Data flow modelling concepts
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Data Flow Modelling Concepts. Data Flow Diagrams External Entities, Data Stores, Processes and Data Flows Elementary Process Descriptions Levelling Document Flow Diagrams. Key Definitions. A process model is a formal way of representing how a business operates

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Data flow modelling concepts

Data Flow ModellingConcepts

  • Data Flow Diagrams

  • External Entities, Data Stores, Processes and Data Flows

  • Elementary Process Descriptions

  • Levelling

  • Document Flow Diagrams


Key definitions

Key Definitions

  • A process model is a formal way of representing how a business operates

  • Data flow diagramming shows business processes and the data that flows between them


Key definitions1

Key Definitions

  • Logical process models describe processes without suggesting how they are conducted

  • Physical models include information about how the processes are implemented


Data flow diagrams aid communication

Data Flow Diagrams aid communication

e

Manager

Purchase Order

1 Stock Clerk

Stock List

Stock

M2

Order

Stock List

P.O.

File

Stock

d

Supplier

Purchase Order

Delivery

2 Stock Clerk

Purchase Order

Receive

Orders

M1

Cabinet

Stock

*

Matched Orders

e

Manager

Delivered Goods

3 Cashier

Stock

Sell

Sold Goods

M2

File

Stock

*

Bought Goods

a

Customer

External Entities

Processes

Data Stores


Data flow diagrams dfd notation

Data Flow DiagramsDFD Notation

  • The DFD is a diagram that consists principally of four symbols, namely the external entity, the dataflow, the process and the datastore

  • Additionally, a physical flow can be shown on the DFD of the current system


Data flow diagrams external entities

Data Flow DiagramsExternal Entities

d

Supplier


Data flow diagrams data flows

Data Flow DiagramsData Flows

Customer Details

Goods

Cosmetics

  • Data Flow (usual)

  • Bi-directional Flow (rare)

  • Flow Between External Entities (for convenience)

  • Resource Flow (for convenience)


Data flow diagrams process

Data Flow DiagramsProcess

Sell

Stock

3

Cashier


Data flow diagrams data stores

Data Flow DiagramsData Stores

D3 Suppliers

Stock

M1

File

T1 Unpaid Invoices

D1 Orders

D1 Orders

  • Digitised

  • Manual

  • Transient

  • Duplicate


Data flow diagrams decomposition

Data Flow DiagramsDecomposition


Data flow modelling concepts

Data Flow Diagrams

Decomposing Data Flow Diagrams

  • Any process on a DFD may be broken up into several sub-processes which, when viewed collectively, make up that process

  • Thus for example we may break-up process 1 of the Small Stock System into that shown on the next slide:


Data flow modelling concepts

e

Manager

1 Order Stock

Stock List

Purchase Order

1.1

1.2

Produce

Record

Stock

Purchase

List

Order

Stock List

Purchase Order

Purchase Order

Stock

M2

M1

Cabinet

File

Data Flow Diagrams

Decomposing Data Flow Diagrams

*

*


Data flow modelling concepts

Data Flow Diagrams

Decomposing Data Flow Diagrams

  • The decomposition of a DFD into lower level DFDs is known as levelling

  • The DFD that shows the entire system is known as the ‘top level’ or ‘level 1’ DFD

  • The DFDs that contain more detailed views of the level 1 processes make up ‘level 2’ DFDs

  • Any level 2 process that is further decomposed gives rise to a level 3 DFD and so on


Data flow modelling concepts

Data Flow Diagrams

Decomposing Data Flow Diagrams

Elementary Process Description

System: SmallStock DFD Type: Current

Process Name: Record Purchase Order Process Id: 1.2

Managers give the stock clerk a ready-made purchase order. The stock clerk places this order in the Purchase Order Cabinet.

It is the managers’ responsibility to send the order directly to the supplier they have chosen. Each purchase order contains product information taken from the supplier’s price list. The date after which a delivery of goods will be unacceptable is also included.


Data flow modelling concepts

1.2

Record

Purchase

Order

Data Flow Diagrams

Decomposing Data Flow Diagrams

*


Data flow modelling concepts

Data Flow Diagrams

Context Diagrams


Data flow diagrams starting from the context diagram

Data Flow Diagrams Starting from the Context Diagram

  • To develop a Context Diagram we carry out the following tasks:

    (i)Identify all sources and recipients of data from the system, i.e. external entities

    (ii)Identify the major data flows to and from the external entities

    (iii)Convert each source or recipient into an external entity symbol

    (iv)Add the data flows between each external entity and a single box representing the entire system


Data flow diagrams starting from the context diagram1

Data Flow Diagrams Starting from the Context Diagram

Supplier s Delivery Note

r Purchase Order

s Delivery Details

s Invoice

Purchaser s P.O. Quantities

r Stock Report

Customer r Dispatch Note

Sales & Marketing s Customer Order

rMatched C.O. #1

AccountsrMatched Invoices

External Entity S or R Data Flow


Data flow diagrams starting from the context diagram2

Data Flow Diagrams Starting from the Context Diagram

a

Supplier

Payment

Delivery Note

Purchase Order

Delivery

Invoice

Details

e

d

Matched Invoice

Despatch Note

Accounts

Customer

Stock Report

Matched C.O.

Customer Order

Copy #1

P.O.Quantities

Customer Order

b

c

Sales and

Purchaser

Marketing

ZigZag

Warehouse

System


Document flow diagrams

Document Flow Diagrams

  • Identify all recipients and sources of documents, whether inside or outside the system boundary

  • Identify the documents that connect them

  • Convert each source and recipient into an external entity symbol

  • Add data flow arrows to represent each connecting document

  • Add the system boundary to exclude the external entities identified in the context diagram


Document flow diagrams1

Document Flow Diagrams

Supplier Invoice P.O.Clerk

Supplier Delivery Times Stock Clerk

Stock Clerk Stock Report Purchaser

Stock Clerk Stock Report Despatch Supervisor

Despatch Clerk Despatch Note Customer

Customer Customer Order Sales & Marketing

Sales & MarketingCustomer OrderDespatch Clerk

Despatch ClerkDespatch ReportDespatch Supervisor

Despatch Super.Matched Dsp RepDespatch Clerk

Despatch ClerkMatched CO #1Sales & Marketing

….

Source Document Recipient


Document flow diagrams2

Document Flow Diagrams

Despatch Supervisor

Sales and

Matched

Marketing

Despatch Rpt

Customer Order

Despatch Report

Matched C.O.

Copy #1

Despatch

Clerk


Data flow diagrams converting document flow diagrams

Data Flow DiagramsConverting Document Flow Diagrams

What process generates this document flow?

What process receives this document flow?

Is the document stored by a process?

Where is the document stored?

Is the document created from stored data?

What business activity triggers the process?

Is the document a source of new data?

To transform the Document Flow Diagram into a DFD we follow each document flow in turn, asking the following questions:


Data flow modelling concepts

e

P.O.

Manager

Stock List

Purchase Order

Matched Orders

d

Small

Stock

Delivery

Supplier

System

Bought Goods

a

Customer

Data Flow Diagrams

Context Diagrams

  • A level higher than level 1, showing the whole system as a single process with external entities around it, is also possible:


Data flow modelling concepts

Data Flow Diagrams

Decomposing Data Flow Diagrams


Data flow modelling concepts

Decomposition Diagram


Data flow diagrams starting from the context diagram3

Data Flow Diagrams Starting from the Context Diagram

  • We can now follow each flow into and identify the elementary process responsible for it

  • A grouping of these elementary processes can then give us a first glimpse of the system’s Data Flow Model


Data flow diagrams converting business activity models

Data Flow DiagramsConverting Business Activity Models

Check

Delivery

2 Goods Receiving

b

Check

Supplier

Delivery

Place Goods

Delivery Note

in Delivery

Dock

P.O. Copy

Matched P.O.

Purchase Orders

M1

Allocate

Stock

T2

Matched P.O.’s

Location

Matched P.O.

Remove Goods

from Delivery

Dock

New Stock

M2 Stock

3 Stock Keeping

Store

Stock

Store

Goods in

Depot


Data flow modelling concepts

2 Goods Receiving

2 Goods Receiving

2 Goods Receiving

b

b

Check

Check

Check

Supplier

Supplier

Delivery

Delivery

Delivery

Delivery Note

Delivery Note

Matched P.O.

Matched P.O.

T2

T2

Matched P.O.’s

Matched P.O.’s

Data Flow DiagramsTips

  • For a process to be complete, it needs to have both an input and an output (shown by data flows going into and coming out of it)


Data flow modelling concepts

f

f

Someone

Someone

2

Something

Do something with it

Same something

Something

M2

M2

A data store

A data store

Data Flow DiagramsTips

WHY?


Creating use cases

Creating Use Cases


Elements of a use case

Elements of a Use Case

  • Trigger -- event that causes the scenario to begin

    • External trigger

    • Temporal trigger

  • All possible inputs and outputs

  • Individual steps

    • Show sequential order

    • Show conditional steps


Scenario template use case

Scenario Template (Use Case)


Creating data flow diagrams

Creating Data Flow Diagrams


Steps in building dfds

Steps in Building DFDs

  • Build the context diagram

  • Create DFD fragments for each scenario

  • Organize DFD fragments into level 0

  • Decompose level 0 DFDs as needed

  • Validate DFDs with user


A dfd fragment example

A DFD Fragment Example


Dfd common errors

DFD – Common Errors

Error

Error

Error


Dfd packet concept

DFD – Packet Concept


A second dfd fragment example

A Second DFD Fragment Example


Level 0 tips

Level 0 Tips

  • Generally move from top to bottom, left to right

  • Minimize crossed lines

  • Iterate as needed

    • The DFD is often drawn many times before it is finished, even with very experienced systems analysts


Composite elementary flows

Composite & Elementary Flows


Tips for level 1 and below

Tips for Level 1 and Below

  • Sources for inputs and outputs listed at higher level

  • List source and destination of data flows to processes and stores within each DFD

  • Depth of DFD depends on overall system complexity

    • Two processes generally don’t need lower level

    • More than seven processes become overly complex and difficult to read


Flows to from data stores

Flows to & from Data Stores


Illegal data flows

Illegal Data Flows


Validating the dfd

Validating the DFD

  • Syntax errors

    • Assure correct DFD structure

  • Semantics errors

    • Assure accuracy of DFD relative to actual/desired business processes

  • User walkthroughs

  • Role-play processes

  • Examine lowest level DFDs

  • Examine names carefully


How should we gather requirements use cases

How should we gather requirements? Use cases?

1. Use cases hold functional requirements in an easy-to-read text format

2. They make a good framework for non-functional requirements & project details.

3. Use cases show only the Functional req’ts.

4. Design is not done only in use case units.


Use cases do not collect formulae state cardinality performance uptime

Use cases do not collect formulae, state, cardinality, performance, uptime, ...

  • Examples:

    1. Order cost = order item costs * 1.06 tax

    2. Promotions may not run longer than 6 months.

    3. Customers only become Preferred after ...

    4. A customer has one and only one sales contact.

    5. Response time is ...

    6. Uptime requirement is ...

    7. Number of simultaneous users will be ...


Use the scenarios directly to design and test the system

Use the scenarios directly to Design and Test the system.

What if...?

Scenarios

“Knows how to...”

“Knows how to...”

“Knows how to...”


Summary

Summary

  • The Data Flow Diagram (DFD) is an essential tool for creating formal descriptions of business processes and data flows.

  • Use cases record the input, transformation, and output of business processes.

  • Eliciting scenario descriptions and modeling business processes are critically important skills for the systems analyst to master.


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