Function Definition

From Investigation to Specification

Defining Functions

The Universal Function Model

Identifying and Documenting Functions

Function Navigation Models

Dialogues

Investigation

DeriveLogical View of

Current

Services

Assemble Investigation

Results

Investigate and Define

Requirements

Develop Business

Activity Model

FEASIBILITY

InvestigateCurrent

Processing

InvestigateCurrent

Data

Feasibility Report

Overview LDSBAMOverview DFM

BAM Requirements Catalogue

BAM

Current E

nvironment

LDM

Requirements Catalogue

Current Physical DFMTask Models

User Catalogue

Logical DFMCurrent Environment LDMRequirements CatalogueTask Models

Specification

Define Required

System

Processing

Develop

Required Data

Model

Enhance

Required Data

Model

Derive System

Functions

Assemble

Requirements

Specification

Develop Users’

Conceptual

Models

Develop

Specification

Prototypes

Develop

Processing

Specification

BSO

Requirements Catalogue

Logical DFM

BSO

Requirements Catalogue

Current Environment LDM

Define Work

Practice Model

BSO

User Catalogue

Task Models

User Catalogue

Task Models

User Roles

Required System DFM

Task Models

User Object ModelsReq. System LDM

Req. System LDM Function Definitions

FunctionDefinitions

User Object Model

Task Models

Requirements Catalogue

Function

Definitions

Window Specifications Window Navigation Models

Task Models

Help System

Specifications

Requirements Catalogue

User Object Models

Dialogue Structures

Entity Life Histories

Required System LDM

Effect Correspondence Diagrams

Enquiry Access Paths

Requirements Catalogue

Investigation

Defining Functions

The technique of Function Definition serves a number of different purposes:

To define the services that the computerised system will offer its users;

To help analysts and users towards a shared understanding of what the system will do;

To provide the basis for the system’s specification;

To ensure that all design products are traceable to requirements.

Defining Functions

The services offered by a computerised information system to its users are known as FunctionsFunctions

The Functions that are available to a given user depend on that user’s roleuser’s role

Functions are usually accessed through a series of one or more menusmenus

Functions usually involve the user in some sort of dialoguedialogue with the system

Defining Functions

For each Function we will have to: Design the human computer interface (and where

applicable interfaces with other computer systems). Specify the database processing that supports each

Function. Consider ways of preventing erroneous data from ending

up in the database. Gather information that can be used to predict and tune the

performance of the Function. Ensure that the work done on each Function is not

duplicated elsewhere and make use of work already done elsewhere.

Ensure that the Function is included in the user’s training manual.

Function Definition Concepts

We can define the concept of a Function in several ways, which together give a good idea of what a Function actually is:

A Function is a set of processing that users wish to carry out at the same time

A Function is a set of processing designed to handle the effects of an event (or group of events) on the system

A Function is the basic unit of processing for input to Physical Design (where programs will be specified to implement it)

Function Types

Enquiry or Update User or System Initiated On-line or off-line

We classify each Function in three main ways:

Function Components

Input Function Input

Process

Events Enquiry Triggers

Control Errors

Syntax Errors

Database

Integrity Errors

Event & Enquiry Output

Function Output Process

Function Error

Process

Valid Output

Error Output

Function Definition Boundary

Update orEnquiry Process

Identifying Functions

1

2

3

1 Maintain Purchase Order

PurchaseOrdersD1

Purchaser

b

DepotD6

SuppliersD7

Supplier

a

ProductsD3

Stock Clerk

i

*

Create Proposed

Purchase Order

1.1

*

ConfirmPurchase

Order

1.2

*

ArrangeDelivery

1.3

Proposed P.O.

Amended P.O.

Purchase Order

P.O.Quantities

Depot Address

SupplierDetails

DeliveryDetails

Product Details

DeliverySchedule

Availability

Amended P.O.

DeliverySchedule

Identifying Functions

Enquiry functions (other than those which form a major part of the systems functionality) will be documented in the Requirements Catalogue, rather than the Required System Data Flow Model. Each enquiry entry in the Requirements Catalogue will initially be identified as a function in its own right.

Documenting Functions

An identifier The Function’s Name The Function’s type A description The business event The business activity Error handling The system event or enquiry The User Role DFD Processes

For each Function we record

I/O Structures & Descriptions and Dialogues

Requirements Tasks User Object Model Windows Common Processing Service Level

Requirements Volumes

Function Navigation Models

the Function is very large and it is more manageable to break it down into several parts

the Function contains more that one event/enquiry: we can define one or more components for each event and enquiry

there are parts of the function that will be used by other functions too: again events and enquiries will often feature in more than one function.

Functions may be broken down into components when:

Function Navigation Models

Web Browse and Buy

Web Browse and Buy

PlaceOrder

PlaceOrder

MakePayment

MakePayment

Identifying Dialogues

A good way of identifying dialogues is to use the Function Definitions and User Role documentation to identify exactly who will require access to each function. The easiest way of documenting this is by using a User Role/Function Matrix

Identifying Dialogues

Delivery Scheduler

Goods In Clerk

Purchaser P. O. Clerk

Stock Keeper

Function

User Role

X

X

X X X

X X X

X

X

Critical Dialogues

Delivery Scheduler

Goods In Clerk

Purchaser P. O. Clerk

Stock Keeper

Function

User Role

X

X

X X X

X X X

X

X

The Place of Function DefinitionD

ecis

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Str

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Dec

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tru

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Pol

icie

s an

d P

roce

du

res

Pol

icie

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Use

r O

rgan

isat

ion

Use

r O

rgan

isat

ion

InvestigationInvestigation

ConstructionConstruction

SpecificationSpecification

Conceptual Model

Internal design

External Design

BAM

RD

WPM

DFM

FD

LDMRDA

BS

O