SE565 Software System Requirements IX Data Flow Diagrams

SE-565 Software System Requirements IX. Data Flow Diagrams Dr. Jiacun Wang Department of Software Engineering Monmouth University 10/21/2021 Jiacun Wang 1

Topics § § Requirements engineering methods Structured techniques for documenting and analyzing requirements § Data-Flow Diagrams § Data Dictionaries 10/21/2021 Jiacun Wang RE-9 2

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 10/21/2021 Jiacun Wang RE-9 3

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 10/21/2021 Jiacun Wang RE-9 4

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 10/21/2021 Jiacun Wang RE-9 5

SA and OOA § § Structured analysis (SA) § focus on functionality § focus on data Object-oriented analysis (OOA) § integrate data and functions 10/21/2021 Jiacun Wang RE-9 6

Data-Flow Modeling § § § One of the most popular structured methods The Data-Flow Diagram (DFD) method (De. Marco, 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). 10/21/2021 Jiacun Wang RE-9 7

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. 10/21/2021 Jiacun Wang RE-9 8

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 10/21/2021 Jiacun Wang RE-9 9

§ 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 10/21/2021 Jiacun Wang RE-9 10

§ 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 User database Update user details 10/21/2021 Jiacun Wang RE-9 11

§ The arc (arrow) § It denotes the flow of information between the other three elements - data flow § Notation Customer’s card details PIN Customer Validate customer access Access map Network database 10/21/2021 Jiacun Wang RE-9 12

The Data-Flow Diagram Notation Summary Data Transformation Terminator Input Output Terminator Data dictionary 10/21/2021 Jiacun Wang RE-9 13

Hierarchical/functional decomposition § The DFD can present the system at different levels of details. (Hierarchical/functional decomposition) details • • • Context level Level 1 Level 2 ………. Level n 10/21/2021 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. Jiacun Wang RE-9 14

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. 10/21/2021 Jiacun Wang RE-9 15

Rules of decomposition A top level process I 1 I 2 A I O 3 O A O I 1 O 3 O 1 O 2 I 1 I I 2 is decomposed into some number of lower level processes I 2 10/21/2021 Jiacun Wang RE-9 16

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 change. Instead, introduce another layer of abstraction by grouping related processes into a higher-level process. 10/21/2021 Jiacun Wang RE-9 17

Example: Simple library system The context - level DFD for Issue library item Library card Library user Requested item 10/21/2021 Issued item Issue library item Jiacun Wang Return date Library assistant RE-9 18

Example: Simple library system Level 1 of the DFD for the Issue library item Library card User database User details Update user details Check user User status Library user Requested item Issued item User ID Check item Item details Item status Item database 10/21/2021 Updated details Jiacun Wang Item ID Issue Return date item Library assistant Update details RE-9 19

Example: A Distribution System Context level data flow diagram Reply to be mailed Greenbelt Depts Dept-Queries Reports to Depts Mgmt-Queries Greenbelt Distribution System Customer Correspondence Customer 10/21/2021 Greenbelt Mgmt-Reports Response to Customer Correspondence with Supplier Response Supplier Discarded correspondence Jiacun Wang RE-9 20

Level 1 data flow diagram Discarded Correspondence Customer Correspondence Reply to be mailed Receiving Correspondence Supplier Response Order items Preparing Shipments Response to Customer 4 Invoice 1 Recorded Customer Order Inventory File Mgmt-Queries supplies Dept-Queries Order Copy Keeping Accounts Stocking Supplies 3 2 Reports to Depts Correspondence with Supplier 10/21/2021 Mgmt-Reports Credit File Jiacun Wang RE-9 21

Level 2 DFD: Receiving correspondence Customer Correspondence Routed Correspondence Record & Route Correspondence Supplier Response 1. 2 1. 1 Read & Sign Draft Read & Evaluate Correspondence Discarded Correspondence Recorded Customer Order Correspondence Demanding Reply Draft of Reply Draft Reply 1. 4 Correspondence File 1. 3 Reply To Be Mailed 10/21/2021 Jiacun Wang RE-9 22

Class exercise Draw the context diagram, level 1 and one level 2 diagrams for modeling the processing of an ATM. 10/21/2021 Jiacun Wang RE-9 23

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 10/21/2021 Jiacun Wang RE-9 24

Data Dictionaries Notation Meta Symbol Meaning + And (concatenation) () Optional {} Iteration […. . |…. . ] Selection of one alternative “” A literal string 10/21/2021 Jiacun Wang RE-9 25

Simple Book Order DFD Customer database Customer Invoice Create Invoice 10/21/2021 Order Credit Status Verify Order is Valid Jiacun Wang RE-9 26

Data Dictionary for the Book Order DFD Name Credit Status Data Flows Generated By Used By Customer Verify Order Data Base Is Valid Invoice Create Invoice Customername Description/Purpose Form An “okay” or “not okay ” that defines the credit worthiness of each customer A bill indicating how much the customer owes [OK| NOTOK] Unique customer identifier Customer name + +billing address+price First-name + (middle-name) + last-name Customer name + { item } + customer address [ shipping-address | billing-address | shipping-address+ billing-address ] {legal character } Legal character [A-Z | a-z | 0 -9 | ‘ | - | |] Order Customer Verify order is valid A contract between customer & book company Customeraddress 10/21/2021 Jiacun Wang RE-9 27

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 10/21/2021 Jiacun Wang RE-9 28

Class exercise: Create a 5 item data dictionary for this DFD User database User details Library card Check Update user details user User status Library user Requested item Update details User ID Item ID Check item Issued item Item status Issue Return date item Item details Library assistant Update details Item database 10/21/2021 Jiacun Wang RE-9 29