1. Software Reuse Main text:
Ian Sommerville, Software Engineering, 8th edition, chapter 18
Additional readings:
Prieto Diaz, "Status Report: Software Reusability," IEEE Software, 10(3):61-66, May, 1993.
Bruce H. Barnes, Terry B. Bollinger, "Making Reuse Cost- Effective", IEEE Software, vol. 8, no. 1, pp , January/February, 1991

2. Software reuse
In most engineering disciplines, systems are designed by composing existing components that have been used in other systems.
Software engineering has been more focused on original development but it is now recognised that to achieve better software, more quickly and at lower cost, we need to adopt a design process that is based on systematic software reuse.
Systematic software reuse vs ad-hoc reuse:
Human nature is inherently lazy => developers are happy to find ready-made solutions
Problem: ad-hoc (or copy-paste) reuse may be harmful !

3. Reuse Taxonomy – what, how, where to reuse ?
From [Prieto-Diaz91]

4. Reuse – by substance
Classification of reuse according to the essence of items to be reused:
Ideas, concepts:
Reuse of abstract concepts – sharing “wisedom”
General solutions to a class of problems: algorithms
Artifacts, components
Reuse of software parts
Processes, procedures, skills
Reusable processes – needs more formalization of software development procedures
Procedure reuse – means also the informal reuse of skills

5. Reuse – by scope
Classification of reuse according to the form and extent of reuse:
Vertical: reuse within the same domain or application area
Goal: to build generic models for families of systems that can be used as templates for building new system in that domain
Needs: effort in domain analysis and domain modelling
Horizontal: reuse of generic parts in different applications
Example: scientific subrutines
Needs: packaging and presentation of parts; cataloging and classification standards for parts;

6. Reuse – by technique
Classification of reuse according to the the approach used to implement reuse:
Compositional
Uses existing components as building blocks for new systems
Generative
Reuse at the specification level by means of application or code generators

7. Reuse – by product
Classification of reuse according to the work products that are reused:
Source code
Design
Specifications
Objects
Text
Architectures

8. What to reuse
Today’s agenda: (mainly based on chap. 18 from [Sommerville])
Software building blocks
Concept reuse

9. Reusing software building blocks
What size are the reused blocks?
Application system reuse
The whole of an application system may be reused:
by incorporating it without change into other systems. (COTS reuse - Detailed in [Sommerville ]
by reusing a common architecture used for developing applications from the same family, tailored for specific customers. (Software product lines - Detailed in [Sommerville ]
Component reuse
Components of an application from sub-systems to single objects may be reused. Detailed in [Sommerville Chapter 19]
Object and function reuse
Software components that implement a single well-defined object or function may be reused.
Who reuses ?
Third party reuse: COTS
Intra-organizational: Software product lines

10. Concept reuse
Concepts are abstract, the reuse process includes an instantiation activity
Design patterns. Detailed in [Sommerville 18.2]
Program generators. Detailed in [Sommerville 18.3]

11. Reuse benefits
Fig [Sommerville]

12. Reuse benefits (cont.)
Fig.18.1 [Sommerville]

13. Reuse problems
Fig from [Sommerville]

14. Reuse problems (cont.)
Fig [Sommerville]

15. The reuse landscape
Although reuse is often simply thought of as the reuse of system components, there are many different approaches to reuse that may be used.
Reuse is possible at a range of levels from simple functions to complete application systems.
The reuse landscape covers the range of possible reuse techniques.

16. The reuse landscape Fig. 18.3 from [Sommerville] Design patterns
Component-based
development
Component
frameworks
Service-oriented
systems C O TS
integ
ration
Application
product lines
Legacy system
wrapping
Prog
ram
libraries
generators
Aspect-oriented
software development
Configurable ver
tical
applications
Fig from [Sommerville]

17. Reuse approaches
Fig from [Sommerville]

18. Reuse approaches (cont)

19. Reuse planning factors
Key question: given this array of techniques for reuse, which one (if any) to choose ?
The key factors to be considered when planning reuse:
The development schedule for the software.
The expected software lifetime.
The background, skills and experience of the development team.
The criticality of the software and its non-functional requirements.
The application domain.
The execution platform for the software.
Conclusion: whether reuse is achieved or not is often a managerial rather than technical issue

20. Reuse Investment Cost . . . Estimated cost w/o reuse
Cost w/ reuse
Cost w/ reuse
Reuse investments
$ Savings
$ Savings
Net profit
Net loss
Reuse producer activities
Reuse consumer activities
From [Barnes&Bollinger91]

21. Reusing parts - Size of reuse units
Possible reuse units:
Objects: often too fine-grained and too specialised for a particular application
Application frameworks: larger-grain abstractions
Application frameworks:
Frameworks are a sub-system design made up of a collection of abstract and concrete classes and the interfaces between them.
The sub-system is implemented by adding components to fill in parts of the design and by instantiating the abstract classes in the framework.
Frameworks are moderately large entities that can be reused.

22. Framework classes System infrastructure frameworks
Support the development of system infrastructures such as communications, user interfaces and compilers.
Middleware integration frameworks
Standards and classes that support component communication and information exchange.
Enterprise application frameworks
Support the development of specific types of application such as telecommunications or financial systems.

23. Extending frameworks
Frameworks are generic and are extended to create a more specific application or sub-system.
Extending the framework involves
Adding concrete classes that inherit operations from abstract classes in the framework;
Adding methods that are called in response to events that are recognised by the framework.
Problem with frameworks is their complexity which means that it takes a long time to use them effectively.

24. Example: Model-view controller
System infrastructure framework for GUI design.
Allows for multiple presentations of an object and separate interactions with these presentations.
MVC framework involves the instantiation of a number of patterns (as discussed earlier under concept reuse).

25. Example: Model-view-controller
Fig from [Sommerville]

26. Application system reuse
Involves the reuse of entire application systems either by configuring a system for an environment or by integrating two or more systems to create a new application.
Reuses large-grain assets
Two approaches covered here:
COTS product integration;
Product line development.

27. COTS product reuse
COTS - Commercial Off-The-Shelf systems – a software system that can be used without change by its buyer
Examples of COTS: database management systems, web browser, er
COTS systems are usually complete application systems that offer an API (Application Programming Interface).
Building large systems by integrating COTS systems is now a viable development strategy for some types of system such as E-commerce systems.
The key benefit is faster application development and, usually, lower development costs.

28. COTS design choices
Which COTS products offer the most appropriate functionality?
There may be several similar products that may be used.
How will data be exchanged?
Individual products use their own data structures and formats.
Writing adapters that convert from one representation to another
What features of the product will actually be used?
Most products have more functionality than is needed. You should try to deny access to unused functionality.

29. COTS Example: E-procurement system
Client W
eb browser
system
Server
E-commerce
system
Ordering and
invoicing system
Adaptor
Convert formats of orders
Convert notifications into s
Fig from [Sommerville]

30. COTS products reused
On the client, standard and web browsing programs are used.
On the server, an e-commerce platform has to be integrated with an existing ordering system.
This involves writing an adaptor so that they can exchange data.
An system is also integrated to generate for clients. This also requires an adaptor to receive data from the ordering and invoicing system.

31. COTS system integration problems
Lack of control over functionality and performance
COTS systems may be less effective than they appear
Problems with COTS system inter-operability
Different COTS systems may make different assumptions that means integration is difficult
No control over system evolution
COTS vendors not system users control evolution
Support from COTS vendors
COTS vendors may not offer support over the lifetime of the product

32. Software product lines
Software product lines or application families are applications with generic functionality that can be adapted and configured for use in a specific context.
All members of the family share a common core:
The application-specific architecture
A set of core components
Adaptation (creation of individual family members) may involve:
Component and system configuration;
Adding new components to the system;
Selecting from a library of existing components;
Modifying components to meet new requirements.

33. Specialisation in a product line
Platform specialisation
Different versions of the application are developed for different platforms.
Environment specialisation
Different versions of the application are created to handle different operating environments e.g. different types of communication equipment.
Functional specialisation
Different versions of the application are created for customers with different requirements.
Process specialisation
Different versions of the application are created to support different business processes.

34. Product line configuration
Software product lines are designed to be reconfigured.
Reconfiguration:
Adding or removing components
Defining parameters and constraints for components
Including knowledge of business processes
Software product lines can be configured at 2 points in the development process:
Deployment time configuration
A generic system is configured by embedding knowledge of the customer’s requirements and business processes. The software itself is not changed.
Design time configuration
A common generic code is adapted and changed according to the requirements of particular customers.

35. Example of deployment-time configuration: Configuration of an ERP system
Fig from [Sommerville]

36. Example: ERP systems
An Enterprise Resource Planning (ERP) system is a generic system that supports common business processes such as ordering and invoicing, manufacturing, etc.
These are very widely used in large companies - they represent probably the most common form of software reuse.
The generic core is adapted by including modules and by incorporating knowledge of business processes and rules.

37. Design time configuration
Software product lines that are configured at design time are instantiations of generic application architectures (see Chapter 13).
Involves changing and extending the source code of the core system
Offers greater flexibility than deployment-time configuration
Generic products usually emerge after experience with specific products.

38. Product line architectures
The decision to design a generic product line becomes explicit at a point, otherwise changes may corrupt the application structure
Architectures must be structured in such a way to separate different sub-systems and to allow them to be modified.
The architecture should also separate entities and their descriptions and the higher levels in the system access entities through descriptions rather than directly.

39. Product line: Vehicle despatching
A specialised resource management system where the aim is to allocate resources (vehicles) to handle incidents.
Versions for: police, fire service, ambulance service
Adaptations include:
At the UI level, there are components for operator display and communications;
At the I/O management level, there are components that handle authentication, reporting and route planning;
At the resource management level, there are components for vehicle location and despatch, managing vehicle status and incident logging;
The database includes equipment, vehicle and map databases.

40. A despatching system
Fig from [Sommerville]

41. Product instance development
Fig from [Sommerville]

42. Product instance development
Elicit stakeholder requirements
Use existing family member as a prototype
Choose closest-fit family member
Find the family member that best meets the requirements
Re-negotiate requirements
Adapt requirements as necessary to capabilities of the software
Adapt existing system
Develop new modules and make changes for family member
Deliver new family member
Document key features for further member development

43. Concept reuse
When you reuse program or design components, you have to follow the detailed design decisions made by the original developer of the component.
This may limit the opportunities for reuse.
One way around is to reuse abstract designs (concepts) that do not include implementation details
You must implement the abstract design for your application
Examples of reusable concepts: algorithms, abstract data types, design patterns
Main approaches to concept reuse are:
Design patterns
Generative programming

44. Design patterns
A design pattern is a way of reusing abstract knowledge about a problem and its solution.
A pattern is a description of the problem and the essence of its solution.
It should be sufficiently abstract to be reused in different settings.
Patterns often rely on object characteristics such as inheritance and polymorphism.

45. Pattern elements Identification of a pattern [GoF]:
Name - A meaningful pattern identifier.
Problem description – when the pattern can be applied
Solution description - Not a concrete design but a template for a design solution that can be instantiated in different ways.
Consequences - The results and trade-offs of applying the pattern.

46. Example: The Observer pattern
Name
Observer.
Description
Separates the display of object state from the object itself.
Problem description
Used when multiple displays of state are needed.
Solution description
See slide with UML description.
Consequences
Optimisations to enhance display performance are impractical.

47. Example: Multiple displays
Fig from [Sommerville]

48. Example: The Observer pattern
Fig from [Sommerville]

49. Reuse through patterns - conclusion
The use of patterns is an effective form of reuse
Disadvantage: only experienced software engineers who have a deep knowledge of patterns can use them effectively
Experience needed to recognise generic situations where a pattern can be applied

50. Generator-based reuse
Concept reuse through patterns:
relies on describing the concept in an abstract way and leaving it up to the software developer to create an implementation
Generator-based reuse:
Program generators involve the reuse of standard patterns and algorithms.
These are embedded in the generator and parameterised by user commands. A program is then automatically generated.
Generator-based reuse is possible when domain abstractions and their mapping to executable code can be identified.
A domain specific language is used to compose and control these abstractions.

51. Reuse through program generation
Fig from [Sommerville]

52. Types of program generators
Generator-based reuse takes advantage of the fact that applications in the same domain have common architectures and carry out similar functions
Areas of use for program generator
Application generators for business data processing
Systems follow an input-process-output model
Usually include operations as data verification and report generation – generic components
Parser and lexical analyser generators for language processing (examples: lex, yacc);
input=the grammar
output=the parser program
Code generators in CASE tools;
input=design diagrams
output=program implementing the design

53. Generator-based reuse
Disadvantages:
Generator-based reuse has a high initial cost in defining and implementing the domain concepts and language
its applicability is limited to a relatively small number of application domains.
Advantages:
It is easier for end-users to develop programs using generators compared to other component-based approaches to reuse.
Future:
Generative programming
K. Czarnecki. Overview of Generative Software Development. In J.-P. Banâtre et al. (Eds.): Unconventional Programming Paradigms (UPP), LNCS 3566, pp. 313–328,
Czarnecki and Eisenecker, Components and generative programming. ESEC/FSE ilmenau.de/~czarn/esec99/esec99.pdf
Aspect-oriented software development
Kiczales et al, An Overview of AspectJ, ECOOP
[Sommerville], chap. 32

54. Generative programming
Combines program generation with component-based development
Applicable for System-family engineering (also known as software product-line engineering)
exploits the commonalities among systems from a given problem domain while managing the variabilities among them in a systematic way
Generative software development is a system-family approach, which focuses on automating the creation of system-family members: a given system can be automatically generated from a specification written in one or more textual or graphical domain-specific languages

55. Spectrum of generative approaches
from [Czarnecki]

56. Aspect-oriented development
Aspect-oriented development addresses a major software engineering problem - the separation of concerns.
Concerns are often not simply associated with application functionality but are cross-cutting - e.g. all components may monitor their own operation, all components may have to maintain security, etc.
Cross-cutting concerns are implemented as aspects and are dynamically woven into a program. The concern code is reuse and the new system is generated by the aspect weaver.

57. Aspect-oriented development
Fig from [Sommerville]

58. A simple AspectJ example
Example 2-1. A simple business logic Java class
package com.oreilly.aspectjcookbook;
public class MyClass {
public void foo(int number, String name)
System.out.println("Inside foo (int, String)"); }
public static void main(String[] args)
// Create an instance of MyClass
MyClass myObject = new MyClass( );
// Make the call to foo
myObject.foo(1, "Russ Miles");
Example 2-2. A simple HelloWorld aspect in AspectJ
package com.oreilly.aspectjcookbook;
public aspect HelloWorld {
pointcut callPointcut( ) :
call(void com.oreilly.aspectjcookbook.MyClass.foo(
int, String));
before() : callPointcut( )
System.out.println(
"Hello World");
"In the advice attached to the call pointcut"); }
HelloWorld defines an aspect that will be applied to the class MyClass.
The aspect captures all calls to the void foo(int, String) method in the MyClass class.
The ajc compiler will produce two .class files; MyClass.class and HelloWorld.class.

59. Key points
Advantages of reuse are lower costs, faster software development and lower risks.
Design patterns are high-level abstractions that document successful design solutions.
Program generators are also concerned with software reuse - the reusable concepts are embedded in a generator system.
Application frameworks are collections of concrete and abstract objects that are designed for reuse through specialisation.
COTS product reuse is concerned with the reuse of large, off-the- shelf systems.
Problems with COTS reuse include lack of control over functionality, performance, and evolution and problems with inter-operation.
Software product lines are related applications developed around a common core of shared functionality.