CS 123 Lecture 02 2023-2024.pdf take it s

CS 123: INTRODUCTIONTO
SOFTWARE ENGINEERING
1
LECTURE 02
SOFTWARE PROCESS MODELS

Topics covered
❖Software process models
❖Process activities
❖Coping with change
❖Process improvement
2

The software process
❖Software development process also known as a Software
development life cycle (SDLC)
✓Software process: A structured set of activities
required to develop a software system.
✓Software lifecycle: Set of activities and their
relationships to each other to support the development
of a software system
❖Software Engineering Institute (SEI) define software
process as A set of activities, methods, practices, and
transformations that people use to develop and maintain
software and the associated products (e.g., project plans,
design documents, code, test cases, and user manuals)
3

The software process
❖Many different software processes but all involve:
✓Software Specification: defining what the system should
do;
✓Software Development (Design and implementation):
defining the organization of the system and implementing
the system;
✓Software Validation: checking that it does what the
customer wants;
✓Software Evolution: changing the system in response to
changing customer needs.
❖A software process model is an abstract representation
of a process. It presents a description of a process from
some particular perspective.
4

Plan-driven and Agile processes
❖Plan-driven processes are processes where all of the
process activities are planned in advance and progress
is measured against this plan.
❖In agile processes, planning is incremental and it is
easier to change the process to reflect changing
customer requirements.
❖In practice, most practical processes include elements
of both plan-driven and agile approaches.
❖There are no right or wrong software processes.
5

Software process models
❖The waterfall model: Plan-driven model. Separate and
distinct phases of specification and development.
❖Incremental development: Specification, development
and validation are interleaved. May be plan-driven or agile.
❖Integration and configuration: The system is assembled
from existing configurable components. May be plan-
driven or agile.
❖In practice, most large systems are developed using a
process that incorporates elements from all of these
models.
7

Waterfall model phases
❖There are separate identified phases in the waterfall
model:
1. Requirements analysis and definition
2. System and software design
3. Implementation and unit testing
4. Integration and system testing
5. Operation and maintenance
❖The main drawback of the waterfall model is the
difficulty of accommodating change after the process is
underway. In principle, a phase has to be complete
before moving onto the next phase.
9

Waterfall model problems
❖Inflexible partitioning of the project into
distinct stages makes it difficult to respond to
changing customer requirements.
✓Therefore, this model is only appropriate in
small project or when the requirements are
well-understood and changes will be fairly
limited during the design process.
▪ Few business systems have stable
requirements.
10

(2) Incremental development
11

Incremental development benefits
❖ The cost of accommodating changing customer requirements is
reduced.
✓The amount of analysis and documentation that has to be
redone is much less than is required with the waterfall model.
❖ It is easier to get customer feedback on the development work
that has been done.
✓Customers can comment on demonstrations of the software and
see how much has been implemented.
❖ More rapid delivery and deployment of useful software to the
customer is possible.
✓Customers are able to use and gain value from the software
earlier than is possible with a waterfall process.
12

Incremental development problems
❖The process is not visible.
✓Managers need regular deliverables to measure
progress. If systems are developed quickly, it is not
cost-effective to produce documents that reflect every
version of the system.
❖System structure tends to degrade as new increments are
added.
✓Unless time and money is spent on refactoring to
improve the software, regular change tends to corrupt
its structure. Incorporating further software changes
becomes increasingly difficult and costly.
13

(3) Integration and configuration
❖Based on software reuse where systems are
integrated from existing components or
application systems (sometimes called COTS -
Commercial-Off-The-Shelf Systems).
❖Reused elements may be configured to adapt
their behaviour and functionality to a user’s
requirements
❖Reuse is now the standard approach for building
many types of business system
14

Reuse-oriented software engineering
15
Key process stages
❖Requirements specification
❖Software discovery and evaluation
❖Requirements refinement
❖Application system configuration
❖Component adaptation and integration

Advantages and disadvantages
❖Reduced costs and risks as less software is developed
from scratch
❖Faster delivery and deployment of system
❖But requirements compromises are inevitable so system
may not meet real needs of users
❖Loss of control over evolution of reused system
elements
16

Process activities
❖Real software processes are inter-leaved sequences of
technical, collaborative and managerial activities with
the overall goal of specifying, designing, implementing
and testing a software system.
❖The four basic process activities of specification,
development, validation and evolution are organized
differently in different development processes.
❖For example, in the waterfall model, they are organized
in sequence, whereas in incremental development they
are interleaved.
18

(1) Software specification
❖The process of establishing what services are required
and the constraints on the system’s operation and
development.
❖Requirements engineering process
✓Requirements elicitation and analysis
▪ What do the system stakeholders require or expect
from the system?
✓Requirements specification
▪ Defining the requirements in detail
✓Requirements validation
▪ Checking the validity of the requirements
19

The requirements engineering process
20

(2) Software design and implementation
(Development)
❖The process of converting the system specification
into an executable system.
❖Software design: Design a software structure that
realises the specification;
❖Implementation: Translate this structure into an
executable program;
❖The activities of design and implementation are
closely related and may be inter-leaved.
21

A general model of the design process
22

Design activities
❖Architectural design, where you identify the overall structure
of the system, the principal components (subsystems or
modules), their relationships and how they are distributed.
❖Database design, where you design the system data structures
and how these are to be represented in a database.
❖Interface design, where you define the interfaces between
system components.
❖Component selection and design, where you search for
reusable components. If unavailable, you design how it will
operate.
23

System implementation
❖The software is implemented either by developing a
program(s) or by configuring an application system.
❖Design and implementation are interleaved activities
for most types of software system.
❖Programming is an individual activity with no standard
process.
❖Debugging is the activity of finding program faults and
correcting these faults.
24

(3) Software validation
❖ Verification and validation (V & V) is intended to show that a
system conforms to its specification and meets the requirements of
the system customer.
❖ Verification: The process of evaluating software to determine
whether the product of a given development phase conforms to
specifications.
✓Correctness of the system with respect to the requirement
✓Did I build what I need?
❖ Validation: The process of evaluating software during or at the end
of the development process to determine whether the software
meets the customer expectations and requirements.
✓Consistency of the requirement with user needs/wants
✓Did I build what I said I would?
25

(3) Software validation
❖Involves checking and review processes and
system testing.
❖System testing involves executing the system
with test cases that are derived from the
specification of the real data to be processed by
the system.
❖Testing is the most commonly used V & V
activity.
26

Testing stages
❖ Component/ Unit Testing
✓Individual components are tested independently;
✓Components may be functions or objects or coherent
groupings of these entities.
❖ System/ System Integration Testing
✓Testing of the system as a whole. Testing of emergent
properties is particularly important.
❖ Customer/ Acceptance Testing
✓Testing with customer data to check that the system meets the
customer’s needs.
27

Testing phases in a plan-driven
software process (V-model)
28

(4) Software evolution
❖ Software is inherently flexible and can change.
❖ As requirements change through changing business
circumstances, the software that supports the business must also
evolve and change.
❖ Although there has been a demarcation between development and
evolution (maintenance) this is increasingly irrelevant as fewer
and fewer systems are completely new.
29
System evolution

Coping with change
❖Change is inevitable in all large software projects.
✓Business changes lead to new and changed system
requirements
✓New technologies open up new possibilities for
improving implementations
✓Changing platforms require application changes
❖Change leads to rework so the costs of change
include both rework (e.g. re-analysing requirements)
as well as the costs of implementing new
functionality
31

Reducing the costs of rework (1/2)
1. Change anticipation, where the software process includes
activities that can anticipate possible changes before
significant rework is required.
✓For example, a prototype system may be developed to
show some key features of the system to customers.
32
System prototyping:- where a version of the
system or part of the system is developed quickly
to check the customer’s requirements and the
feasibility of design decisions. This approach
supports change anticipation.

Reducing the costs of rework (2/2)
2. Change tolerance, where the process is designed so that
changes can be accommodated at relatively low cost.
✓This normally involves some form of incremental
development. Proposed changes may be implemented in
increments that have not yet been developed.
✓If this is impossible, then only a single increment (a small
part of the system) may have be altered to incorporate the
change.
33
Incremental delivery:- where system increments
are delivered to the customer for comment and
experimentation. This supports both change
avoidance and change tolerance.

Software prototyping
❖ A prototype is an initial version of a system used to demonstrate
concepts and try out design options.
❖ A prototype can be used in the following scenarios:
1. The requirements engineering process to help with
requirements elicitation and validation;
2. In design processes to explore options and develop a UI
design;
3. In the testing process to run back-to-back tests.
❖ Benefits of Software prototyping
✓ Improved system usability.
✓ A closer match to users’ real needs.
✓ Improved design quality.
✓ Improved maintainability.
✓ Reduced development effort.
34

Prototype Development
❖ May be based on rapid prototyping languages or tools
❖ May involve leaving out functionality
✓Prototype should focus on areas of the product that are not
well-understood;
✓Error checking and recovery may not be included in the
prototype;
✓Focus on functional rather than non-functional requirements
such as reliability and security
35

Throw-away prototypes
❖Prototypes should be discarded after development as
they are not a good basis for a production system:
✓It may be impossible to tune the system to meet
non-functional requirements;
✓Prototypes are normally undocumented;
✓The prototype structure is usually degraded through
rapid change;
✓The prototype probably will not meet normal
organizational quality standards.
36

Incremental delivery
❖Rather than deliver the system as a single delivery, the
development and delivery is broken down into
increments with each increment delivering part of the
required functionality.
❖User requirements are prioritised and the highest
priority requirements are included in early increments.
❖Once the development of an increment is started, the
requirements are frozen though requirements for later
increments can continue to evolve.
37

Incremental development and delivery
❖ Incremental development
✓Develop the system in increments and evaluate each
increment before proceeding to the development of the next
increment;
✓Normal approach used in agile methods;
✓Evaluation done by user/customer proxy.
❖ Incremental delivery
✓Deploy an increment for use by end-users;
✓More realistic evaluation about practical use of software;
✓Difficult to implement for replacement systems as increments
have less functionality than the system being replaced.
38

Incremental delivery advantages
❖Customer value can be delivered with each increment
so system functionality is available earlier.
❖Early increments act as a prototype to help elicit
requirements for later increments.
❖Lower risk of overall project failure.
❖The highest priority system services tend to receive
the most testing.
40

Incremental delivery problems
❖Most systems require a set of basic facilities that are used
by different parts of the system.
✓As requirements are not defined in detail until an
increment is to be implemented, it can be hard to
identify common facilities that are needed by all
increments.
❖The essence of iterative processes is that the specification
is developed in conjunction with the software.
✓However, this conflicts with the procurement model of
many organizations, where the complete system
specification is part of the system development contract.
41

Process improvement
❖Many software companies have turned to software
process improvement as a way of enhancing the
quality of their software, reducing costs or
accelerating their development processes.
❖Process improvement means understanding existing
processes and changing these processes to increase
product quality and/or reduce costs and
development time.
43

Approaches to improvement
❖The process maturity approach, Focuses on improving
process and project management and introducing good
software engineering practice.
✓The level of process maturity reflects the extent to which
good technical and management practice has been adopted
in organizational software development processes.
❖The agile approach, Focuses on iterative development and
the reduction of overheads in the software process.
✓The primary characteristics of agile methods are rapid
delivery of functionality and responsiveness to changing
customer requirements.
44

The process improvement cycle
45
❖ Process measurement
✓ You measure one or more attributes of the
software process or product. These
measurements forms a baseline that helps you
decide if process improvements have been
effective.
❖ Process analysis
✓ The current process is assessed, and process
weaknesses and bottlenecks are identified.
Process models (sometimes called process maps)
that describe the process may be developed.
❖ Process change
✓ Process changes are proposed to address some of
the identified process weaknesses. These are
introduced and the cycle resumes to collect data
about the effectiveness of the changes.

Process measurement
❖Wherever possible, quantitative process data should be
collected
✓However, where organisations do not have clearly
defined process standards this is very difficult as you
don’t know what to measure. A process may have to be
defined before any measurement is possible.
❖Process measurements should be used to assess process
improvements
✓But this does not mean that measurements should drive
the improvements. The improvement driver should be the
organizational objectives.
46

Process metrics
❖Time taken for process activities to be completed
✓E.g. Calendar time or effort to complete an activity
or process.
❖Resources required for processes or activities
✓E.g. Total effort in person-days.
❖Number of occurrences of a particular event
✓E.g. Number of defects discovered.
47

Capability maturity levels
48
❖ Initial: Essentially uncontrolled
❖ Repeatable: Product management
procedures defined and used
❖ Defined: Process management procedures
and strategies defined and used
❖ Managed: Quality management strategies
defined and used
❖ Optimising: Process improvement
strategies defined and used
Software Engineering
Institute (SEI) Capability
Maturity Model (CMM)
The SEI process maturity
framework identifies
maturity levels that
essentially correspond to
the use of good software
engineering practice.

Capability maturity levels
49
❖CMM is about how an IT department works on all
projects.
❖An IT department that is immature, spends a lot of time
dealing with issues it was not able to predict nor control.
❖A mature IT department is aware of risks, various ways
to do projects, what needs to be done to ensure quality,
etc.
❖The mature IT department makes certain that work is
done in such a way that it’s processes increase the
likelihood of excellent products.

Key points
❖Software processes are the activities involved in
producing a software system. Software process models
are abstract representations of these processes.
❖General process models describe the organization of
software processes.
✓Examples of these general models include the
‘waterfall’ model, incremental development, and
reuse-oriented development.
❖Requirements engineering is the process of developing a
software specification.
50

Key points
❖Design and implementation processes are concerned
with transforming a requirements specification into an
executable software system.
❖Software validation is the process of checking that the
system conforms to its specification and that it meets
the real needs of the users of the system.
❖Software evolution takes place when you change
existing software systems to meet new requirements.
The software must evolve to remain useful.
❖Processes should include activities such as prototyping
and incremental delivery to cope with change.
51

Key points
❖Processes may be structured for iterative development
and delivery so that changes may be made without
disrupting the system as a whole.
❖ The principal approaches to process improvement are
agile approaches, geared to reducing process overheads,
and maturity-based approaches based on better process
management and the use of good software engineering
practice.
❖The SEI process maturity framework identifies maturity
levels that essentially correspond to the use of good
software engineering practice.
52

53
THANK YOU
END
CS 123: LECTURE 02

CS 123 Lecture 02 2023-2024.pdf take it s

More Related Content

Similar to CS 123 Lecture 02 2023-2024.pdf take it s (20)

Recently uploaded (20)

CS 123 Lecture 02 2023-2024.pdf take it s