Systems Development Life Cycle

SDLC: A simplified introduction

Stages of the Cycle1. Problem definition2. Feasibility study3. Analysis4. Design5. Implementation6. Maintenance

Why do we need the SDLC? The software crisis of the late ‘60s and early

‘70s Systems were delivered years late They were over budget Unreliable Difficult to maintain Did not do what was required

What does the SDLC do? Systems Development Life Cycle was an

attempt to establish a structured approach to systems development.

For management, each stage of the life cycle was a milestone with an associated date and set of deliverables.

Deliverables Each stage has an associated set of

deliverables Sets of documents produced at each stage in

the life cycle

1. Problem Definition The problem definition forms the basis of the

problems and requirements list (see later) It records all problems and requirements

mentioned by clients in interviews, or which are subsequently discovered during analysis of the system.

Problem Definition Report – the purpose To provide a written statement of the user's

current problems and requirements; to get agreement with the user.

To ensure that the right problem is being tackled

To force the user to become involved To define the current state of the system and

the required end state

Problem Definition Report – the sections Problem Objectives Scope Preliminary ideas Recommended action

2. Feasibility Study Feasibility study - is there a practical solution to the problem

outlined in the initial problem definition.

In particular, the feasibility study examines the technical, financial and organisational feasibility of the project: Can it be done?

Can we afford it?

Will the proposed new system fit in with existing procedures?

Feasibility study report Presented by the system developer to the user

Decision is made whether or not to proceed.

3. Analysis Deliverable from the analysis stage is the

‘Specification of requirements’ Logical model of the required system States WHAT the system is to do Says nothing about HOW to implement it Includes

◦ Data flow diagrams◦ Data dictionary◦ Process definitions◦ E/R model◦ Entity life histories or state diagrams

4. DesignThis has two stages: Provides several different solutions to the

problem Selects one solution and specifies it in detail

Design – Alternative solutions A very cheap solution which does the job and

no more. A medium price solution which does the job

well and is convenient for the user; A high cost, all-singing, all dancing solution

How solutions may differ System boundaries; Automation boundaries; could remain manual. Hardware Software Design strategies User interface Costs

Design – Selecting a solutionSpecification may include:Program design (e.g. structure charts) and

specificationSpecification of the user man/machine

interface Specification of the layout of reports and

other system outputs File and record specificationsHardware specifications, including costs Implementation schedule

5. ImplementationProgram listings, test plans and supporting

documentationManual of operating proceduresManual of clerical proceduresUser manualHardware on which the system will runThe system must be installed at the clients'

site on their equipment Changeover from the old to the new system

supervised Often a hand-holding period

6. Maintenance Starts as soon as the system is handed over Term maintenance often euphemism for

finding and correcting errors True maintenance is modifying the system to

meet evolving client requirements System developer must start again at the

beginning of the cycle

An example, simplified system

The System Requirements

BackgroundErrors in requirements may account for

approximately 50 per cent of the total cost of debugging a software system.

Many of the traditional system development methods merely pay lip service to identifying, describing and validating the client’s requirements for the system.

Today requirements engineering is recognised as a crucial stage in the development of software.

Requirements – what are they?No consensus of opinion as to what is meant by

requirements System requirements - the client’s needs

and wishes. Software requirements - constraints put on

the system development, such as hardware, software and design methods.

Requirements EngineeringCovers three phases: elicitation (identifying requirements) specification (describing them in an

appropriate language or notation) validation (checking with the client that the

description accurately records his or her needs and wishes).

Different types of requirements Functional requirements: what the

system has to do what its inputs and outputs are and how these are linked.

Non-functional requirements: the attributes of the system as it performs its job; can be divided into 1. non-functional requirements of the system and2. non-functional requirements arising from

external sources.

Non-functional System Requirements Usability Performance Reliability Security

ElicitationThis covers several different activities: Observation of the users at work Study of relevant documents User questionnaires Talking to the people involved in the system

The Requirements Specification A cornerstone of a system development

project Encapsulates the shared understanding and

intentions of all the stakeholders May be used as a vehicle for communication

between developers, users and other stakeholders

The Requirements Specification May form the basis of a legal contract

between developer and client Guides the programmers in their

implementation of the system Desirable qualities of a requirements

specification can be found in the IEEE Recommended Practice for Software Requirements Specifications from the IEEE Standard 831–1993.

Requirements Validation Technically feasible to confirm that the

requirements specification document is of the desired quality

Not easy to ascertain whether the requirements expressed in the specification are really what the client wants and needs

The client may not know what he or she wants

Requirements Validation Prototyping allows the client and users to get

some feeling for how their ideas would work once implemented in a computer system.

Talk through the requirement specification with the client and users.

SummaryStage Stage ContentContent DeliverableDeliverable

1.1. Problem DefinitionProblem Definition What is the problem?What is the problem? Problem Definition Report - statement of Problem Definition Report - statement of problems, scope and objectives problems, scope and objectives

objectives of new systemobjectives of new system

2.2. Feasibility Study Feasibility Study Is there a feasible Is there a feasible solution – quick look solution – quick look ahead to see if you can ahead to see if you can do something about the do something about the

problemproblem

Feasibility Study Report - rough cost benefit Feasibility Study Report - rough cost benefit analysisanalysis

- system scope and objectives cost benefit - system scope and objectives cost benefit analysisanalysis

3. Analysis3. Analysis What What must be done to must be done to solve the problemsolve the problem

Specification of Requirements –logical Specification of Requirements –logical model of required systemmodel of required system

4.4. DesignDesign HowHow should the problem should the problem be solvedbe solved

Technical Design Specification –includes Technical Design Specification –includes program specifications, hardware program specifications, hardware specifications, cost estimates and an specifications, cost estimates and an implementation scheduleimplementation schedule

5.5. ImplementationImplementation Do itDo it Working system, includes program listings and Working system, includes program listings and documentation, test plan, hardware, operating documentation, test plan, hardware, operating procedures, clerical proceduresprocedures, clerical procedures

6.6. MaintenanceMaintenance Modify system as Modify system as necessarynecessary

Working system - Operational system, modified Working system - Operational system, modified and documented as requiredand documented as required

Tasks Using one or two A4 sides, list each stage of

the SDLC, using illustrations and diagrams where appropriate

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