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DFD Overview 1 Data Flow Diagram (DFD) Overview Example Why used in SSADM DFD conventions and components Hierarchical structure What for and when.

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Presentation on theme: "DFD Overview 1 Data Flow Diagram (DFD) Overview Example Why used in SSADM DFD conventions and components Hierarchical structure What for and when."— Presentation transcript:

1 DFD Overview 1 Data Flow Diagram (DFD) Overview Example Why used in SSADM DFD conventions and components Hierarchical structure What for and when

2 DFD Overview 2 Example of a Level 1 DFD a Customer Close Accounts * Customer details 2 Record Deposits and Withdrawals b Manager a Customer d Clerk D2 Bank Accounts Amount/ Balance Ex Customer Ex Bank Account Notification of Closure Notification of Closure c Employer 1 Register New Customer * D1 Customer Details Account No Amount/ Balance Amount of Salary Paid Deposit/ Withdrawal Balance 3 Notification of Closure Customer details

3 DFD Overview 3 Representation and Communication DFDs help to represent and communicate – information and data flows – processes that change or handle data – stored data – system boundaries – sources and recipients of information and data At many levels of detail

4 DFD Overview 4 Benefits of DFDs Provide a pictorial, non-technical representation Are easy to understand Quick to produce and easy to amend Use limited number of symbols with specific meanings – concise system descriptions Use a simple, top-down expansion – easy progression between levels of detail

5 DFD Overview 5 DFD Components and Conventions Represents sources and/or recipients of information or data outside the system described by the DFD e.g. external persons or organisations, system users, physical systems in the same organisation, or other information systems If duplicated on same diagram a Customer a Customer External Entity

6 DFD Overview 6 DFD Components and Conventions (continued) Process Processes transform or handle data – may be clerical, automated or a mixture – can be decomposed to several levels of detail Name should be a simple imperative statement Identifier numbers do NOT indicate sequence Control logic is NOT shown on a DFD N.B. a DFD is NOT a flowchart 1Manager Register New Customer

7 DFD Overview 7 DFD Components and Conventions (continued) Data Store Represents data retained by the system (long and short term) – manual data stores referenced by M1, M2, M3 etc – transient data stores (short term) referenced by T1, T2, T3 etc – computer data stores referenced by D1, D2, D3 etc – transient manual data stores by T1(M), T2(M) T3(M) etc e.g computer files, record books, box of cards, filing cabinets D1 Customer Details or where repeated

8 DFD Overview 8 DFD Components and Conventions (continued) Data Flows Show the input and/or output of information or data to and from Processes Arrows show the direction of the flow Description normally given to the data flow Identified by source & destination and data flow description e.g. data flow a-1 goes from External Entity a to Process 1 Show the flow of information or data NOT storage media Usually flow to or from Processes – an exception might be e.g. letters between external entities. Amount of deposit

9 DFD Overview 9 DFD Components and Conventions (continued) Data flows show the direction of information Update Customer Update Customer D1Customers Customer Details D1Customers Customer Details Customer for Amendment WRONG RIGHT Read Customer Read Customer D1Customers Customer Details D1Customers Customer Details WRONG RIGHT Name of Required Customer

10 DFD Overview 10 Hierarchical Structure of DFDs Start with Level 1 (or top-level) DFD Each process on Level 1 can be expanded into a level 2 DFD Level 2 processes can be further expanded until sufficiently detailed Any process may be a window into another diagram, holding greater detail Processes which dont decompose further are marked /* in corner Lowest level processes may be described further by Elementary Process Descriptions Elementary Process Descriptions use narrative, Structured English, mathematical formulae, decision tables or trees as appropriate

11 DFD Overview 11 Hierarchical Structure Level 1 Level 2 a a a - 1 a a - 1.3

12 DFD Overview 12 Example of a Level 2 DFD 2 Record Deposits and Withdrawals Check and Record Withdrawal Record Deposit Accept Pay Deposit d Clerk d Clerk c Employer D2 Bank Accounts D2 Bank Accounts Amount of Deposit Amount of Deposit Balance of account Amount Requested Balance of Account Amount of Deposit Amount of Salary Paid * **

13 DFD Overview 13 Example of a Level 2 DFD Name and identifier of higher level Process shown at top of lower level diagram Frame represents the boundary of the Process Data flows across the frame must relate to data flows at the higher level Data stores used by only one process are usually shown as internal to that process at the lower level Processes with no further decomposition are marked /*


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