Download - Introduction and life cycle models
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What is S/W Engineering?
It discusses systematic and cost effective techniques to software development.
The techniques are developed from 1.New innovations2.Past mistakes so that they are not
repeated in future while developing similar software.
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Program versus S/W product
Programs are small in size with limited functionality. It is for individual’s personal use. It doesn’t have good user interface and proper documentation.
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A software product has large no. of users, properly designed, good user-interface, proper user manuals or user guides. They are developed by a group of engineers in a team.
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Software Life Cycle Models(SDLC)
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A software life cycle model is a descriptive and diagrammatic representation of the software life cycle.
It maps the different activities performed while developing a s/w.
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Classical Waterfall Model
Classical waterfall model divides life cycle into phases: feasibility study, requirements analysis and
specification, design, coding and unit testing, integration and system testing, maintenance.
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Classical Waterfall Model
Feasibility Study
Req. Analysis
Design
Coding
Testing
Maintenance
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Feasibility Study
Main aim of feasibility study:determine whether developing the product financially worthwhile
technically feasible. First roughly understand what the
customer wants: different data which would be input to the
system, processing needed on these data, output data to be produced by the system, various constraints on the behavior of the
system.
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Activities during Feasibility Study Work out an overall
understanding of the problem. Formulate different solution
strategies. Examine alternate solution
strategies in terms of: resources required, cost of development, and development time.
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Activities during Feasibility Study Perform a cost/benefit analysis:
to determine which solution is the best.
you may determine that none of the solutions is feasible due to: high cost, resource constraints, technical reasons.
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Requirements Analysis and Specification Aim of this phase:
understand the exact requirements of the customer,
document them properly. Consists of two distinct
activities: requirements gathering and
analysis requirements specification.
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Goals of Requirements Analysis
Collect all related data from the customer: analyze the collected data to
clearly understand what the customer wants,
find out any inconsistencies and incompleteness in the requirements,
resolve all inconsistencies and incompleteness.
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Requirements Gathering
Gathering relevant data: usually collected from the
end-users through interviews and discussions.
For example, for a business accounting software: Interview/discuss with all the accountants of the organization to find out their requirements.
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Requirements Analysis (CONT.)
The data you initially collect from the users: would usually contain several contradictions and ambiguities:
each user typically has only a partial and incomplete view of the system.
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Requirements Analysis (CONT.)
Ambiguities and contradictions: must be identified resolved by discussions with the
customers.
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Requirements specification
Next, requirements are organized: into a Software Requirements
Specification (SRS) document.
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Requirements specification(CONT.)
Engineers doing requirements analysis and specification: are designated as
analysts.
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Design
Design phase transforms requirements specification: into a form suitable for implementation in some programming language.
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Design In technical terms:
during design phase, software architecture is derived from the SRS document.
Two design approaches: traditional approach, object oriented approach.
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Traditional Design Approach
Identify all the functions to be performed.
Identify data flow among the functions.
Decompose each function recursively into sub-functions. Identify data flow among the
subfunctions as well.
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Structured Analysis (CONT.)
Carried out using Data flow diagrams (DFDs).
After structured analysis, carry out structured design: architectural design (or high-
level design) detailed design (or low-level
design).
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Object Oriented Design
First identify various objects (real world entities) occurring in the problem: identify the relationships among the
objects. For example, the objects in a pay-roll
software may be: employees, managers, pay-roll register, Departments, etc.
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coding and unit testing Purpose of coding and unit testing phase: translate software design into source code.
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coding and unit testing During the implementation
phase: each module of the design is
coded, each module is unit tested
tested independently as a stand alone unit, and debugged,
each module is documented.
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Integration and System Testing
Different modules are integrated in a planned manner: modules are almost never
integrated in one shot. Normally integration is carried out
through a number of steps.
During each integration step, the partially integrated system is
tested.
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Integration and System Testing
M1
M4M3
M2
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System Testing
After all the modules have been successfully integrated and tested: system testing is carried out.
Goal of system testing: ensure that the developed system functions according to its requirements as specified in the SRS document.
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Types of testing
Alpha-testing- performed by the development team.
Beta-testing- performed by friendly set of customers
Acceptance testing- performed by the customer himself after the product is delivered to him/her.
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Maintenance
Maintenance of any software product: requires much more effort than the effort to develop the product itself.
development effort to maintenance effort is typically 40:60.
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Relative Effort for Phases
Phases between feasibility study and testing known as development
phases.
Among all life cycle Among all life cycle phasesphases maintenance phase maintenance phase
consumes maximum consumes maximum effort. Maintenance for effort. Maintenance for long lasting softwares long lasting softwares like OS requires lots of like OS requires lots of time. time.
0
10
20
30
40
50
60
Req
. S
p
Desig
n
Cod
ing
Test
Main
tnce
Relative Effort
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For some software like certain business For some software like certain business application and the business itself gets obsolete. In application and the business itself gets obsolete. In that case, proportion of maintenance effort will be that case, proportion of maintenance effort will be low.low.
Among development phases, testing phase consumes the maximum effort.
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Maintenance (CONT.)
Corrective maintenance: Correct errors which were not discovered
during the product development phases. Perfective maintenance:
Improve implementation of the system as suggested by customer ex: GUI
enhance functionalities of the system. Adaptive maintenance:
Port software to a new environment, e.g. to a new computer or to a new operating
system.
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Shortcomings of classical waterfall model
It assumes that no defect is introduced during any development activity but in practice defects do get introduced in almost every phase of the life cycle.
Developer need to go back to the phase where defect had occurred and redo some work and also in later phases.
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Classical Waterfall Model
Feasibility Study
Req. Analysis
Design
Coding
Testing
Maintenance
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Cntd…
It assumes that all the requirements are defined in the beginning of the project, but customer requirements keep on changing.
All the phases might not be sequential i.e., two phases might overlap. Example design and testing phase might overlap after requirements have been specified.
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Iterative Waterfall Model (CONT.)
Once a defect is detected: we need to go back to the phase
where it was introduced redo some of the work done during
that and all subsequent phases.
Therefore we need feedback paths in the classical waterfall model.
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Iterative Waterfall Model (CONT.)
Feasibility study
Req. Analysis
Design
Coding
Testing
Maintenance
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Iterative Waterfall Model (CONT.)
There is no feedback path to the feasibility stage as feasibility study errors cant be corrected.
Errors should be detected in the same phase in which they are introduced.
For example: if a design problem is detected in the
design phase itself, the problem can be taken care of much more easily than if it is identified at the end of the integration and system testing phase.
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Phase containment of errors
Reason: rework must be carried out not only to the design but also to code and test phases.
The principle of detecting errors as close to its point of introduction/occurrence as possible is known as phase containment of errors.
Iterative waterfall model is by far the most widely used model. Almost every other model is derived
from the waterfall model.
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Shortcomings of I.W.M.
Cannot handle satisfactorily various risks that a real life project suffer from. Example: it assumes that all the requirements are specified completely before development starts but it is not so every time.
Rigid phase sequence may lead to resource or man power wastage.
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Prototyping Model
Before starting actual development, a working prototype of the system
should first be built. A prototype is a toy implementation
of a system: limited functional capabilities, low reliability, inefficient performance.
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Reasons for developing a prototype
Illustrate to the customer: input data formats, messages, or
reports.
It helps in gaining better understanding of customers requirement.
Especially useful in developing the GUI part of the system. It becomes easier with working model rather than just imagining.
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For example, we are developing compiler for a command language and none of the team member have written a compiler before. In that case, writing a compiler program for a very small language first helps us to understand the issues associated with developing a compiler.
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Examine technical issues associated with product development: Often major design decisions depend
on issues like: response time of a hardware
controller, efficiency of a sorting algorithm, etc.
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Prototyping Model (CONT.)
The third reason for developing a prototype is: it is impossible to ``get it right''
the first time, we must plan to throw away
the first product if we want to develop a good product.
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Prototyping Model (CONT.)
RequirementsGathering Quick Design
Refine Requirements
Build Prototype
Customer Evaluation of Prototype Design
Implement
Test
Maintainance
Customer
acceptance
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Prototyping Model (CONT.)
Start with approximate requirements.
Carry out a quick design. Prototype model is built using
several short-cuts: Short-cuts might involve using inefficient,
inaccurate, or dummy functions. A function may use a table look-up rather than
performing the actual computations.
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Prototyping Model (CONT.)
The developed prototype is submitted to the customer for his evaluation: Based on the user feedback, requirements
are refined. This cycle continues until the user approves
the prototype. The actual system is developed using
the iterative waterfall approach.
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Prototyping Model (CONT.)
Requirements analysis and specification phase becomes redundant: final working prototype (with all user
feedbacks incorporated) serves as an animated requirements specification.
Design and code for the prototype is usually thrown away: However, the experience gathered from
developing the prototype helps a great deal while developing the actual product.
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Prototyping Model (CONT.)
Even though construction of a working prototype model involves additional cost --- overall development cost might be lower for: systems with unclear user requirements, systems with unresolved technical issues.
Many user requirements get properly defined and technical issues get resolved: This minimizes the change requests from
customer and massive redesign costs.
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Evolutionary Model
Evolutionary model (aka successive versions or incremental model): Software requirements is broken down
into several modules which can be incrementally implemented and delivered.
First develop the core modules (those which don’t need service from other modules) of the system.
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The initial product skeleton is refined into increasing levels of capability: by adding new functionalities in
successive versions.
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Evolutionary Model (CONT.)
Successive version of the product: functioning systems capable of
performing some useful work. A new release may include
new functionality:also existing functionality in the current release might have been enhanced.
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Evolutionary Model (CONT.)
AB
C
A AB
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Advantages of Evolutionary Model
Users get a chance to experiment with a partially developed system much before the full working version is released
Helps finding exact user requirements: much before fully working system is
developed. requests for changes after complete s/w
becomes less
Core modules get tested thoroughly: reduces chances of errors in final product.
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Disadvantages of Evolutionary Model
Often, difficult to subdivide problems into functional units: which can be incrementally
implemented and delivered.
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This model is suited for……
1. very large problems, where it is easier to find modules for incremental implementation.
2. Situations when the customer prefers to receive the product in increments so that he/she can start using different features as and when they are developed rather than waiting all the time for full product.
3. Object-oriented software development projects. 57
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Spiral Model
Proposed by Boehm in 1988. Each loop of the spiral represents a
phase of the software process: the innermost loop might be concerned
with system feasibility, the next loop with system requirements
definition, the next one with system design, and so
on. There are no fixed phases in this
model, that is why it is much more flexible compared to other models.
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Spiral Model (CONT.)
Determine Objectives
Identify & Resolve Risks
Develop Next Level of Product
Customer Evaluation and plan for next phase
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Spiral Model (CONT.)
The team must decide: how to structure the project into
phases. Start work using some generic
model: add extra phases
for specific projects or when problems are identified during a project.
Each loop in the spiral is split into four sectors (quadrants).
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Objective Setting (First Quadrant)
Identify objectives of the phase, they are investigated, elaborated, and analyzed.
Examine the risks associated with these objectives. Risk:
any adverse circumstance that might hamper/hinder successful completion of
a software project.
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ex: the risk involved in accessing data from a remote database can be that the data access rate might be too slow.
Find alternate solutions possible.
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Spiral Model (CONT.)
Determine Objectives
Identify & Resolve Risks
Develop Next Level of Product
Customer Evaluation and plan for next phase
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Risk Assessment and Reduction (Second Quadrant)
For each identified project risk, a detailed analysis is carried out.
Steps are taken to reduce the risk. For example, if there is a risk that
the requirements are inappropriate: a prototype system may be developed. Ex: database risk can be resolved by
building a prototype of data access subsystem and experimenting with the exact data access rate.
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Spiral Model (CONT.)
Development and Validation (Third Third quadrantquadrant): identified features are implemented. develop and validate the next level of the
product.
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Spiral Model (CONT.)
Determine Objectives
Identify & Resolve Risks
Develop Next Level of Product
Customer Evaluation and plan for next phase
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Review and Planning (Fourth Fourth quadrantquadrant): review the results achieved so far with
the customer and plan the next iteration around the spiral.
With each iteration around the spiral: progressively more complete version of
the software gets built.
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The radius of the spiral at any point represent the cost incurred in the project so far, and the angular dimension would represent the progress made so far in the current phase.
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Pros and Cons of spiral model
It is more complex model to follow, since it is risk-driven and more complicated than other models.
Requires more knowledgeable staff. Adv: for project involving many
unknown risk, this model is more appropriate as risks will be known and resolved as and when identified.
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Spiral Model as a Meta model
Subsumes all discussed models: a single loop spiral represents waterfall model. uses an evolutionary approach --
iterations through the spiral are evolutionary levels. enables understanding and reacting to risks
during each iteration along the spiral. uses:
prototyping as a risk reduction mechanism retains the step-wise approach of the waterfall
model.
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Spiral Model (CONT.)
Determine Objectives
Identify & Resolve Risks
Develop Next Level of Product
Customer Evaluation and plan for next phase
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Comparison of Different Life Cycle Models
Iterative waterfall model most widely used model. But, suitable only for well-understood
problems. not for very large projects and for
those which involves risks.
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Prototype model is suitable for projects not well understood: user requirements technical aspects all risks identified before project starts. especially popular for development of
user interface part of project.
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Comparison of Different Life Cycle Models (CONT.)
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Comparison of Different Life Cycle Models (CONT.)
Evolutionary model is suitable for large problems: can be decomposed into a set of
modules that can be incrementally implemented,
incremental delivery of the system is acceptable to the customer.
Object oriented development projects.
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The spiral model: suitable for development of technically
challenging software products that are subject to several kinds of risks that are difficult to discover at the start of the project.
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Comparison of Different Life Cycle Models (CONT.)
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Which life cycle model would you follow?
A well-understood data processing application.
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Classical Waterfall model Iterative Waterfall model Evolutionary model Prototype model Spiral model
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A new software product that would connect computers through satellite communication. Assume that your team has no previous experience in developing satellite communication software.
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Which life cycle model would you follow?
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A compiler for a new language.
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Which life cycle model would you follow?
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An object oriented s/w development effort.
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The graphical user interface part of a large s/w product.
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A new text editor.
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An extremely large s/w that would provide monitor, and control cellular communication among its subscriber using a set of revolving satellites.
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If the company has already experienced in developing payroll s/w for different organizations and now want to develop a payroll system for some organization.
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