The Spiral Model EEE493 2000

1
The Spiral Model EEE493 2000
Royal Military College of Canada Electrical and
Computer Engineering

• Major Greg Phillips
• greg.phillips_at_rmc.ca
• 1-613-541-6000 ext. 6190

Dr. Scott Knight knight-s_at_rmc.ca 1-613-541-6000
ext. 6190
2
Review Waterfall model
System requirements
Software requirements
Analysis
Design
Coding
Testing
Maintenance
3
Review Prototyping cycle
4
Review Prototyping in Waterfall
System requirements
Software requirements
Analysis
Design
5
Review Evolutionary prototyping
Gather requirements from customer
Build prototype
Customer evaluates prototype
Quick design
Derive design
Refine prototype
Tune system
Operate and maintain
6
Boehms observation (1988)

• both the waterfall model and evolutionary
  prototyping models are aimed a reducing risk
• waterfall reduces risk of requirements/design
  thrashing by insisting they are finalized and
  signed off early
• evolutionary prototyping model reduces risk of
  misunderstanding user needs by giving user
  constant feedback (as prototypes) while system
  evolves
• relative risks vary by project, change through
  project life span
• so, develop software lifecycle model that
  explicitly recognizes risk as key driver

7
Spiral model
2. Evaluate product and process alternatives,
identify and resolve risks
1. Establish next-level objectives, constraints,
alternatives
5. Review progress, confirm commitment to continue
3. Develop, verify next-level product
4. Plan next phases
8
Example Spiral model project
9
Starting and stopping

• start with a hypothesis
• a particular user need can be met
  cost-effectively by the development of a software
  system
• stop when either
• system complete and fielded
• must test hypothesis by determining whether
  software system actually meets need
• hypothesis demonstrated to be false
• system determined to be too expensive,
  unnecessary alternative becomes available etc

10
Maintenance

• spiral model directly supports maintenance
• maintenance hypothesis
• a modification to this software system is a
  cost-effective way of meeting a particular user
  need

11
Key features

• Development of documentation and other work
  products non-uniform
• highest risk items addressed first, more heavily
  documented
• Incorporates prototyping as a risk-reduction
  activity
• Provides an integral framework for rework by
  allowing previous decisions to be revisited in a
  principled manner

12
WinWin Spiral Model
2. Identify stakeholders win conditions
3a. Reconcile win conditions
1. Identify next-level stakeholders
3b. Establish next-level objectives, constraints,
alternatives
7. Review and commit
4. Evaluate product and process alternatives,
identify and resolve risks
6. Validate product and process
5. Develop next level of product and process,
including partitions
13
Advantages

• flexibility
• accommodates a wide range of development
  approaches and can be used for software with a
  wide range of intrinsic characteristics
• focuses early attention on options involving the
  reuse of existing software
• accommodates preparation for lifecycle evolution,
  growth, and changes of the software product
• provides a mechanism for incorporating software
  quality objectives into software product
  development
• focuses on eliminating errors and unattractive
  alternatives early
• for each source of activity or expenditure,
  answers the question "How much is enough?"
• provides a unified approach for new development
  and maintenance
• provides a viable framework for integrated
  hardware-software system development.

14
Difficulties

• Contracting
• the Spiral model is difficult (but not
  impossible) to use in a contracting situation,
  both for the contractor and for the customer
• difficult to accommodate the required flexibility
  
• difficult to contract without specifying
  deliverables in advance
• Risk assessment expertise
• successful use of the model requires the
  effective identification and resolution of risks,
  something many managers and developers are poorly
  trained in
• Need for further elaboration
• as defined, the model is quite general and is
  difficult to effectively apply without
  significant expertise

15
Next ClassPlanning