Formal Methods in

Software Engineering

Adding Formal Methods to a Project

Lecture 32

1

Adding Formal Methods to a Project

• Remember using formal methods is not an all or nothing process

• The level of rigor used should be tailored to fit the specific project with respect to– system criticality level

– budget

– schedule

– technical environments

2

Best Use of Formal Methods

• New system components– adaptive or corrective maintenance

• Poorly understood requirements– perfective maintenance

• Highly critical system components– preventative maintenance

3

Management Considerations - part 1

• Project staff expertise– Formal Methods Expert (seeks to match applications

with appropriate methods, tools, and techniques)

– Project Domain Expert (evaluates candidate application and identifies the best to experiment with)

• Project scale– best to only try applying formal methods on 1 or 2

components the first time out

– can be viewed as a training exercise

– demonstrate value of formal methods with low risk4

Management Considerations - part 2• Project training

– use existing staff with formal methods expertise

– provide in-house, hands-on training with formal methods languages and support tools

– outside experts provide training and advice in early project stages

• Process integration strategy– few changes needed if requirements analysis procedure

are well-defined

– formal methods can complement existing process steps

5

Management Considerations - part 3

• Project guidelines and standards– writing formal specifications requires guidelines

similar to those found in existing • configuration management procedures

• coding style guidelines

• documentation standards

• Guidelines will have the greatest impact on the project if they are in place before the project or training begins

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Technical Considerations - part 1

• Type of application– applications with greater complexity will benefit more

from formal methods use than simple applications

– logic and discrete math applications benefit more than numerical applications

• Size of application– optimal code size is between 4K LOC and 25 KLOC

• Type of formal methods used– project objectives (better documentation or early defect

detection)7

Types of Analysis/Formal Methods

The preferred type of analysis and method is strongly influenced by the project objectives

L 4

Project Objectives for the Use of Formal Methods

Modest(e.g. formalspecifications fordocumentation)

Moderate(e.g. early defectdetection)

Ambitious(e.g. assure correctness of criticalproperties or algorithms)

Low Level of Rigor

Moderate Level of Rigor

High Level of Rigor

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Technical Considerations - part 2

• Level of rigor for formal methods– usually determined by the methods dependence on

automate tool support (higher rigor = more dependent)

• Scope of formal methods use affected by– number of system components

– degree of system functionality

– number of life cycle phases

9

Level of Rigor for Formal Methods

Increasing rigor is usually associated with increasing dependence on automated support tools

L 4

manualreviews,inspections

Levels of Rigor

low

high

medium

informal

fully formal specificationlanguagewith rigoroussemantics andproof checking

modeling using logicand discretemathematics

formal specificationlanguagewith type and syntax checking

require formal methods tools

10

Scope of Formal Methods UseThree dimensions of scope of formal method use

L 4

Number of Phasesin Life Cycle

Number of System Components

Degree ofSystem Functionality

selected component(s)

all components

most important function(s)

all functionssingle phase

all phases

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Technical Considerations - part 3

• Type of formal methods tools used– must take project objectives, level or rigor, and scope

into account when selecting a formal methods tool

– other important factors

• available training

• history of use

• ease of learning and ease of use

• effective support

• match with problem domain

12

Plan for Introducing Formal Methods on a Project - part 1

• Identify formal methods and domain expertise– expertise in both needed

• Define scale of formal methods involvement– trial, partial, or full project?

• Choose an application • Select suitable formal methods to be use

13

Plan for Introducing Formal Methods on a Project - part 2

• Select formal methods tools to use– consider application and available resources

• Implement formal methods training• Develop project guidelines

– analogous to those for conventional software engineering processes

• Track and document process changes– update and revise process using measurement-based

project feedback

14

Cost Considerations

• The act of formalizing specifications can be considered to be cost-effective if you consider the cost associated with fixing defects later in the software life cycle

• Proof checking may be less cost-effective, so choose the problem domain wisely

• The highest level of rigor should be reserved for mission critical and highly complex system components

15

Limitations of Formal Methods

• Formal methods are not a magic solution to all software development problems

• Need to use formal methods in suitable project environments if benefits are to exceed their costs

• Formal methods and domain expertise must be fully integrated to achieve positive results

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Benefits of Formal Methods

• Formal methods can help detect defects earlier in the software life cycle

• Formal methods can be applied with various levels of resource investment

• They can be integrated into existing process models with minimal disruption

• They can improve software quality

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Prerequisites to Using Formal Methods

• Need a reasonably mature, disciplined development environment

• Environment should emphasize quality (in fact quality ceilings may have been reached using traditional methods)

• Project staff must have adequate expertise, training, and support

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