DFDs

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Context Diagram

• Shows the boundaries of the system• Shows the overall business process as just ONE

process• Shows all the outside entities that receive

information from or contribute information to the system

• No Data store

2

Level 0 Diagram

• Shows all the processes that comprise the overall system

• Shows how information moves from and to each process

• Adds data stores

3

Level 1 Diagram

• Shows all the processes that comprise a single process on the level 0 diagram

• Shows how information moves from and to each of these processes

• Level 1 diagrams may not be needed for all level 0 processes

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Guidelines for Drawing a Context

Read the case study from start to finish a number of times until you have a fair idea of what the system as a whole does.

Try to make a list of potential external entities. A person or place is an external entity under two conditions. ◦ Firstly, if it gives something to the system without explanation of

the processing involved in its creation. ◦ Secondly, if it receives something from the system with no

explanation of what it does with it. Establish what data flows are sent to external entities from

the system and what data flows are received into the system from them.

Having identified external entities and flows to and from them, the context diagram can be drawn.

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Guidelines for Drawing Level 0 Take one sentence at a time and decide if it is background information

or if it is an activity which must be represented by a process in the diagram. Processes can be identified by the verb in the sentence.

Make a list of all potential processes ◦ group these potential processes so that you end up with approximately 3 to

10 processes. Identify and list data flows

◦ these are generally documents but could also be phone calls, physical items such as goods etc. Anything that moves around a system can be considered a data flow.

Identify and list data stores. Having identified all the components of the DFD, draw the diagram

using the appropriate conventions (notation). Validate the level 0 diagram against the context diagram to ensure that

they are consistent. ◦ a difference in the number of flows or the names of those flows will result in

the wrong system being modeled. 6

Guidelines for Drawing Level 1• For each level 0 process make a list of sub-processes, each

one of which will become a process on the corresponding level 1 diagram.

• Validate the level 1 diagram against the level 0 diagram to ensure that they are consistent.

• Remember the process identifier from the level 0 process is used as the first part of the identifier for the lower level processes.

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Points to Remember when Drawing DFDs When drawing DFDs, you are concerned with the logical

components of the system, i.e. what are the activities which when linked together make up the system under investigation as a whole.

When listing processes, think of what activities are carried out, not who does them or where they are done.

Make sure that the beginning and the end of a flow touch its source and destination.

All flows must be labeled. No two flows on the same diagram can have the same name (unless they are identical).

All words in a flow name must be connected by underscores, e.g. sales_invoice. Keep flow names short but meaningful. 8

Points to Remember when Drawing DFDs Make sure that no section of a diagram is separate from the rest,

i.e. there must be a logical connection through the entire diagram. ◦ If one section is not joined to the rest then this part of the diagram

will never be triggered and consequently the system the diagram is modeling will be incomplete and inaccurate.

Make sure that the correct arrow type is used to join a process with a store: ◦ if a process is updating information then the arrow head is at data

store. ◦ if a process is retrieving information from a store then the arrow

head is at the process. Make sure the processes are correctly numbered. Check the consistency of flows across diagrams (Balancing), ◦ The same number of flows coming in and out of a level 0 as a context

and also that the names are the same. Make sure that each diagram is properly labeled i.e. its 'title' should

be clearly stated. For example, 'Level 1 Order Processing'. 9

Rules governing DFDs

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Q1 (a): Q1 (a): Identify Potential Violations of rules & Identify Potential Violations of rules & guidelines guidelines

A1 (a):

1.Process 1.0 (P2) has only inputs, making it a black hole; 2.data flow DF5 should not move directly from source E1 to data store DS1 without first going through a process;3.data flow DF1 should not move directly from source E1 to sink E2 without first going through a process.4.Other peculiarities (such as Process 1.0 has label P2 and the data store has only a label, not a number)

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Q1 (b): Q1 (b): Identify Potential Violations of rules & Identify Potential Violations of rules & guidelinesguidelines

A1 (b):

1.No data store in context diagram2.Payment- extra dataflow in level-03.Customer data– from source to data store directly4.Order- bidirectional dataflow5.Order details & inventory data – wrong forked dataflow (different data)6.Process 3.0 – No output

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Q2: Draw the context Diagram from the Level 0 DFD for Hiring System

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A2:

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Q3: Student Administration System • Student Administration System : The enrolment process

works as follows:• Students send in an application form containing their personal

details, and their desired course. The university checks that the course is available and that the student has necessary academic qualifications. If the course is available the student is enrolled in the course, and the university confirms the enrolment by sending a confirmation letter to the student. If the course is unavailable the student is sent a rejection letter.

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A3: Context DFD (pre-steps)• Context DFD for the Student Administration System

• Step 1• Familiarize yourself with the case study.

• Step 2 (identify potential external entities)• Student.

• Step 3 (identify flows to/from system and external entities)• Student send: Application details• Student receive: Confirmation/ rejection details

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A3: Context DFD• Step 4 (draw your context diagram)

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A3: Level-0 DFD(pre-steps)• Draw a Level 0 DFD for the Student Administration System .

• Steps 1 & 2 (identify potential processes)• Verify availability, Enroll student, Confirm registration.

• Step 4 (identify data flows)• Requested-courses, Accepted/rejected selection, registration,

confirmation letter, open-course, course-details , course- enrollment, student-details,

• Step 5 (identify data stores)• Course file , student file

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A3: Level-0 DFD

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• Step 6 (draw your level-0 diagram)

Q4: Precision Tools System• Precision Tools System: Precision Tools sells a line of high-

quality woodworking tools. When customers place orders on the company’s Web site, the system checks to see if the items are in stock, issues a status message to the customer, and generates a shipping order to the warehouse, which fills the order. When the order is shipped, the customer is billed. The system also produces various reports.

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A4:Context DFD(Pre-steps)• Context DFD for the Precision Tools System

• Step 1• Familiarize yourself with the case study.

• Step 2 (identify potential external entities)• Customer, Warehouse, Accounting

• Step 3 (identify flows to/from system and external entities)• Customer sends: order, payment• Customer receives: status message , invoice• Warehouse sends: shipping confirmation• Warehouse receives: in stock request, shipping order • Accounting receives : inventory reports

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A4 – Context DFD• Step 4 (draw your context diagram)

24ACCOUNTING

WAREHOUSECUSTOMER

0

Order System

Order

Payment

In-StockRequest

StatusMessage

Invoice Shipping Confirmation

Shipping Order

Inventory Reports

A4: Level-0 DFD(Pre-steps) Draw a Level 0 DFD for the Precision Tools System◦ Steps 1 & 2 (identify potential processes)• Check Status ,Issue Status Messages, Generate Shipping Order, Manage

Accounts Receivable, Produce Reports◦ Step 4 (identify data flows)◦ Order, In-Stock Request, Order Data, Status Data, Status Message,

Shipping Order, Order Data, Invoice , Shipping Confirmation, Payment, Accounting Data, Accounts Receivable Data, Order Data, Inventory Reports

◦ Step 5 (identify data stores)• Pending Orders, Accounts Receivable◦ Step 6 (draw level 0 diagram)

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A4: Level-0 DFD

1.0

CheckStatus

2.0

IssueStatus

Messages

3.0

GenerateShipping

Order

ACCOUNTING

CUSTOMER WAREHOUSE

4.0

Manage Accounts

Receivable5.0

ProduceReports

Order In-Stock Request

Status Data

Status Message

PendingOrdersD1

Order Data

Order Data

Shipping Order

Shipping Confirmation

Invoice

Payment

Accounts ReceivableD2

Accounting Data Accounts Receivable Data

Order Data

Inventory Reports 26