Software engineering Unit-2

Software Engineering UNIMAK
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Unit-2
2.0 Software Requirements
The requirements for software are the descriptions of what the software should do, the services
that it provides and the constraints on its operation. These requirements reflect the needs of
customers for software that serves a certain purpose such as controlling a device, placing an
order, or finding information. The process of finding out, analyzing, documenting and checking
these services and constraints is called requirements engineering (RE), or software requirements
are description of features and functionalities of the target system. Requirements convey the
expectations of users from the software product. The requirements can be clear or hidden, known
or unknown, expected or unexpected from client’s point of view.
The term ‘requirement’ is not used consistently in the software industry. In some cases, a
requirement is simply a high-level, abstract statement of a service that a system should provide
or a constraint on a system. At the other extreme, it is a detailed, formal definition of a system
function. Davis (1993) explains why these differences exist:
If a company wishes to let a contract for a large software development project, it must define its
needs in a sufficiently abstract way that a solution is not predefined. The requirements must be
written so that several contractors can bid for the contract, offering, perhaps, different ways of
meeting the client organization’s needs. Once a contract has been awarded, the contractor must
write a system definition for the client in more detail so that the client understands and can
validate what the software will do. Both of these documents may be called the requirements
document for the system.
Some of the problems that arise during the requirements process are a result of failing to make a
clear separation between these different levels of description. Davis distinguish between them by
using the term ‘user requirements’ to mean the high-level abstract requirements and ‘system
requirements’ to mean the detailed description of what the system should do.

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1. User Requirement
User requirements are statements, in a natural language plus diagrams, of what services the
system is expected to provide to system users and the constraints under which it must operate.
2. System Requirement
System requirements are more detailed descriptions of the software system’s functions, services,
and operational constraints. The system requirements document (sometimes called a functional
specification) should define exactly what is to be implemented. It may be part of the contract
between the system buyer and the software developers.
Different levels of requirements are useful because they communicate information about the
system to different types of reader. The figure below illustrates the distinction between user and
system requirements.
Example: This example is from a mental health care patient management system (MHC-PMS)
show how a user requirement may be expanded into several system requirements. You can see
from the figure that the user requirement is quite general. The system requirements provide more
specific information about the services and functions of the system that is to be implemented.
User Requirement Definition
System Requirements Specification
1. The MHC-PMS shall generate monthly management reports showing the cost of
drugs prescribed by each clinic during that month.
the cost of drugs prescribed by each clinic during that month.
1. On the last working day of each month, a summary of the drugs prescribed, their
cost, and the prescribing clinics shall be generated.
2. The system shall automatically generate the report for printing after 17.30 on the last
working day of the month.
3. A report shall be created for each clinic and shall list the individual drug names, the
total number of prescriptions, the number of doses prescribed, and the total cost of
the prescribed drugs.
4. If drugs are available in different dose units (e.g., 10 mg, 20 mg) separate reports shall
be created for each dose unit.
5. Access to all cost reports shall be restricted to authorized users listed on a
management access control list.

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2.0 Functional and non-functional Requirement
Software system requirements are often classified as functional requirements or non-functional
requirements.
2.1 Functional Requirement
The functional requirements for a system describe what the system should do. These
requirements depend on the type of software being developed, the expected users of the software,
and the general approach taken by the organization when writing requirements. When expressed
as user requirements, functional requirements are usually described in an abstract way that can
be understood by system users. However, more specific functional system requirements describe
the system functions, its inputs and outputs, exceptions, etc., in detail.
Functional system requirements vary from general requirements covering what the system
should do to very specific requirements reflecting local ways of working or an organization’s
existing systems. Here are examples of functional requirements.
Example-1
 Search option given to user to search from various invoices.
 User should be able to mail any report to management.
 Users can be divided into groups and groups can be given separate rights.
 Should compile business rules and administrative functions.
Example-2
Requirements for the MHC-PMS system, used to maintain information about patients receiving
treatment for mental health problems:
1. A user shall be able to search the appointments lists for all clinics.
2. The system shall generate each day, for each clinic, a list of patients who are expected to
attend appointments that day.
3. Each staff member using the system shall be uniquely identified by his or her employee
number.

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These functional user requirements define specific facilities to be provided by the system. These
have been taken from the user requirements document.
2.2 Non-functional Requirement
Non-functional requirements, as the name suggests, are requirements that are not directly
concerned with the specific services delivered by the system to its users. They may relate to
growing system properties such as reliability, response time, and store tenancy. Alternatively,
they may define constraints on the system implementation such as the capabilities of I/O devices
or the data representations used in interfaces with other systems.
Non-functional requirements, such as performance, security, or availability, usually specify or
constrain characteristics of the system as a whole. Non-functional requirements are often more
critical than individual functional requirements. System users can usually find ways to work
around a system function that doesn’t really meet their needs.
However, failing to meet a non-functional requirement can mean that the whole system is
unusable. For example, if an aircraft system does not meet its reliability requirements, it will not
be certified as safe for operation; if an embedded control system fails to meet its performance
requirements, the control functions will not operate correctly.
Although it is often possible to identify which system components implement specific functional
requirements (e.g., there may be formatting components that implement reporting requirements),
it is often more difficult to relate components to non-functional requirements. The
implementation of these requirements may be diffused throughout the system. There are two
reasons for this:
1. Non-functional requirements may affect the overall architecture of a system rather than
the individual components. For example, to ensure that performance requirements are
met, you may have to organize the system to minimize communications between
components.
2. A single non-functional requirement, such as a security requirement, may generate a
number of related functional requirements that define new system services that are

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required. In addition, it may also generate requirements that restrict existing
requirements.
Examples
 Security
 Logging
 Storage
 Configuration
 Performance
 Cost
 Interoperability
 Flexibility
 Disaster recovery
 Accessibility
Non-functional requirements arise through user needs, because of budget constraints,
organizational policies, the need for interoperability with other software or hardware systems, or
external factors such as safety regulations or privacy legislation.
2.2 Requirement Engineering Process
The process to gather the software requirements from client, analyze, and document them is
known as requirement engineering. The goal of requirement engineering is to develop and
maintain sophisticated and descriptive ‘System Requirements Specification’ document.
It is a four step process, which includes
1. Feasibility Study
2. Requirement Gathering
3. Software Requirement Specification
4. Software Requirement Validation

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1. Feasibility study
When the client approaches the organization for getting the desired product developed, it comes
up with a rough idea about what all functions the software must perform and which all features
are expected from the software.
Referencing to this information, the analysts do a detailed study about whether the desired
system and its functionality are feasible to develop. This feasibility study is focused towards goal
of the organization. This study analyzes whether the software product can be practically
materialized in terms of implementation, contribution of project to organization, cost constraints,
and as per values and objectives of the organization. It explores technical aspects of the project
and product such as usability, maintainability, productivity, and integration ability.
The output of this phase should be a feasibility study report that should contain adequate
comments and recommendations for management about whether or not the project should be
undertaken.
2. Requirement Gathering
If the feasibility report is positive towards undertaking the project, next phase starts with
gathering requirements from the user. Analysts and engineers communicate with the client and
end-users to know their ideas on what the software should provide and which features they want
the software to include.
3. Software Requirement Specification (SRS)
SRS is a document created by system analyst after the requirements are collected from various
stakeholders. SRS defines how the intended software will interact with hardware, external
interfaces, speed of operation, response time of system, portability of software across various
platforms, maintainability, speed of recovery after crashing, Security, Quality, Limitations etc.
The requirements received from client are written in natural language. It is the responsibility of
the system analyst to document the requirements in technical language so that they can be
comprehended and used by the software development team.

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SRS should come up with the following features:
 User Requirements are expressed in natural language.
 Technical requirements are expressed in structured language, which is used inside the
organization.
 Design description should be written in Pseudo code.
 Format of Forms and GUI screen prints.
 Conditional and mathematical notations for DFDs etc.
4. Software Requirement Validation
After requirement specifications are developed, the requirements mentioned in this document are
validated. User might ask for illegal, impractical solution or experts may interpret the
requirements inaccurately. This results in huge increase in cost if not compressed in the budget.
Requirements can be checked against the following conditions:
 If they can be practically implemented
 If they are valid and as per functionality and domain of software
 If there are any ambiguities
 If they are complete
 If they can be demonstrated
2.3 Requirement Elicitation and Management
This is the process of deriving the system requirements through observation of existing systems,
discussions with potential users and procurers, task analysis, and so on. This may involve the
development of one or more system models and prototypes. These help you understand the
system to be specified.
Requirement elicitation process can be depicted using the following diagram

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 Requirements gathering - The developers discuss with the client and end users and
know their expectations from the software.
 Organizing Requirements - The developers prioritize and arrange the requirements in
order of importance, urgency and convenience.
 Negotiation & discussion - If requirements are ambiguous or there are some conflicts in
requirements of various stakeholders, it is then negotiated and discussed with the
stakeholders. Requirements may then be prioritized and reasonably compromised. The
requirements come from various stakeholders. To remove the ambiguity and conflicts,
they are discussed for clarity and correctness. Unrealistic requirements are negotiated
reasonably.
 Documentation - All formal and informal, functional and non-functional requirements
are documented and made available for next phase processing.
2.3.1 Requirement Elicitation Techniques
There are various ways to discover requirements. Some of them are listed below
 Interviews
 Surveys
 Questionnaires
 Task analysis
 Domain Analysis
 Brainstorming
 Prototyping
 Observation
2.4 Software Prototyping
A prototype is an initial version of a software system that is used to demonstrate concepts, try out
design options, and find out more about the problem and its possible solutions. Rapid, iterative
development of the prototype is essential so that costs are controlled and system stakeholders can
experiment with the prototype early in the software process.

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A software prototype can be used in a software development process to help anticipate changes
that may be required:
1. In the requirements engineering process, a prototype can help with the elicitation and
validation of system requirements.
2. In the system design process, a prototype can be used to explore particular software
solutions and to support user interface design.
Software prototypes allow users to see how well the software supports their work. They may get
new ideas for requirements, and find areas of strength and weakness in the software. They may
then propose new system requirements. Furthermore, as the prototype is developed, it may reveal
errors and omissions in the requirements that have been proposed.
A software prototype may be used while the software is being designed to carry out design
experiments to check the feasibility of a proposed design. For example, a database design may be
prototyped and tested to check that it supports efficient data access for the most common user
queries. Prototyping is also an essential part of the user interface design process. Because of the
dynamic nature of user interfaces, textual descriptions and diagrams are not good enough for
expressing the user interface requirements. Therefore, rapid prototyping with end-user
involvement is the only sensible way to develop graphical user interfaces for software systems.
2.4.1 Prototyping in the software process------Presentation Assignment
2.4.2 Rapid Prototyping Techniques -----------Presentation Assignment
2.4.3 User Interface Prototyping
User Interface (UI) is an important part of any software or hardware or hybrid system. Software
is widely accepted if it is
 Easy to operate
 Quick in response
 Effectively handling operational errors
 Providing simple yet consistent user interface

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User acceptance majorly depends upon how user can use the software. UI is the only way for
users to perceive the system. A well performing software system must also be equipped with
attractive, clear, consistent, and responsive user interface. Otherwise the functionalities of
software system cannot be used in a convenient way. A system is said to be good if it provides
means to use it efficiently. User interface requirements are briefly mentioned below
 Content presentation
 Easy Navigation
 Simple interface
 Responsive
 Consistent UI elements
 Feedback mechanism
 Default settings
 Purposeful layout
 Strategical use of color and texture.
 Provide help information
 User centric approach
 Group based view settings.
2.5 Software document
The software requirements document (sometimes called the software requirements specification
or SRS) is an official statement of what the system developers should implement. It should
include both the user requirements for a system and a detailed specification of the system
requirements. Sometimes, the user and system requirements are integrated into a single
description. In other cases, the user requirements are defined in an introduction to the system
requirements specification. If there are a large number of requirements, the detailed system
requirements may be presented in a separate document.
Requirements documents are essential when an outside contractor is developing the software
system. However, agile development methods argue that requirements change so rapidly that a
requirements document is out of date as soon as it is written, so the effort is largely wasted.

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Rather than a formal document, approaches such as Extreme Programming (Beck, 1999) collect
user requirements incrementally and write these on cards as user stories. The user then prioritizes
requirements for implementation in the next increment of the system.
For business systems where requirements are unstable, I think that this approach is a good one.
However, I think that it is still useful to write a short supporting document that defines the
business and dependability requirements for the system; it is easy to forget the requirements that
apply to the system as a whole when focusing on the functional requirements for the next system
release.
The requirements document has a diverse set of users, ranging from the senior management of
the organization that is paying for the system to the engineers responsible for developing the
software. Figure below, shows outline of a software document.
Chapter Description
Preface This should define the expected readership of the document and describe
its version history, including a rationale for the creation of a new version
and a summary of the changes made in each version.
Introduction This should describe the need for the system. It should briefly describe
the system’s functions and explain how it will work with other systems.
It should also describe how the system fits into the overall business or
strategic objectives of the organization commissioning the software.
Glossary This should define the technical terms used in the document. You should
not make assumptions about the experience or expertise of the reader.
User requirements
definition
Here, you describe the services provided for the user. The non-functional
system requirements should also be described in this section. This
description may use natural language, diagrams, or other notations that
are understandable to customers. Product and process standards that must
be followed should be specified.

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System architecture This chapter should present a high-level overview of the anticipated
system architecture, showing the distribution of functions across system
modules. Architectural components that are reused should be highlighted.
System models This might include graphical system models showing the relationships
between the system components, the system, and its environment.
Examples of possible models are object models, data-flow models, or
semantic data models.
System evolution This should describe the fundamental assumptions on which the system
is based, and any anticipated changes due to hardware evolution,
changing user needs, and so on. This section is useful for system
designers as it may help them avoid design decisions that would
constrain likely future changes to the system.
Appendices These should provide detailed, specific information that is related to the
application being developed; for example, hardware and database
descriptions. Hardware requirements define the minimal and optimal
configurations for the system. Database requirements define the logical
organization of the data used by the system and the relationships between
data.
Index Several indexes to the document may be included. As well as a normal
alphabetic index, there may be an index of diagrams, an index of
functions, and so on.

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2.6 Software Analysis
Software analysis and design includes all activities, which help the transformation of
requirement specification into implementation. Requirement specifications specify all functional
and non-functional expectations from the software. These requirement specifications come in the
shape understandable documents, to which a computer has nothing to do.
Software analysis and design is the intermediate stage, which helps human readable requirements
to be transformed into actual code.
Let us see few analysis and design tools used by software designers
 Data Flow Diagram
 Structured English
 Pseudo-Code
 Decision Tables
 Entity-Relationship Model
 Data Dictionary
 Hierarchical Input Process Output (HIPO) Diagram----Presentation Assignment
 Structure Charts---------Presentation Assignment
2.7 Software modeling------Presentation Assignment
2.7.1 Data model----------Presentation Assignment
2.7.2 Functional model----Presentation Assignment
2.7.3 Behavioral Model---Presentation Assignment
2.8 Structured Analysis—Reading Assignment

Software engineering Unit-2

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Software engineering Unit-2