Software Evolution1 Introduction to Software Evolution and Maintenance Software Evolution Chapter 1:...

Software Evolution 1

Introduction to Software Evolution and Maintenance

Software EvolutionChapter 1: History and Challenges of Software Evolution

Tom Mens

Software Engineering, 7th EditionChapter 21: Software Evolution

Ian Sommerville


Outline

Overview Software evolution processes The research landscape


Importance of evolution

Organizations have huge investments in their software systems - they are critical business assets.

To maintain the value of these assets to the business, they must be changed and updated.

The majority of the software budget in large companies is devoted to evolving existing software rather than developing new software.

Studies indicate that up to 75% of all software professionals are involved in some form of maintenance/evolution activity.


Software change

Software change is inevitable• New requirements emerge when the software is used;• The business environment changes;• Errors must be repaired;• New computers and equipment is added to the

system;• The performance or reliability of the system may have

to be improved. A key problem for organizations is implementing

and managing change to their existing software systems.


Repair software faults• Changing a system to correct deficiencies in the way meets

its requirements. Adapt software to a different operating environment

• Changing a system so that it operates in a different environment (computer, OS, etc.) from its initial implementation.

Add to or modify the system’s functionality• Modifying the system to satisfy new requirements.

Improve the program structure and system performance• Rewriting all or parts of the system to make it more efficient

and maintainable.

Types of changes


Software evolution and software maintenance

No standard definitions.

Broad definition of evolution: Generally, software evolution refers to the study and management of the process of making changes to software over time. • In this definition, software evolution comprises:

• Development activities• Maintenance activities• Reengineering activities

Narrow definition of evolution: Sometimes, software evolution is used to refer to the activity of adding new functionality to existing software.

Maintenance refers to the activity of modifying software after it has been put to use in order to maintain its usefulness.


Repair software faults• Changing a system to correct deficiencies in the way meets

its requirements. Adapt software to a different operating environment

• Changing a system so that it operates in a different environment (computer, OS, etc.) from its initial implementation.

Add to or modify the system’s functionality• Modifying the system to satisfy new requirements.

Improve the program structure and system performance• Rewriting all or parts of the system to make it more efficient

and maintainable.

Types of changes“M

aint

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“Evo

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n”

“Ree

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History

1960s – 1970s• Inclusion of maintenance in waterfall lifecycle after delivery of the software

product.• Perception that post-delivery activities only consisted of bug fixes and minor

adjustments.• Did not account for the need to add functionality due to new and changed

requirements. 1970s

• Lehman postulated the initial laws of program evolution.• Stressed the need for continuous evolution due to changes in the software’s

operational environment. Late 1970s – 1980s

• Initial process models that handled change requests. 1990s

• General acceptance of software evolution.• Development of new process models that accounted for evolution activities:

evolutionary development, spiral model, agile software development.


Outline



Evolution processes

Processes for evolving a software product depend on• The type of software being maintained;• The development processes used;• The skills and experience of the people involved.

Proposals for change are the drivers for system evolution. Change identification and evolution continue throughout the system lifetime.

Sommerville, Ch. 21


Change identification and evolution

Change proposalsNew system

Change identificationprocess

Software evolutionprocess

Sommerville, Ch. 21


The system evolution process

Releaseplanning

Changeimplementation

Systemrelease

Impactanalysis

Changerequests

Platformadaptation

Systemenhancement

Fault repair

Sommerville, Ch. 21


Change implementation

Requirementsupdating

Softwaredevelopment

Requirementsanalysis

Proposedchanges

Sommerville, Ch. 21


Legacy systems

For many systems, the software evolution process is not as straightforward as described.

• Associated models and documentation of the software may be missing or hopelessly outdated.

• The new requirements may not be anticipated by the design of the software, making the resulting changes difficult to implement correctly.

Legacy systems are old systems that have become significantly difficult to modify.

• Accumulation of changes have eroded the modularity of the original design.• The documentation has not been maintained and has become obsolete.• One or more pieces of its underlying technologies have become obsolete.

Two complementary techniques are employed to support the continued evolution of legacy systems:

• Reverse engineering.• Reengineering.


Obsolete system components

Hardware - may be obsolete mainframe hardware. Support software - may rely on support software from

suppliers who are no longer in business. Application software - may be written in obsolete

programming languages. Application data - often incomplete and inconsistent. Business processes - may be constrained by software

structure and functionality. Business policies and rules - may be implicit and

embedded in the system software.


Forward and reverse engineering and reengineering

“Forward Engineering is the traditional process of moving from high-level abstractions and logical, implementation-independent designs to the physical implementation of a system.”

“Reverse Engineering is the process of analyzing a subject system to identify the system’s components and their interrelationships and create representations of the system in another form or at a higher level of abstraction.”

“Reengineering ... is the examination and alteration of a subject system to reconstitute it in a new form and the subsequent implementation of the new form.”

— Chikofsky and Cross [R.S. Arnold, Software Reengineering, IEEE CS Press, 1993]


Reverse engineering

In many legacy systems, the only reliable information about the system is the source code.

Reverse engineering reconstructs requirements, design models, test cases and user documentation consistent with the current state of the source code.

Reverse engineering encompasses several activities: program comprehension, software visualization, static and dynamic slicing, etc.

Reverse engineering is often the initial activity in a reengineering project.


System reengineering

Rewriting parts or all of a legacy system to make it more evolvable, so that it can more easily accommodate future change requests.• Some authors [e.g., Sommerville] define it more strictly as the

process of restructuring legacy software without changing its functionality.

• Others include a forward engineering phase as part of reengineering.

Reengineering is applicable where some but not all sub-systems of a larger system require frequent maintenance.

Reengineering involves adding effort to make them easier to maintain. The system may be restructured and redocumented.

Sommerville, Ch. 21


Advantages of reengineering

Reduced risk• There is a high risk in new software development.

There may be development problems, staffing problems and specification problems.

Reduced cost• The cost of re-engineering is often significantly less

than the costs of developing new software.

Sommerville, Ch. 21


The reengineering process

Reverseengineering

Programdocumentation

Datare-engineering

Original data

Programstructure

improvement

Programmodularization

Structuredprogram

Re-engineereddata

Modularizedprogram

Originalprogram

Source codetranslation

Sommerville, Ch. 21


Reengineering process activities

Source code translation• Convert code to a new language.

Reverse engineering• Analyze the program to understand it;

Program structure improvement• Restructure automatically for understandability;

Program modularization• Reorganize the program structure;

Data reengineering• Clean-up and restructure system data.

Sommerville, Ch. 21


Outline



Research landscape

Two aspects of software evolution research Reverse engineering and reengineering

techniques Techniques for dealing with change Process and change management Evolution of software artifacts

Mens & Demeyer, Ch. 1


Two aspects of software evolution

“What and why”• Focuses on software evolution as a scientific discipline. • Studies the nature of the software evolution phenomenon to

understand its driving factors.• Key interests include the formulation and refinement of

fundamental theories and laws of software evolution. “How”

• Focuses on software evolution as an engineering discipline.• Studies how to support the daily tasks of the software developer

or project manager. • Key interests include the development and validation of tools

and techniques to guide, implement and control software evolution.



Techniques for dealing with change

Program comprehension• Understanding the existing program in order to change it.

Change impact analysis• Identification of the parts of the system that will be affected by a

proposed change. Change propagation

• Making sure that all affected parts are changed correctly. Restructuring/Refactoring

• Improving the software structure or architecture without changing the behavior.

Regression testing• Efficiently verifying that the change preserved the behavior of

functionalities that should not be impacted.



Management

Economics of software evolution• Developing economic models to support evolution-related

management decisions.• Comparing the cost of different strategies for changes.• Assessing the cost-benefits of investing in reengineering.

Software metrics• Measuring the quality of a change.• Measuring the degree of modularity.

Configuration management• Change management processes.• Management of multiple versions.• Merging versions together.• Release management.



Evolution of software artifacts

Requirements evolution• Managing requirements changes.

Architecture evolution• Reengineering the architectures of legacy systems.• Migration to distributed architectures, e.g., service-oriented

architectures.• Maintenance issues with modern architectures.

Design evolution• Evolution of design models.

Test case evolution• Adding and modifying test cases to verify that the system

behavior was changed as intended. Traceability management

• How to assure the consistency of the different artifacts.



Other evolution issues

Data evolution• Migrating to a new database schema.• Verifying that the information in the existing databases are

preserved. Runtime evolution

• How to modify a system without stopping it.• Encompasses runtime reconfiguration, dynamic adaptation,

dynamic upgrading. Language evolution

• Dealing with changes in the programming language definition.• Especially an issue in multi-language systems.• Designing languages to make them more robust to evolution.



Our research interests

Empirical studies of software evolution Mining software repositories Semantic change analysis

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