2.4 Spiral model

2.4.0.0 Phases of the spiral model  

• Objective setting
  • Specific objectives for the project phase are identified

• Risk assessment and reduction
  • Key risks are identified, analysed and information is sought to reduce these risks

• Development and validation
  • An appropriate model is chosen for the next phase of development.

• Planning
  • The project is reviewed and plans drawn up for the next round of the spiral

The spiral model is a meta-process model:

• Large systems are usually made up of several sub-systems
• The same process model need not be used for all subsystems
• By filling in the arms of spiral, we get different models
  • e.g., Waterfall model for well-understood developments

2.4.0.0 Template for a spiral round  

• Objectives: goal of the analysis
• Constraints: factors that limit possibilities
• Alternatives: ways to achieve objectives
• Risks: dangers with the alternatives
• Risk resolution: strategies to reduce the risks
• Results: outcome of risk reduction
• Plans: how to approach next phase
• Commitment: management decisions on continuation

Determine objective, alternatives, contraints:

• Objectives
  • Procure software component catalogue

• Constraints
  • Within a year
  • Must support existing component types
  • Total cost less than $100, 000

• Alternatives
  • Buy existing information retrieval software
  • Buy database and develop catalogue using database
  • Develop special purpose catalogue

• Risks
• Risk resolution
• Results
• Plans
• Commitment

Evaluate alternatives; identify, resolve risks

• Objectives
• Constraints
• Alternatives
  • Buy existing information retrieval software
  • Buy database and develop catalogue using database
  • Develop special purpose catalogue

• Risks
  • May be impossible to procure within constraints
  • Catalogue functionality may be inappropriate

• Risk resolution
  • Develop prototype catalogue (using existing 4GL and an existing DBMS) to clarify requirements
  • Commission consultants report on existing information retrieval system capabilities.
  • Relax time constraint

• Results
• Plans
• Commitment

Prototype is developed and studied.

Develop, verify next-level product

• Objectives
• Constraints
• Alternatives
  • Buy existing information retrieval software
  • Buy database and develop catalogue using database
  • Develop special purpose catalogue

• Risks
• Risk resolution
• Results
  • Information retrieval systems are inflexible. Identified requirements cannot be met.
  • Prototype using DBMS may be enhanced to complete system
  • Special purpose catalogue development is not cost-effective

• Plans
• Commitment

Prototype is developed and studied.

Plan next phase

• Objectives
• Constraints
• Alternatives
  • Buy existing information retrieval software
  • Buy database and develop catalogue using database
  • Develop special purpose catalogue

• Risks
• Risk resolution
• Results
• Plans
  • Develop catalogue using existing DBMS by enhancing prototype and improving user interface

• Commitment
  • Fund further 12 month development

2.4.0.0 Risk management  

• Perhaps the principal task of a manager is to minimise risk
• The "risk" inherent in an activity is a measure of the uncertainty of the outcome of that activity
• High-risk activities cause schedule and cost overruns
• Risk is related to the amount and quality of available information. The less information, the higher the risk