Kendall7e_ch07 Using Dataflow Diagrams

8/10/2019 Kendall7e_ch07 Using Dataflow Diagrams

1/48

Using Dataflow

Diagrams

Systems Analysis and Design, 7eKendall & Kendall

7

2008 Pearson Prentice Hall


2/48

Kendall & Kendall 7-2

Learning Objectives

Comprehend the importance of using logical andphysical data flow diagrams (DFDs) to graphicallydepict movement for humans and systems in an

organization Create, use, and explode logical DFDs to capture andanalyze the current system through parent and childlevels

Develop and explode logical DFDs that illustrate the

proposed system Produce physical DFDs based on logical DFDs youhave developed

Understand and apply the concept of partitioning ofphysical DFDs


3/48


Data Flow Diagrams

Graphically characterize data processesand flows in a business system

Depict:

System inputs

Processes

outputs


4/48


Major Topics

Data flow diagram symbols

Data flow diagram levels

Creating data flow diagrams

Physical and logical data flow diagrams

Partitioning Communicating Using Data Flow

Diagrams


5/48


Advantages of the Data Flow

Approach Freedom from committing to the

technical implementation too early

Understanding of the interrelatednessof systems and subsystems

Communicating current system

knowledge to users

Analysis of the proposed system


6/48


Basic Symbols

A double square for an external entity

An arrow for movement of data from

one point to anotherA rectangle with rounded corners for

the occurrence of a transforming

processAn open-ended rectangle for a data

store


7/48Kendall & Kendall 7-7

Figure 7.1The four basic symbols used in dataflow diagrams, their meanings, and examples



External Entities

Represent another department, abusiness, a person, or a machine

A source or destination of data, outsidethe boundaries of the system

Should be named with a noun



Data Flow

Shows movement of data from onepoint to another

Described with a noun

Arrowhead indicates the flow direction

Represents data about a person, place,or thing



Process

Denotes a change in or transformation ofdata

Represents work being performed in thesystem

Naming convention Assign the name of the whole system when

naming a high-level process

To name a major subsystem attach the wordsubsystem to the name Use the form verb-adjective-noun for detailed

processes



Data Store

A depository for data that allows examination,addition, and retrieval of data

Named with a noun, describing the data Data stores are usually given a unique

reference number, such as D1, D2, D3

Represents a:

Filing cabinet Database

Computerized file



Figure 7.2Steps in developing

data flow diagrams



Creating the Context Diagram

The highest level in a data flow diagram

Contains only one process, representingthe entire system

The process is given the number 0

All external entities, as well as Majordata flows are shown



Figure 7.3Context diagram


15/48


Drawing Diagram 0

The explosion of the context diagram

May include up to nine processes Each process is numbered

Major data stores and all external

entities are included


16/48


Drawing Diagram 0 (Continued)

Start with the data flow from an entityon the input side

Work backwards from an output dataflow

Examine the data flow to or from a data

storeAnalyze a well-defined process

Take note of any fuzzy areas


17/48


Figure 7.3Note the greater

detail in diagram 0


18/48


Data Flow Diagram Levels

Data flow diagrams are built in layers

The top level is the Context level

Each process may explode to a lowerlevel

The lower level diagram number is the

same as the parent process number Processes that do not create a child

diagram are called primitive


19/48


Creating Child Diagrams

Each process on diagram 0 may beexploded to create a child diagram

A child diagram cannot produce outputor receive input that the parent processdoes not also produce or receive

The child process is given the same

number as the parent process Process 3 would explode to Diagram 3


20/48


Creating Child Diagrams

(Continued) Entities are usually not shown on the

child diagrams below Diagram 0

If the parent process has data flowconnecting to a data store, the childdiagram may include the data store aswell

When a process is not exploded, it iscalled a primitive process


21/48


Figure 7.4Differences between the parentdiagram (above) and the child diagram (below)


22/48


Checking the Diagrams for Errors

Forgetting to include a data flow orpointing an arrow in the wrong direction


23/48


Checking the Diagrams for

Errors (Continued) Connecting data stores and external

entities directly to each other


24/48



Errors (Continued) Incorrectly labeling processes or data

flow

Including more than nine processes ona data flow diagram


25/48



Errors (Continued) Omitting data flow

Creating unbalanced decomposition (or

explosion) in child diagrams


26/48


Figure 7.5Typical errors that can occurin a data flow diagram (payroll example)


27/48


Logical and Physical Data FlowDiagrams

Logical Focuses on the business and how the business

operates

Not concerned with how the system will beconstructed

Describes the business events that take place andthe data required and produced by each event

Physical Shows how the system will be implemented

Depicts the system


28/48


Figure 7.7Features common of logicaland physical data flow diagrams


29/48


Figure 7.8The progression ofmodels from logical to physical


30/48


Developing Logical Data Flow

Diagrams Better communication with users

More stable systems

Better understanding of the business byanalysts

Flexibility and maintenance

Elimination of redundancy and easiercreation of the physical model


31/48


Developing Physical Data FlowDiagrams

Clarifying which processes are performed byhumans and which are automated

Describing processes in more detail Sequencing processes that have to be done ina particular order

Identifying temporary data stores

Specifying actual names of files and printouts Adding controls to ensure the processes are

done properly


32/48


Figure 7.10Physical data flow diagramscontain many items not found in logical data

flow diagrams


33/48


Event Modeling and Data FlowDiagrams

An input flow from an external entity issometimes called a trigger because it starts

the activities of a process Events cause the system to do something and

act as a trigger to the system

An approach to creating physical data flowdiagrams is to create a data flow diagramfragment for each unique system event


34/48


Event Response Tables

An event table is used to create a dataflow diagram by analyzing each event

and the data used and produced by theevent

Every row in an event table represents

a data flow diagram fragment and isused to create a single process on adata flow diagram


35/48


Figure 7.12An event responsetable for an Internet storefront


36/48


Figure 7.13Data flow diagrams for the firstthree rows of the Internet storefront event

response table


37/48


Use Cases and Data Flow

Diagrams Each use case defines one activity and

its trigger, input, and output

Allows the analyst to work with users tounderstand the nature of the processesand activities and then create a single

data flow diagram fragment


38/48


Partitioning Data Flow Diagrams

Partitioning is the process of examining adata flow diagram and determining how it

should be divided into collections of manualprocedures and computer programs

A dashed line is drawn around a process orgroup of processes that should be placed in a

single computer program


39/48


Reasons for Partitioning

Different user groups

Timing

Processes may be separated into differentprograms for security

Similar tasks

Efficiency Consistency

Security


40/48


Partitioning Web Sites

Improves the way humans use the site

Improves speed of processing

Ease of maintaining the site

Keep the transaction secure


41/48


Communicating UsingData Flow Diagrams

Use unexploded data flow diagramsearly when ascertaining information

requirements Meaningful labels for all data

components


42/48


Summary

Data flow diagrams Structured analysis and design tools that

allow the analyst to comprehend the system

and subsystems visually as a set ofinterrelated data flows

DFD symbols Rounded rectangle

Double square An arrow Open-ended rectangle


43/48


Summary (Continued)

Creating the logical DFD

Context-level data flow diagram

Level 0 logical data flow diagram Child diagrams

Creating the physical DFD

Create from the logical data flow diagram Partitioned to facilitate programming


44/48


Summary (Continued)

Partitioning data flow diagrams Whether processes are performed by

different user groups Processes execute at the same time

Processes perform similar tasks

Batch processes can be combined for

efficiency of data Processes may be partitioned into different

programs for security reasons

Example 1:


45/48

Example 1:

Lincoln Pizzas ordering system

Lincoln Pizza is a small restaurant on Lincoln Ave. Most of its customers are students. Inthe current manual ordering system, the restaurant employees have to go through threedifferent activities in order to process a pizza order.

The first activity, called Receiving the order, consists in getting customers information(such as name, phone number, and address), and getting orders information (such as the

pizza size, the type of crust, and the ingredients needed to make the pizza). The sameprocess also checks the availability of the ingredients needed before setting the status ofthe pizza order as valid. When a pizza order is valid, the employee in charge of Receivingthe order provides the valid order information to the cooks who make the pizza.

The second activity, Making the pizza, consists in getting the ingredients needed (fromcontainers in the kitchen) and actually cooking the pizza. At the end of that process, the

cooks have to update the manual file that keeps track of the ingredients. They, then,provide the drivers in charge of delivering the pizza with the (completed) status of theorder.

The final activity, Delivering the pizza, done by the drivers consists in getting address andpayment information from the employee in charge of Receiving the order, and actuallydelivering the pizza. The drivers provide the customers with a receipt and get the payment.



46/48



47/48



48/48

Example 2:

Scargill Ltd is a distributor of imported ironmongery with a manual accounting system inwhich sales orders and invoicing are carried out as follows

Orders are received by the sales order/invoicing department, mainly by telephone.

On receipt, an official order form is prepared and passed to the sales ledger clerk to be

vetted for creditworthiness of the customer.

From there it is passed to the dispatch department to prepare pre-numbered dispatchdocuments and assemble the goods for dispatch.

The dispatch department then prepares a delivery schedule for delivering the variousorders by its own transport. After delivery a copy of each delivery note, signed by thecustomer, is returned to the dispatch department for matching with the original.

The dispatch department accounts for all dispatch note numbers and passes executednotes to the sales order/invoicing department for invoice typing.

The invoicing department dispatches top copy invoices direct to the customers and thesecond copies go to the ledger clerk for entry into the sales journal and posting to thesales ledger.

Kendall7e_ch07 Using Dataflow Diagrams

Documents

Transcript of Kendall7e_ch07 Using Dataflow Diagrams