TDT 4242

Inah Omoronyia and Tor Stålhane

Requirements Handling

TDT 4242

Institutt for datateknikk oginformasjonsvitenskap

TDT 4242

Requirements Handling - 1

Characteristics of an effective RE process:• Minimizes the occurrence of requirements errors

• Mitigates the impact of requirements change

• Is critical to the success of any development project.

The goal of the RE process is to ensure that requirement for a system can be allocated to a particular software component that assumes responsibility for satisfying the requirement. When such allocation is possible:• The resulting software is well modularized.• The modules have clear interfaces • All requirements are clearly separated.

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Requirements Handling – 2

Criteria for good requirements handling• Handle the view points of the system-to-be

• Handle non-functional requirements and soft goals

• Handles the identification and handling of crosscutting and non-crosscutting requirements

• Handles the impact of COTS, outsourcing and sub-contracting

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Viewpoints, perspectives and views• Viewpoint is defined as a standing position used by an individual when examining a universe of discourse – in our case the combination of the agent and the view that the agent holds

• A perspective is defined as a set of facts observed and modelled according to a particular aspect of reality• A view is defined as an integration of these perspectives•A viewpoint language is used to represent the viewpoints

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Example: Train break viewpointsConsider the requirements for a system to be installed on a

train which will automatically stop the train if it goes through a red light • Driver Requirements from the train driver on the

system

• Trackside equipment Requirements from trackside equipment which must interface with the system to be installed

• Safety engineer Safety requirements for the system

• Existing on-board systems Compatibility requirements

• Braking characteristics Requirements which are derived from the braking characteristics of a train.

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Example: ATM Viewpoints

• Bank customers• Representatives of other banks• Hardware and software maintenance engineers• Marketing department• Bank managers and counter staff• Database administrators and security staff• Communications engineers• Personnel department

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Types of viewpointsData sources or sinks

Viewpoints that are responsible for producing or consuming data. Analysis involves checking that data is produced and consumed and that assumptions about the source and sink of data are valid

Representation frameworksViewpoints that represent particular types of system

model (e.g. State machine representation). Particularly suitable for real-time systems

Receivers of servicesViewpoints that are external to the system and

receive services from it. Most suited to interactive systems

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The VORD method – 1

VORD is a method designed as a service-oriented framework for requirements elicitation and analysis.

Viewpoint Identificatio

n

Viewpoint Structuring

Viewpoint Documentatio

n

Viewpoint System mapping

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The VORD method – 2

1.Viewpoint identificationDiscover viewpoints which receive system services

and identify the services provided to each viewpoint

2.Viewpoint structuringGroup related viewpoints into a hierarchy. Common

services are provided at higher-levels in the hierarchy

3.Viewpoint documentationRefine the description of the identified viewpoints

and services4.Viewpoint-system mapping

Transform the analysis to an object-oriented design

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VORD standard forms

Viewpoint template

Reference: The view point name

Attributes: Attributes providing viewpoint information

Events: A reference to a set of event scenarios describing how the system reacts to viewpoint events

Services: A reference to a set of service descriptions

Sub-VPs: The names of sub-viewpoints

Service template

Reference: The service name

Rationale: Reason why the service is provided.

Specification: Reference to a list of service specifications. These may be expressed in different notations.

Viewpoints: A List of viewpoint names receiving the service

Non-functional requirements: Reference to a set of non-functional requirements which constrain the service.

Provider: Reference to a list of system objects which provide the service.

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Viewpoint: Service Information

ACCOUNT HOLDER BANK TELLER

FOREIGN CUSTOMER

Service list

Withdraw cash

Query balance

Order checks

Send message

Transaction list

Order statement

Transfer funds

Service list

Withdraw cash

Query balance

Service list

Run diagnostics

Add cash

Add paper

Send message

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Viewpoint hierarchy

Services

Query balance

Withdraw cash

Services

Order checks

Send message

Transaction list

Order statement

Transfer funds

Account holder

Account holder

Teller

Manager Engineer

Bank staff

Customer

All Viewpoints

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Customer/cash withdrawal

Reference: CustomerAttributes: Account number;

PIN; Start transactionEvents: Select service;

Cancel transaction; End transaction

Services: Cash withdrawal Balance inquirySub-Viewpoints:

Account holderForeign customer

Reference: Cash withdrawal

Rationale: To improve customer service and reduce paperwork

Specification: Users choose this service by pressing the cash withdrawal button. They then enter the amount required. This is confirmed and, if funds allow, the balance is delivered.

Viewpoints: Customer

Non-functional requirements: Deliver cash within 1minute of amount being confirmed

Provider: __________

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Requirement handling – Viewpoint

Advantages of viewpoint-oriented approaches in requirements handling:

Assist in understanding and controlling the complexity by separating interests of various actors

Explicitly recognise the diversity of sources of requirements

Provide a mechanism for organising and structuring this diverse information

Imparts a sense of thoroughness (completeness)

Provide a means for requirements sources or stakeholders to identify and check their contribution to the requirements

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NFR and soft goals – 1 Scenario:Imagine that you have been asked by your client to conduct a requirements analysis for a new system intended to support several office functions within the organization, including scheduling meetings.

Clients success criterion: The new system should be highly usable, flexible and adaptable to the work patterns of individual users and that its introduction should create as little disruption as possible.

Question:how are you going to deal with the client’s objectives of having a usable and flexible system?

Challenge:We need some way to represent flexibility and usability concern, along with their respective interrelationships.

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NFR and soft goals – 2 The concept of goal is used extensively in AI where a goal is satisfied absolutely when its subgoals are satisfied.

NFRF is centered around the notion of soft goals which do not have a clear-cut criterion for their satisfaction

•Soft goals are satisficed when there is sufficient positive and little negative evidence for this claim, and that they are unsatisficeable when there is sufficient negative evidence and little positive support for their satisficeability.

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NFR Framework – 1

Soft goals are not analyzed independently of one another, but rather in relation to each other.

Softgoal relationships

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NFR Framework – 2

Non-functional requirements analysis: • Step1: Begins with soft goals that represent non-functional

requirements agreed upon by the stakeholders, say Usability, Flexibility, etc.

• Step 2: Each soft goal is then refined by using decomposition methods.

Decomposition can be based on :• General expertise/knowledge about security, flexibility etc.

• Domain-specific knowledge

• Project-specific knowledge – decided upon jointly by the stakeholders of the project

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Non-functional requirements analysis: Example (partial) result of flexibility soft goal decomposition for of nonfunctional requirements analysis for an office support system

✔

Access of otherstaff’s files

✔

Flexibility

✔Flexible work

patterns

✔

Access ofdatabase

✔

Sharing ofInformation

✔

Task switching

✔

Future Growth

✔

Separate PerformanceStandards

✔

Design forExtra Terminals

✔

Design forModularity

Flexibility soft goal decomposition

NFR Framework – 3

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NFR Framework – 4 Non-functional requirements analysis: Also involves finding lateral relationship the soft goals of individual soft goal trees

✔

Access of otherstaff’s files

✔

Flexibility

✔Flexible work

patterns

✔

Access ofdatabase

✔

Sharing ofInformation ✔

Task switching

✔

Future Growth

✔Separate PerformanceStandards

✔

Design forExtra Terminals

✔

Design forModularity

Flexibility soft goal decomposition and interference with softgoals belonging to different soft goal tree structures

Usability Performan

ce

Security

Security

Profitability

Maintainability

Performance

--+

--

+

+ -

-

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Advantages of NFR Framework

NFR are obtained by gathering knowledge about the domain for which a system will be built.

NFRF focuses on clarifying the meaning of non-functional requirements

NFRF provides alternatives for satisfying soft goals to the highest possible level, considering the conflicts between them.

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Cross-cutting requirements – 1 How do we deal with cross-cutting concerns in goals requirements and constraints? A sub-goal, concrete requirements, etc. can be involved in the satisfaction of more than one higher level goal representation.An agent in most cases is involved in executing a number of system behaviors.

Goal

Sub goals

Concrete requirements, design constraints, assumptions

Agents

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Cross-cutting requirements – 2 Cross cutting requirements and constraints come from several sources. Example: embedded systems, IS, COTS- Commercial, off-the-shelf)

Problem domain Solution Space

Proposed

solutionProblem definitio

n

Organisational context

Operational context

Requirements /Constraints

Requirements, Constraints, Problems and Solutions in RE

Market forces

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Cross-cutting requirements – 3

The cross cutting attribute results in requirements without clear distinct/atomic allocation to modules.Many non-functional requirements fall into this category.

Example:Performance is a factor of the system architecture and its operational environment. We cannot develop a performance module independent of other parts of a software system.

Such requirements are termed crosscutting (or aspectual) requirements. Examples of such properties include security, mobility, availability and real-time constraints.

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Cross-cutting requirements – 4

Aspects oriented requirements engineering is about identifying cross-cutting concerns early during requirements engineering and architecture design phase rather than during implementation. This involves four basic steps:

• Identify• Capture• Compose• Analyze.

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Cross-cutting requirements – 5

Aspects oriented requirements engineering

Example scenario: Consider a banking system with many requirements, include the following:

Requirement A1. Pay interest of a certain percent on each account

making sure that the transaction is fully completed and an audit history is kept.

2. Allow customers to withdraw from their accounts, making sure that the transaction is fully completed and an audit history is kept.

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Cross-cutting requirements – 6Central concerns revealed in requirement A:

• “pay interest,” “withdrawal,” “complete in full,” and “auditing”

• Of those concerns, “pay interest” and “withdrawal” are described in separate requirements.

• However, “complete in full” and “auditing” are each described in both requirements 1 and 2.

Main challenge in requirement A:• Concerns are scattered across the requirement set

• If we want to find out which transactions should be fully completed or audited, we must sift through the whole requirements set for references to transactions and auditing.

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Cross-cutting requirements – 7Attempt to rewrite requirement A to remove scattered concepts:

Requirement B1. Pay interest of a certain percent on each account.

2. Allow customers to withdraw from their accounts.

3. Make sure all transactions are fully completed.

4. Keep an audit history of all transactions.

Main challenge in requirement B:

• This rewriting introduces implicit tangling between the newly separated concerns (“auditing” and “complete in full”) and the other concerns (“pay interest” and “withdrawal”).

• You can’t tell, without an exhaustive search, which transactions the “complete in full” and “auditing” properties affect.

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Cross-cutting requirements – 8

Example scenario: The broadly scoped concerns are considered as aspects (i.e. “complete in full” and “auditing” properties)

Requirement C – Aspect Oriented (AO) solution

1Δ Pay interest of a certain percent on each account.2Δ Allow customers to withdraw from their accounts.3Δ To fully complete a transaction…3A List of transactions that must be fully completed: {1Δ, 2Δ}4Δ To audit…4A List of transactions that must leave an audit trial: {1Δ, 2Δ}

The AO solution is to make the impact explicit by modularizing aspects into two sections:

• one describes the requirements of the aspect concern itself (3Δ, 4Δ)

• one describes the breadth of its impact (3A, 4A).

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Cross-cutting requirements – 8Advantages of early aspects:

Captures the core or base concerns (“withdrawal” and “pay interest”): 1Δ, 2Δ

Captures cross-cutting concerns as aspects: 3Δ, 4Δ

Describes Impact requirements: a requirement describing the influence of one concern over other concerns: 3A, 4A

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Requirements for COTS – 1

• As the size and complexity of systems grow, the use of commercial off the shelf (COTS) components is being viewed as a possible solution.

• In this case requirements are constrained by the availability of suitable COTS component.

• Early evaluation of candidate COTS software products is a key aspect of the system development lifecycle.

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Requirements for COTS – 2

The impact of using COTS based components is expected to vary with the domain:

• For business applications a large, pervasive COTS product may be used to deliver one or more requirements (e.g., MS Office, Oracle, Netscape, etc.).

• For embedded real time or safety critical domains, the COTS components are expected to be small and require large amounts of glue code to integrate the COTS components with the rest of the system

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Requirements for COTS – 3Problems with COTS:

• An organizations have limited access to product’s internal design.

• The description of commercial packages is sometimes incomplete and confusing.

• Customers have limited chance to verify in advance whether the desired requirements are met.

• Most selection decisions are based on subjective judgments, such as current partnerships and successful vendor marketing.

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Requirements for COTS – 4Advantages of COTS:

• We get a product that has been tested many times by real-world users with consequent improvement in software quality.

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Requirements for COTS - example

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Requirements for outsourcing – 1

This is a management strategy by which an organization outsources/contracts out major, non-core functions to specialized, efficient service providers and third parties.It is a rapidly growing market all over the world.

• Onshore outsourcing: outsourcing a project within own country

• Offshore outsourcing:Includes outsourcing services offered by countries outside Europe, typically overseas

• Nearshore outsourcing:E.g., for Scandinavian countries nearshore might be Baltic countries

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Requirements for outsourcing – 2

Phases:• Selection: This is about selecting the subcontractor

and is synonymous to tendering.

• Monitoring: This phase starts with the signed contract and follows the subcontractor’s work till the product is delivered.

• Completion: It includes acceptance and installation of the product, and in many cases also the maintenance of the product over its lifetime

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Requirements for outsourcing – 3

Advantages:• Cost savings• Improving service delivery and quality (is

gaining in importance)• Keeping pace with technological innovation

Disadvantage:• Companies will lose control over business

process and in-house expertise.

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ConclusionThere are several approaches for identifying and handling

requirements that are inherently complex, interdependent and multi-faceted.

• Viewpoints aims to explicitly model the interest of various actors.

• Non-functional requirements framework focuses on modeling of soft goals and clarifying their meaning

• Early aspects focuses on identifying cross-cutting concerns in requirements at the early phase of a project lifecycle.

• There is additional requirements handling consideration when using COTS components, outsourcing or sub-contracting