PROCESS MODELLING DATA FLOW DIAGRAMS

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CSE1204 - Information Systems 1

Process Modelling Data Flow Diagrams

Process Modelling Process modelling aims to graphically represent the processes which capture, manipulate, store and distribute data. • • • •

data flow diagrams function decomposition structured English decision tables and decision trees

Data flow Diagrams Data flow diagrams model the flow of data into, through, and out of an information system: • show the processes that change or transform data • show the movement of data between processes • represent a system as a network of processes which transform data flowing between them

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Example DFD DFDs fit neatly onto a single page • the user is not overwhelmed by a DFD

WAREHOUSE

ORDERS

invalid orders CUST OMERS

shipping details order details 1. RECEIVE ORDER

orders

books 2. SHIP BOOKS

c ustomer name, c ustomer address

CUST OMERS

billing information

c ustomer name, c ustomer address

books

INVOICES

c ustomer name, invoic e details

3. COLLECT PAYMENT S

invoic es, statements

CUST OMERS payments, enquiries

Data Flow Diagrams

Data flow diagrams (or DFDs): • • • •

a well-known process modelling technique easily understood a good communication tool model both manual and automated processes

Components of a DFD 2

1.

Process calculate price

2.

Data flow

loan application

3.

Data store

Products

4.

External agent

Suppliers

(Source/Sink)

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1. Process • transforms incoming data flows into outgoing flows: the work performed on data which changes it

data

• each process has a unique number and name 2 calculate price

1. Process • name each process using a verb and a noun phrase eg.

calculate price check customer details accept supplier delivery

• the name of a process should describe what the process does • avoid vague names where possible e.g. "process data"

2. Data Flow • a data flow represents data in motion, moving from one place in the system to another • describes a "packet" of data or data that moves together: a data flow may consist of many individual, related pieces of data that move together to a common destination loan application

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• name each data flow using a noun or noun phrase eg. customer order • the name of a data flow should describe the contents of the data "packet" • the name should include as much information as possible about the data flow eg."customer payment" rather than just "payment"

customer order

2 validate customer order

valid customer order

invalid customer order

3. Data Store

• a data store represents a collection of data flows "at rest" • each data store has a unique name • the name should describe the contents of the data store • a data store may represent many different types of physical locations of data • a data store may be a temporary or a permanent repository of data

The Data Store different notations

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data flows to and from a data store can remain unlabelled if all attributes in the store are moving, i.e. if an entire data packet (or packets) is going into or out of the data store 2 sales order

check sales order

Sales orders 3 produce weekly sales totals

weekly sales totals

4. External Agent (Source/Sink) • an external agent represents an entity with which the system communicates and which is outside the scope of the system eg.

an outside organisation or individual, another department or another system, a person or group within the department supported by the system who interacts with the system

• an external agent is a source if it is an origin of data coming into the system • an external agent is a sink if it is a destination of data leaving the system

Suppliers

4. External Agent (Source/Sink)

• data flows connecting the external agents to the processes within the system represent the interface between the system and its environment • external agents are outside the system and define its boundaries • an external agent may be both a source and a sink what a sink does with data it receives from the system and how a source produces data which it inputs to the system are outside the boundary of the system and are not shown on the data flow diagram

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Example DFD sales order

Sales Order System

purchase order Suppliers

goods returned

goods unavailable notice

Inventory System

shipping slip

supplier delivery

checked supplier invoice

Warehouse

Accounts Department

Example Data Flow Diagram Customers

sales order 1 check sales order

Suppliers Sales orders 2 produce purchase order

purchase order

Guidelines for Drawing DFDs • each object on a data flow diagram must have a unique name • each process must have at least one data flow coming in (input) and at least one data flow going out (output) • the inputs to a process are different from the outputs of that process • a process must be able to build its outputs using only the information in its input data flows plus any constant information

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Guidelines for Drawing DFDs data flows are permitted: • between processes • from a data store to a process • from a process to a data store • from a source to a process • from a process to a sink

Guidelines for Drawing DFDs data flows are NOT permitted: •

between external agents



between data stores



from an external agent to a data store

Identify six errors in this abstract DFD Data Store 2 EE1 Data store 1

1 process2

EE2 2

EE3

Process1

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Levelling Data Flow Diagrams Any "real" system is too large to represent as a single data flow diagram • the solution is to decompose the system into a hierarchy of levels of processing • the process model of the system then consists of a set of levelled data flow diagrams • levelling of DFDs improves their readability and usefulness as a communication tool

Levelling of DFDs Levelling creates a hierarchical decomposition of the processing within the system Context diagram • Level 0 diagram Level 1 diagrams Level 2 diagrams . . . Level n diagrams

Levelling of DFDs

Context diagram

Level 0 diagram

Level 1 diagrams Level 2 1.1.1 diagrams

1.1

1.1.2

1

1.2

2

1.3

2.1

3

2.2

3.1

4

3.2

3.2.1

4.1

4.2

4.3

3.2.2

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Context Diagram • the highest level data flow diagram is the context diagram • the context diagram shows the interaction of the system with its environment in terms of data flows • the context diagram defines the boundary of the system (the scope of the system) • only the data flows which leave the system and the data flows which come from outside the system are shown

Context Diagram • the entire system is represented as a single process • all external agents (sources and sinks) are shown • no data stores are shown: they are inside the boundary of the system

EASY GO HOTEL

Example Context Diagram sales order

Sales Order System

purchase order Suppliers

goods returned

goods unavailable notice

shipping slip

Warehouse

Inventory System

supplier delivery

checked supplier invoice

Accounts Department

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Level Zero Diagram • the level zero data flow diagram is the diagram at the level immediately below the context diagram • it "expands" the single process on the context diagram to show the major, high-level processes (or functions) within the system

4.0

1.0

2.0 3.0

Level Zero Diagram • all external agents (sources and sinks) are included because the level zero diagram, like the context diagram, represents the entire system • the number of each process ends in .0 which corresponds to the level of the diagram: e.g. 1.0, 2.0, 3.0 etc. often just 1, 2, 3 etc. are used

Example Level Zero Diagram Order Entry System

purchase order

Suppliers

sales order 2.0 Monitor Stock levels 3.0

1.0 out of stock notice

Fill Sales Orders

Accept Deliveries

supplier delivery

Stock file

shipping slip

Warehouse

checked supplier invoice

Accounts Department

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Level 1 and lower level diagrams each Level 1 diagram and diagrams at lower levels (e.g. Level 2, Level 3) show only a part of the processing Level 1 processes are numbered 1.1, 1.2, 1.3, and 2.1, 2.2, 2.3 etc that part is shown in more detail than on the Level zero diagram no external agents are shown on a Level 1 or lower diagrams, as the entire system is not being represented

Level 1 diagrams a set of data flow diagrams is created at Level 1 there is one Level 1 diagram for each of the processes at Level zero each diagram decomposes a Level zero process into several processes

2.4 2.1

2.3

2.5 2.2

Levelling DFDs

1 2 Context diagram

3 Level zero diagram 2.1

3.1

2.2

3.2 Diagram 3 (level 1)

Further diagrams or process descriptions

Diagram 3 (level 1)

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Example Level zero diagram SUPPLIERS CUSTOMERS Customer details Product details product qty product holding 1

p.o. number

INVENTORY

product delivered qty

product product holding order qty

Record and Invoice Sales

SALES ORDERS

customer invoice

p.o. invoice

product order qty 2

delivery qty

Maintain Product Inventory

product order qty

Example Level 1 diagram Check Stock Available

1.1 Check Customer Credit

Product details

customer credit status

product qty

product

product holding INVOICES

available qty

SALES ORDERS 1.4

1.3 product qty

INVENTORY

1.2

Customer details

Record Sales Demand

Create Sales Invoice

customer invoice

Guidelines for Levelling DFDs • numbering: when a process is decomposed, its diagram is given the same number as that process • balancing of DFDs: all data flows entering and leaving a process must appear on the corresponding diagram which decomposes that process • external agents: are only included on the two diagrams which represent the entire system, i.e. the context and level zero diagrams

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Guidelines for Levelling DFDs balancing data flows: if data flows are decomposed at lower levels then the contents of the data flows across levels must be balanced sales order

customer shipping details

1 out of stock notice

customer order items 1.1

Fill Sales Orders

1.2

Check shipping details

Check stock available

shipping slip

sales order

=

customer + shipping details

customer order items

Guidelines for Levelling DFDs The access to data stores across levels of diagrams must be consistent: •the direction of accesses must match and all accesses on higher level diagrams must appear on corresponding lower level diagrams •a data store is first shown on the highest level diagram where it is accessed by more than one process •it can then appear on all lower level diagrams where it is accessed

1 1.1

1.2 1.3

2

Guidelines for Levelling DFDs partition processes to: • form cohesive, related groups of activities • minimise the data flows between them

all parts of the system need not be decomposed to the same level

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Guidelines for Levelling DFDs How many levels should be in a set of DFDs? • each diagram usually has between 3 and 7 processes • level the diagrams until bottom level or primitive processes are reached primitive processes cannot be further decomposed as a data flow diagram

An example - Context Diagram Line

Applicant

Manager Address Position Spec

Applicant name Position Acknowledgement

Successful Applicant

Employment System

Decision

Payroll System

Position

An example - Level zero diagram Applicant Position Address

Acknowledgement

1

Applicant name

Line Manager

Ackn’d Appn

Accept Application

Position Spec Decision

2 Evaluate Applicants

Applicant Position

Unsuccessfuls file

Evaluation results

Successful Applicant

Payroll System

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Level 1- Diagram 1 Address

Applicant name

Position 1.1

Acknowledgement

Verified Application 1.2

Verify for Completeness

Acknowledge Application

Acknowledged Application

Level 1 - Diagram 2 Ackn’d Appn

Position Spec

Qualified applicant

2.1 Screen Applicants

unqualified applicant

Decision

2.2 Reject Unsuitabl es

2.3 Schedule Evaluation

Decision Unsuccessfuls file

Evaluation results

Successful Applicant

References WHITTEN, J.L., BENTLEY, L.D. and DITTMAN, K.C. (2001) 5th ed., Systems Analysis and Design Methods, Irwin/McGraw-HilI, New York, NY. Chapters 8 HOFFER, J.A., GEORGE, J.F. and VALACICH (2005) 4th ed., Modern Systems Analysis and Design, Benjamin/Cummings, Massachusetts. Chapter 7

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