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Data Flow Diagram (DFD) Overview

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Presentation on theme: "Data Flow Diagram (DFD) Overview"— Presentation transcript:

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

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

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 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 Components and Conventions
External Entity a Customer 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

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 1 Manager Register New Customer

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 or where repeated D1 Customer Details D1 Customer Details

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 Components and Conventions (continued)
Data flows show the direction of information Update Customer D1 Customers Details Customer for Amendment WRONG RIGHT Read Name of Required

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 don’t 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 Hierarchical Structure
Level 1 1 a - 1 a 1.1 1.2 a - 1.1 1.3 1.4 a - 1.3 a Level 2

12 Example of a Level 2 DFD 2 Record Deposits and Withdrawals d Clerk d
Balance of account Amount of Deposit Amount Requested 2..1 2..2 Check and Record Withdrawal Record Deposit Amount Requested * * Amount of Deposit Balance of Account Bank Accounts D2 Bank Accounts 2..3 Amount of Deposit D2 Accept Pay Deposit * Amount of Salary Paid c Employer

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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