower levels, not on the context diagram. 7. Name each process as a concise action: verb + object 8. Number the processes uniquely and hierarchically. On level 1 diagram number each process with an integer. If you create a child DFD for process 2, number the processes in that child diagram 2.1, 2.2, etc. 9. Don’t show more than 10 processes on a single diagram because it becomes harder to understand and change. Instead, introduce another layer of abstraction by grouping related processes into a higher-level process. Jiacun Wang

18. Example: Simple library system The context - level DFD forIssue library item Library card Library user Issue library item Library assistant Requested item Return date Issued item Jiacun Wang

19. Example: Simple library system Level 1 of the DFD for the Issue library item User database User details Updated details Library card Update user details Check user Library user User status User ID Item ID Requested item Check item Item status Return date Library assistant Issue item Issued item Item details Item database Update details Jiacun Wang

20. Dept-Queries Greenbelt Depts Greenbelt Mgmt Mgmt-Reports Reports to Depts Mgmt-Queries Greenbelt Distribution System Correspondence with Supplier Customer Correspondence Supplier Customer Supplier Response Response to Customer Example: A Distribution SystemContext level data flow diagram Reply to be mailed Discarded correspondence Jiacun Wang

21. Receiving Correspondence Preparing Shipments 1 4 Inventory File Stocking Supplies Keeping Accounts 2 Credit File 3 Level 1 data flow diagram Discarded Correspondence Customer Correspondence Reply to be mailed Response to Customer Order items Supplier Response Invoice Recorded Customer Order Mgmt-Queries supplies Dept-Queries Order Copy Reports to Depts Mgmt-Reports Correspondence with Supplier Jiacun Wang

22. Record & Route Correspondence 1.1 Correspondence File Read & Sign Draft Draft Reply 1.4 1.3 Level 2 DFD: Receiving correspondence Customer Correspondence Discarded Correspondence Routed Correspondence Read & Evaluate Correspondence Recorded Customer Order 1.2 Supplier Response Correspondence Demanding Reply Draft of Reply Reply To Be Mailed Jiacun Wang

23. Class exercise Draw the context diagram, level 1 and one level 2 diagrams for modeling the processing of an ATM. Jiacun Wang

24. Data Dictionaries • Repositories to store information about all data items or attributes used in the application. • Typical information include: • Name of data item • Aliases (other names for the item) • Description/Purpose (meaning) • Related data items (formulas) • Range of values • Data flows (generated_by/used_by) • Data structure definition/form Jiacun Wang

25. Data Dictionaries Notation Meta Symbol Meaning + And (concatenation) () Optional {} Iteration [ …..| …..| …..] Selection of one alternative “” A literal string Jiacun Wang

26. Simple Book Order DFD Customer Customer database Invoice Order Credit Status Create Invoice Verify Order is Valid Jiacun Wang

27. Name Data Flows Description/Purpose Form Generated By Used By Credit Status Customer Verify Order An “okay” or “not okay ” [OK| NOTOK] Data Base Is Valid that defines the credit worthiness of each customer Invoice Create Customer A bill indicating how Customer name + Invoice much the customer owes +billing address+price Customer- name Unique customer identifier First-name + (middle-name) + last-name Order Customer Verify order is valid A contract between customer & book company Customer name + { item } + customer address [ shipping-address | billing-address | shipping-address+ billing-address ] Customer- address First-name {legal character } Legal character [A-Z | a-z | 0-9 | ‘ | - | |] Data Dictionary for the Book Order DFD Jiacun Wang

28. Data Dictionaries (cont.) • Integration problems are reduced if all developers respect the data dictionary definitions. • The data dictionary complements the project glossary • Begin collecting data definitions as you encounter them during the elicitation • If you keep the data dictionary current, it will remain a valuable tool throughout the system’s maintenance life Jiacun Wang

29. Class exercise: Create a 5 item data dictionary for this DFD User database User details Update details Library user Library card Check user Update user details User status User ID Item ID Requested item Check item Item status Return date Issue item Library assistant Issued item Item details Update details Item database Jiacun Wang