3-Requirements.ppt

Requirements 1
Software Requirements
Analysis and Specification

Requirements 2
Background
 Problem of scale is a key issue for SE
 For small scale, understand and specifying
requirements is easy
 For large problem - very hard; probably the
hardest, most problematic and error prone
 Input : user needs in minds of people
 Output : precise statement of what the future
system will do

Requirements 3
Background..
 Identifying and specifying req necessarily
involves people interaction
 Cannot be automated
 Requirement (IEEE)= A condition or capability
that must be possessed by a system
 Req. phase ends with a software requirements
specification (SRS) document
 SRS specifies what the proposed system
should do

Requirements 4
Background..
 Requirements understanding is hard
 Visualizing a future system is difficult
 Capability of the future system not clear, hence
needs not clear
 Requirements change with time
 …
 Essential to do a proper analysis and
specification of requirements

Requirements 5
Need for SRS
 SRS establishes basis of agreement
between the user and the supplier.
 Users needs have to be satisfied, but user
may not understand software
 Developers will develop the system, but
may not know about problem domain
 SRS is the medium to bridge the commn.
gap and specify user needs in a manner
both can understand

Requirements 6
Need for SRS…
 Helps user understand his needs.
 users do not always know their needs
 must analyze and understand the potential
 the goal is not just to automate a manual system,
but also to add value through IT
 The req process helps clarify needs
 SRS provides a reference for validation of the
final product
 Clear understanding about what is expected.
 Validation - “ SW satisfies the SRS “

Requirements 7
Need for SRS…
 High quality SRS essential for high Quality SW
 Requirement errors get manifested in final sw
 to satisfy the quality objective, must begin with
high quality SRS
 Requirements defects are not few
 25% of all defects in one case; 54% of all defects found
after UT
 80 defects in A7 that resulted in change requests
 500 / 250 defects in previously approved SRS.

Requirements 8
Need for SRS…
 Good SRS reduces the development cost
 SRS errors are expensive to fix later
 Req. changes can cost a lot (up to 40%)
 Good SRS can minimize changes and errors
 Substantial savings; extra effort spent
during req. saves multiple times that effort
 An Example
 Cost of fixing errors in req. , design ,
coding , acceptance testing and operation
are 2 , 5 , 15 , 50 , 150 person-months

Requirements 9
Need for SRS…
 Example …
 After req. phase 65% req errs detected in design ,
2% in coding, 30% in Acceptance testing, 3%
during operation
 If 50 requirement errors are not removed in the
req. phase, the total cost
32.5 *5 + 1*15 + 15*50 + 1.5*150 = 1152 hrs
 If 100 person-hours invested additionally in req to
catch these 50 defects , then development cost
could be reduced by 1152 person-hours.
 Net reduction in cost is 1052 person-hours

Requirements 10
Requirements Process
 Sequence of steps that need to be performed
to convert user needs into SRS
 Process has to elicit needs and requirements
and clearly specifies it
 Basic activities
 problem or requirement analysis
 requirement specification
 validation
 Analysis involves elicitation and is the hardest

Requirements 11
Requirements Process..
needs
Analysis
Specification
Validation

Requirements 12
Requirement process..
 Process is not linear, it is iterative and
parallel
 Overlap between phases - some parts
may be analyzed and specified
 Specification itself may help analysis
 Validation can show gaps that can lead
to further analysis and spec

Requirements 13
Requirements Process…
 Focus of analysis is on understanding the
desired systems and it’s requirements
 Divide and conquer is the basic strategy
 decompose into small parts, understand each part
and relation between parts
 Large volumes of information is generated
 organizing them is a key
 Techniques like data flow diagrams, object
diagrams etc. used in the analysis

Requirements 14
Requirements Process..
 Transition from analysis to specs is hard
 in specs, external behavior specified
 during analysis, structure and domain are
understood
 analysis structures helps in specification,
but the transition is not final
 methods of analysis are similar to that of
design, but objective and scope different
 analysis deals with the problem domain,
whereas design deals with solution domain

Requirements 15
Problem Analysis
 Aim: to gain an understanding of the needs,
requirements, and constraints on the software
 Analysis involves
 interviewing client and users
 reading manuals
 studying current systems
 helping client/users understand new possibilities
 Like becoming a consultant
 Must understand the working of the
organization , client and users

Requirements 16
Problem Analysis…
 Some issues
 Obtaining the necessary information
 Brainstorming: interacting with clients to
establish desired properties
 Information organization, as large amount
of info. gets collected
 Ensuring completeness
 Ensuring consistency
 Avoiding internal design

Requirements 17
Problem Analysis…
 Interpersonal issues are important
 Communication skills are very important
 Basic principle: problem partition
 Partition w.r.t what?
 Object - OO analysis
 Function - structural analysis
 Events in the system – event partitioning
 Projection - get different views
 Will discuss few different analysis techniques

Requirements 18
Characteristics of an SRS
 What should be the characteristics of a good
SRS? Some key ones are
 Complete
 Unambiguous
 Consistent
 Verifiable
 Ranked for importance and/or stability

Requirements 19
Characteristics…
 Correctness
 Each requirement accurately represents some
desired feature in the final system
 Completeness
 All desired features/characteristics specified
 Hardest to satisfy
 Completeness and correctness strongly related
 Unambiguous
 Each req has exactly one meaning
 Without this errors will creep in
 Important as natural languages often used

Requirements 20
Characteristics…
 Verifiability
 There must exist a cost effective way of checking
if sw satisfies requirements
 Consistent
 two requirements don’t contradict each other
 Ranked for importance/stability
 Needed for prioritizing in construction
 To reduce risks due to changing requirements

Requirements 21
Components of an SRS
 What should an SRS contain ?
 Clarifying this will help ensure
completeness
 An SRS must specify requirements on
 Functionality
 Performance
 Design constraints
 External interfaces

Requirements 22
Functional Requirements
 Heart of the SRS document; this forms the
bulk of the specs
 Specifies all the functionality that the system
should support
 Outputs for the given inputs and the
relationship between them
 All operations the system is to do
 Must specify behavior for invalid inputs too

Requirements 23
Performance Requirements
 All the performance constraints on the
software system
 Generally on response time ,
throughput etc => dynamic
 Capacity requirements => static
 Must be in measurable terms
(verifiability)
 Eg resp time should be xx 90% of the time

Requirements 24
Design Constraints
 Factors in the client environment that
restrict the choices
 Some such restrictions
 Standard compliance and compatibility with
other systems
 Hardware Limitations
 Reliability, fault tolerance, backup req.
 Security

Requirements 25
External Interface
 All interactions of the software with
people, hardware, and sw
 User interface most important
 General requirements of “friendliness”
should be avoided
 These should also be verifiable

Requirements 26
Specification Language
 Language should support desired char
of the SRS
 Formal languages are precise and
unambiguous but hard
 Natural languages mostly used, with
some structure for the document
 Formal languages used for special
features or in highly critical systems

Requirements 27
Structure of an SRS
 Introduction
 Purpose , the basic objective of the system
 Scope of what the system is to do , not to do
 Overview
 Overall description
 Product perspective
 Product functions
 User characteristics
 Assumptions
 Constraints

Requirements 28
Structure of an SRS…
 Specific requirements
 External interfaces
 Functional requirements
 Performance requirements
 Design constraints
 Acceptable criteria
 desirable to specify this up front.
 This standardization of the SRS was done by
IEEE.

Requirements 29
Use Cases Approach for
Functional Requirements
 Traditional approach for fn specs – specify
each function
 Use cases is a newer technique for specifying
behavior (functionality)
 I.e. focuses on functional specs only
 Though primarily for specification, can be
used in analysis and elicitation
 Can be used to specify business or org
behavior also, though we will focus on sw
 Well suited for interactive systems

Requirements 30
Use Cases Basics
 A use case captures a contract between
a user and system about behavior
 Basically a textual form; diagrams are
mostly to support
 Also useful in requirements elicitation as
users like and understand the story
telling form and react to it easily

Requirements 31
Basics..
 Actor: a person or a system that interacts with the
proposed system to achieve a goal
 Eg. User of an ATM (goal: get money); data entry operator;
(goal: Perform transaction)
 Actor is a logical entity, so receiver and sender actors
are different (even if the same person)
 Actors can be people or systems
 Primary actor: The main actor who initiates a UC
 UC is to satisfy his goals
 The actual execution may be done by a system or another
person on behalf of the Primary actor

Requirements 32
Basics..
 Scenario: a set of actions performed to
achieve a goal under some conditions
 Actions specified as a sequence of steps
 A step is a logically complete action performed
either by the actor or the system
 Main success scenario – when things go
normally and the goal is achieved
 Alternate scenarios: When things go wrong
and goals cannot be achieved

Requirements 33
Basics..
 A UC is a collection of many such
scenarios
 A scenario may employ other use cases
in a step
 I.e. a sub-goal of a UC goal may be
performed by another UC
 I.e. UCs can be organized hierarchically

Requirements 34
Basics…
 UCs specify functionality by describing
interactions between actors and system
 Focuses on external behavior
 UCs are primarily textual
 UC diagrams show UCs, actors, and dependencies
 They provide an overview
 Story like description easy to understand by
both users and analysts
 They do not form the complete SRS, only the
functionality part

Requirements 35
Example
Use Case 1: Buy stocks
Primary Actor: Purchaser
Goals of Stakeholders:
Purchaser: wants to buy stocks
Company: wants full transaction info
Precondition: User already has an account

Requirements 36
Example …
 Main Success Scenario
1. User selects to buy stocks
2. System gets name of web site from user for
trading
3. Establishes connection
4. User browses and buys stocks
5. System intercepts responses from the site and
updates user portfolio
6. System shows user new portfolio stading

Requirements 37
Example…
 Alternatives
 2a: System gives err msg, asks for new
suggestion for site, gives option to cancel
 3a: Web failure. 1-Sys reports failure to
user, backs up to previous step. 2-User
exits or tries again
 4a: Computer crashes
 4b: web site does not ack purchase
 5a: web site does not return needed info

Requirements 38
Example 2
 Use Case 2: Buy a product
 Primary actor: buyer/customer
 Goal: purchase some product
 Precondition: Customer is already
logged in

Requirements 39
Example 2…
 Main Scenario
1. Customer browses and selects items
2. Customer goes to checkout
3. Customer fills shipping options
4. System presents full pricing info
5. Customer fills credit card info
6. System authorizes purchase
7. System confirms sale
8. System sends confirming email

Requirements 40
Example 2…
 Alternatives
 6a: Credit card authorization fails
 Allows customer to reenter info
 3a: Regular customer
 System displays last 4 digits of credit card no
 Asks customer to OK it or change it
 Moves to step 6

Requirements 41
Example – An auction site
 Use Case1: Put an item for auction
 Primary Actor: Seller
 Precondition: Seller has logged in
 Main Success Scenario:
 Seller posts an item (its category, description, picture,
etc.) for auction
 System shows past prices of similar items to seller
 System specifies the starting bid price and a date when
auction will close
 System accepts the item and posts it
 Exception Scenarios:
 -- 2 a) There are no past items of this category
* System tells the seller this situation

Requirements 42
Example – auction site..
 Use Case2: Make a bid
 Primary Actor: Buyer
 Precondition: The buyer has logged in
 Buyer searches or browses and selects some item
 System shows the rating of the seller, the starting bid, the
current bids, and the highest bid; asks buyer to make a bid
 Buyer specifies bid price, max bid price, and increment
 Systems accepts the bid; Blocks funds in bidders account
 System updates the bid price of other bidders where needed,
and updates the records for the item

Requirements 43
 Exception Scenarios:
 -- 3 a) The bid price is lower than the current
highest
* System informs the bidder and asks to rebid
 -- 4 a) The bidder does not have enough funds in
his account
* System cancels the bid, asks the user to get
more funds

Requirements 44
Example –auction site..
 Use Case3: Complete auction of an item
 Primary Actor: Auction System
 Precondition: The last date for bidding has been
reached
 Select highest bidder; send email to selected bidder and seller
informing final bid price; send email to other bidders also
 Debit bidder’s account and credit seller’s account
 Transfer from seller’s account commission amount to
organization’s account
 Unblock other bidders funds
 Remove item from the site; update records
 Exception Scenarios: None

Requirements 45
Example – summary-level Use Case
 Use Case 0 : Auction an item
 Primary Actor: Auction system
 Scope: Auction conducting organization
 Precondition: None
 Seller performs put an item for auction
 Various bidders make a bid
 On final date perform Complete the auction of
the item
 Get feed back from seller; get feedback from
buyer; update records

Requirements 46
Requirements with Use Cases
 UCs specify functional requirements
 Other req identified separately
 A complete SRS will contain the use
cases plus the other requirements
 Note – for system requirements it is
important to identify UCs for which the
system itself may be the actor

Requirements 47
Developing Use Cases
 UCs form a good medium for
brainstorming and discussions
 Hence can be used in elicitation and
problem analysis also
 UCs can be developed in a stepwise
refinement manner
 Many levels possible, but four naturally
emerge

Requirements 48
Developing…
 Step 1: Identify actors and goals
 Prepare an actor-goal list
 Provide a brief overview of the UC
 This defines the scope of the system
 Completeness can also be evaluated
 Step 2: Specify main Success Scenarios
 For each UC, expand main scenario
 This will provide the normal behavior of the
system
 Can be reviewed to ensure that interests of all
stakeholders and actors is met

Requirements 49
Developing…
 Step 3: Identify failure conditions
 List possible failure conditions for UCs
 For each step, identify how it may fail
 This step uncovers special situations
 Step 4: Specify failure handling
 Perhaps the hardest part
 Specify system behavior for the failure conditions
 New business rules and actors may emerge

Requirements 50
Other Approaches to Analysis

Requirements 51
Data Flow Modeling
 Widely used; focuses on functions
performed in the system
 Views a system as a network of data
transforms through which the data flows
 Uses data flow diagrams (DFDs) and
functional decomposition in modeling
 The SSAD methodology uses DFD to
organize information, and guide analysis

Requirements 52
Data flow diagrams
 A DFD shows flow of data through the
system
 Views system as transforming inputs to
outputs
 Transformation done through transforms
 DFD captures how transformation occurs
from input to output as data moves
through the transforms
 Not limited to software

Requirements 53
Data flow diagrams…
 DFD
 Transforms represented by named
circles/bubbles
 Bubbles connected by arrows on which
named data travels
 A rectangle represents a source or sink and
is originator/consumer of data (often
outside the system)

Requirements 55
DFD Conventions
 External files shown as labeled straight lines
 Need for multiple data flows by a process
represented by * (means and)
 OR relationship represented by +
 All processes and arrows should be named
 Processes should represent transforms,
arrows should represent some data

Requirements 56
Data flow diagrams…
 Focus on what transforms happen , how
they are done is not important
 Usually major inputs/outputs shown,
minor are ignored in this modeling
 No loops , conditional thinking , …
 DFD is NOT a control chart, no
algorithmic design/thinking
 Sink/Source , external files

Requirements 57
Drawing a DFD
 If get stuck , reverse direction
 If control logic comes in , stop and restart
 Label each arrows and bubbles
 Make use of + & *
 Try drawing alternate DFDs
Leveled DFDs :
 DFD of a system may be very large
 Can organize it hierarchically
 Start with a top level DFD with a few bubbles
 then draw DFD for each bubble
 Preserve I/O when “ exploding”

Requirements 58
Drawing a DFD for a system
 Identify inputs, outputs, sources, sinks for the
system
 Work your way consistently from inputs to
outputs, and identify a few high-level
transforms to capture full transformation
 If get stuck, reverse direction
 When high-level transforms defined, then
refine each transform with more detailed
transformations

Requirements 59
Drawing a DFD for a system..
 Never show control logic; if thinking in
terms of loops/decisions, stop & restart
 Label each arrows and bubbles;
carefully identify inputs and outputs of
each transform
 Make use of + & *
 Try drawing alternate DFDs

Requirements 60
Leveled DFDs
 DFD of a system may be very large
 Can organize it hierarchically
 Start with a top level DFD with a few bubbles
 then draw DFD for each bubble
 Preserve I/O when “ exploding” a bubble so
consistency preserved
 Makes drawing the leveled DFD a top-down
refinement process, and allows modeling of
large and complex systems

Requirements 61
Data Dictionary
 In a DFD arrows are labeled with data items
 Data dictionary defines data flows in a DFD
 Shows structure of data; structure becomes
more visible when exploding
 Can use regular expressions to express the
structure of data

Requirements 62
Data Dictionary Example
 For the timesheet DFD
Weekly_timesheet – employee_name + id +
[regular_hrs + overtime_hrs]*
Pay_rate = [hourly | daily | weekly] +
dollar_amt
Employee_name = last + first + middle
Id = digit + digit + digit + digit

Requirements 63
DFD drawing – common errors
 Unlabeled data flows
 Missing data flows
 Extraneous data flows
 Consistency not maintained during
refinement
 Missing processes
 Too detailed or too abstract
 Contains some control information

Requirements 64
Prototyping
 Prototyping is another approach for
problem analysis
 Discussed it earlier with process – leads
to prototyping process model

Requirements 65
Requirements Validation
 Lot of room for misunderstanding
 Errors possible
 Expensive to fix req defects later
 Must try to remove most errors in SRS
 Most common errors
 Omission - 30%
 Inconsistency - 10-30%
 Incorrect fact - 10-30%
 Ambiguity - 5 -20%

Requirements 66
Requirements Review
 SRS reviewed by a group of people
 Group: author, client, user, dev team rep.
 Must include client and a user
 Process – standard inspection process
 Effectiveness - can catch 40-80% of req.
errors

Requirements 67
Summary
 Having a good quality SRS is essential for Q&P
 The req. phase has 3 major sub phases
 analysis , specification and validation
 Analysis
 for problem understanding and modeling
 Methods used: SSAD, OOA , Prototyping
 Key properties of an SRS: correctness,
completeness, consistency,unambiguousness

Requirements 68
Summary..
 Specification
 must contain functionality , performance ,
interfaces and design constraints
 Mostly natural languages used
 Use Cases is a method to specify the
functionality; also useful for analysis
 Validation - through reviews

3-Requirements.ppt

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