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Data Flow Diagram(DFD)

Introduction to DFD

When it comes to conveying how information data flows through systems (and how that data is transformed in the process), data flow diagrams (DFDs) are the method of choice over technical descriptions for three principal reasons.

1. DFDs are easier to understand by technical and nontechnical audiences 2. DFDs can provide a high level system overview, complete with boundaries and connections to other systems 3. DFDs can provide a detailed representation of system components 5/2/2020

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DFD Symbol

Process Data store Source/Sink Data flow

Some Rules for External Entities

• External people, systems and data stores • Reside outside the system, but interact with system • Either a) receive info from system, b) trigger system into motion, or c) provide new information to system • e.g. Customers, managers • Not clerks or other staff who simply move data External Entity 5/2/2020

Some Rules for Data Stores

• Internal to the system • Data at rest • Include in system if the system processes

transform

– Store, Add, Delete, Update the data • Every data store on DFD should correspond to an entity on an ERD • Data stores can come in many forms: – Hanging file folders – Computer-based files – Notebooks

D1 Data Stores

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Some Rules for Data Flows

• Data in motion, moving from one place to another in the system – From external entity (source) to system – From system to external entity (sink) – From internal symbol to internal symbol, but always either start or end at a process

Data Flow

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Some Rules for Processes

• Always internal to system • Law of conservation of data: #1: Data stays at rest unless moved by a process.

#2: Processes cannot consume or create data – – –

Must have at least 1 input data flow Must have at least 1 output data flow Should have sufficient inputs to create outputs 0.

Processes

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Data Flow Diagram Types

• Data Flow Diagram is of two types:

1. Physical Data Flow Diagram: Physical data flow diagrams

are implementation-dependent and show the actual devices, department, people, etc., involved in the current system.

•

2. Logical or Conceptual Data Flow Diagram: Logical data

flow diagram represents business functions or processes. It describes the system independently of how it is actually implemented, and focuses rather on how an activity is accomplished.

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Physical Data Flow Diagram

• Describes about “How a system works”.

• Describes the flow of physical data components between physical operations in a system.

CUSTOMER ORDER Reject

1

Ordering Processing Clerk

2

Enter into computer file

3

Run LOCATE program Delivery node Customer Order 5/2/2020

Logical Data Flow Diagram

• Describes about “What a system does”.

• Describes the flow of logical data components between logical operations in a system.

Reject

1

Receive customer order

2

Find Locations of parts

3

Arrange delivery Delivery order Customer order 5/2/2020

Asynchronous operation

(a) Asynchronous operation

• The two processes are connected through a data store. Hence, the operations of the two bubbles are independent.

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Synchronous operation

Synchronous operation

• The above two processes are directly connected by a data flow. This means that the ‘validate-number’ process can start only after the ‘read-number’ process had supplied data to it.

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Data Flow Diagramming Rules

• Specific rules to DFD is as follows – Context Diagram (or Level-0 DFD) – Level-1 and lower decompositions – Numbering of the processes – Balancing across levels 5/2/2020

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Context Diagram of College Database System

Want to view records about students/fac ulty Add a record Show the specified records The specified record is added User

College Database System 0

issue the specified book for the user if available The specified record is deleted Want to issue books Delete a record Administrator

Level-1 DFD College Database System

Wants to view Records about students/ faculties add or delete student records User Shows the specified record Wants to view records about books/want to issue books Add or delete the specified data to database

Educational Dept 0.1

Administrator Shows records/issue the specified book if available Wants to view records

Library 0.2

Accounts section 0.3

Shows records as specified 5/2/2020

Level-2 DFD College Database System

Ask to Give records Ask to add or delete record Give records as specified User

Student 0.1.1

Give records as specified Ask to Give records

Faculty 0.1.2

Add or delete record as specified Administrator 5/2/2020

Balancing DFD

• The data that flow into or out of a bubble must match the data flow at the next level of DFD.

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Balancing DFD

An example showing balanced decomposition

In the next level, bubble 0.1 is decomposed.

(a) Level 1 DFD

In the level 1 of the DFD, data items d1 and d3 flow out of the bubble 0.1 and the data item d2 flows into the bubble 0.1.

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Balancing DFD continued….

(b) Level 2 DFD

The decomposition is balanced, as d1 and d3 flow out of the level 2 diagram and d2 flows in.

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What is a Good DFD?

• DFDs have a number of features which can be used to ensure that they are self explanatory, complete and unambiguous. The features are: • • • The absence of flowchart structures Conservation of data and Good naming convention 5/2/2020

A good DFD should

• Have no data flows that split up into a number of other data flows • Have no crossing lines • Not include flowchart loops of control statements and • Not include data flows that act as signals to activate processes 5/2/2020

Advantages of DFD

• DFD is a graphical text used for requirement analysis.

• DFD depicts information software without explicit representation of procedural logic • DFD also depicts real time systems with the help of some extended notations 5/2/2020