TDT 4242

Torbjørn Skramstad

Requirements Specification and TestingAn introduction

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Institutt for datateknikk oginformasjonsvitenskap

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Challenges in Requirements Engineering

What is a requirement?•What a system must do (functional): System requirements•How well the system will perform its functions (non-functional): System quality attributes

definedoperationalcapabilities

businessneeds

satisfy

The RE process:

Ultimately:

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How the customer explained it

How the architects and developers understood it

What the customer really needed

Software specification in natural language

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Challenges in Requirements Engineering

Source: Benoy R Nair (IBS software services)

Importance of getting requirements right:1/3 budget to correct errors originate from requirements

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Challenges in Requirements Engineering

Source: Benoy R Nair (IBS software services)

Factors that make a software project challenging:

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Challenges in Requirements Engineering

Source: Benoy R Nair (IBS software services)

Why projects are cancelled:

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Requirements Development - 1Requirements Elicitation:The process of discovering the requirements for a system by communication with customers, system users and others who have a stake in the system development.

Requirements gathering techniques

• Systematic extraction of concrete requirements from high level goals

• Requirements quality metrics

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The Ariane 5 accident, 1996 Single root cause failure!

The ”bug”: attitude deviation stored as 2-byte integer (max value 65,535) in stead of 4-byte (max value 4,294,967,295)

SW module was reused from Ariane 4

Insufficient V&V of detailed requriements: larger attitude deviation tolerated in Ariane 5 than in Ariane 4

Ariane 5 production cost 10 years and $7 billion; luckily no victims because it was unmanned. 65,535 = 00000000 00000000 11111111 11111111

65,536 = 00000000 00000001 00000000 00000000

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Other software related accidents & incidents Accidents & incidents

Alarm flooding, power distribution failure, BP Grangemouth Scotland, 29th May - 10th June 2000

failure in a data bus + faulty logic in the software => engine power loss, Airbus A340-642, 2005

failed accelerometer + software bug => Faulty air speed metering, Boeing 777-200, 2005

Software bug => shutdown of radio communication between ATC and aircraft, 2004. disrupted 800 flights

Safety related software flaws => recall of 200,000 pacemakers in 1990-2000

radiotherapy machines attacked by computer viruses, 2005

Buggy software in corporate computer + connection between corporate and control systems networks => shutdown of nuclear power plant, USA 2008

In general, most software failures are actually ”bugs” in the detailed requirement specifications, i.e. poor understanding of the very detailed requirements

Korean Air 747 in Guam, 200 deaths (1997): incorrect configuration of ”minimum altitude” warning system

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Requirements Development – 3 Effects of Inadequate Requirements development – Airbus:

Requirement: Reverse thrust may only be used, when the airplane is landed.

Translation: Reverse thrust may only be used while the wheels are rotating.

Implementation: Reverse thrust may only be used while the wheels are rotating fast enough.

Situation: Rainstorm – aquaplaning

Result: Crash due to overshooting the runway!

Problem: erroneous modeling in the requirement phase

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Problem world and machine solution

The problem to be solved is rooted in a complex organizational, technical or physical world.

• The aim of a software project is to improve the world by building some machine (HW/SW) expected to solve the problem.

• Problem world and machine solution each have their own phenomena while sharing others.

• The shared phenomena defines the interface through which the machine interacts with the world.

E-commerce world

Requirements engineering is concerned with the machine’s effect on the surrounding world and the assumption we make about that world.

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Formulation of requirements statements

Statement scope:

• Phenomenon of train physically moving is owned by environment. It cannot be directly observed by software phenomenon

• The phenomenon of train measured speed being non-null is shared by software and environment. It is measured by a speedometer in the environment and observed by the software.

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Two types of requirements statements

• Descriptive statements: state properties about the system that holds regardless of how the system behaves. E.g. if train doors are open, they are not closed.

• Prescriptive statements: States desirable properties about the system that may hold or not depending on how the system behaves

• Need to be enforced by system components• E.g. train doors shall always remain closed when the

train is moving

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Formulation of system requirement

A prescriptive statement enforced by the software-to-be.

• Possibly in cooperation with other system components• Formulated in terms of environment phenomena

Example: All train doors shall always remain closed while the train is moving

In addition to the software-to-be we also require the cooperation of other components:

• Train controller being responsible for the safe control of doors.

• The passenger refraining from opening doors unsafely

• Door actuators working properly

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Formulation of software requirement

A prescriptive statement enforced solely by the software-to-be. Formulated in terms of phenomena shared between the software and the environment.

The software “understands” or “senses” the environment through input data

Example: The doorState output variable shall always have the value ‘closed’ when the measuredSpeed input variable has a non-null value

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Domain properties

A domain property:• Is a descriptive statement about the problem world• Should hold invariably regardless of how the system behaves• Usually corresponds to some physical laws

Example:

A train is moving if and only if its physical speed is non-null.

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Goal orientation in requirements engineering – 1

A goal is an objective that the system under consideration shall achieve.

– Ranges from high-level strategic to low-level technical concerns over a system

– System consist of both the software and its environment. Interaction between active components, i.e. devices, humans, software etc. also called Agents

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Goal orientation in requirements engineering – 2

Goals can be stated at different levels of granularity:– High-level goal: A goal that requires the cooperation of

many agents. They are normally stating strategic objective related to the business, e.g. “the system’s transportation capacity shall be increased by 50%”

– Requirement: A goal under the responsibility of a single agent in the software-to-be.

– Assumption (expectation): A goal under the responsibility of a single agent in the environment of the software-to-be. Assumptions cannot be enforced by the software-to-be

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Goal orientation in requirements engineering – 3

We shall make software for a toll road gate. The system’s transmitter sends a signal to an approaching car which returns the car-owner’s id. The payment is invoiced to the car-owner.

• Assumption: “The car returns the car-owner’s id”. This is the responsibility of a single agent in the environment of the software-to-be• Requirement: “The system’s transmitter sends a signal to an approaching car”. This is the responsibility of a single agent in the software-to-be

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Goal statement typology

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Goal types

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Behavioral goal specialization

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Goal categorization – 1

Goal categories are similar to requirements categories:

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Goal categorization – 2

Functional goal: States the intent underpinning a system service

• Satisfaction: Functional goals concerned with satisfying agent request

• Information: Functional goals concerned with keeping agents informed about important system states

• Stimulus-response: Functional goals concerned with providing appropriate response to specific event

Example: The on-board controller shall update the train’s acceleration to the commanded one immediately on receipt of an acceleration command from the station computer

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Goal categorization – 3

Non-functional goal: States a quality or constraint on service provision or development.

Accuracy goal: Non-functional goals requiring the state of variables controlled by the software to reflect the state of corresponding quantities controlled by environment agent

E.g: The train’s physical speed and commanded speed may never differ by more than X miles per hour

Soft goals are different from non-functional goals. Soft goals are goals with no clear-cut criteria to determine their satisfaction.

E.g: The ATM interface should be more user friendly

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Goal refinement

A mechanism for structuring complex specifications at different levels of concern.

A goal can be refined in a set of sub-goals that jointly contribute to it.

Each sub-goal is refined into finer-grained goals until we reach a requirement on the software and expectation (assumption) on the environment.

NB: Requirements on software are associated with a single agent and they are testable

Assumptions / Expectations example

The notation above shows that we expect the measurement equipment to measure a value corresponding to the reality.

It is not something we, at this level, can do anything about.

It will, however, at some point in time, be e.g. a sensor requirement

Measurement = reality

Measurement equipment

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Goal refinement: Example

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Goal refinement tree – 1

Refinement links are two way links: One showing goal decomposition, the other showing goal contribution

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Goal refinement tree – 2

Goal feature annotation

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Where do the goals come from?

We get goals from:

• Preliminary analysis of the current system.

• Systematically by searching intentional keywords in documents provided, interview transcripts etc. E.g. ‘objective’, ‘purpose’, ‘in order to’.

• Iterative refinement and abstraction of high-level goals: By asking the how and why question. Results in a goal refinement tree

• Approaches: KAOS – Goal driven requirements acquisition. http://www.info.ucl.ac.be/~avl/ReqEng.html

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Goals – Summary

Goals can be defined at different levels of abstraction

There are two types of goals: Behavioral or soft goal

There are several categories of goals, e.g. • Functional and non-functional• Goal refinement provides a natural mechanism

for structuring complex specifications at different levels of concern:

• Goal refinement tree

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Requirements quality metrics – 1

Qualitative Goal-Requirements tracing:An approach to requirements

refinement/abstraction that makes it less likely to generate trace links that are ambiguous, inconsistent, opaque, noisy, incomplete or with forward referencing items

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Requirements quality metrics – 2

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Requirements quality metrics – 3Forward Referencing: Requirement items that make

use of problem world domain features that are not yet defined.E, C and D need to be mapped to a requirement item

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Requirements quality metrics – 4Opacity: Requirement items for which rational or

dependencies are invisible.

Multiple unrelated concept mapping. A is not related to B

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Requirements quality metrics – 5

Noise: Requirement items that yield no information on problem world features. X refers to a concept undefined in the domain

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Requirements quality metrics – 6Completeness: The needs of a prescribed

system are fully covered by requirement items without any undesirable outcome.

No requirement item mentions the goal concept Z

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RQM – Steam boiler example – 1

P

ControlUnit Feed

water

230 V AC

Process Steam

ControlUnit

To air

P

LT

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RQM – Steam boiler example – 2

The steam boiler has three sensors – a temperature senor (T) a pressure sensor(P) and a water level sensor (L)

The water level sensor gives info to the pump control unit

The temperature sensor and the pressure sensor gives info to the heating unit.

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Examples • A requirement that contains a reference to a

sensor is ambiguous since there are three sensors involved.

• The requirement “The pump control unit will read a value from the temperature sensor” is inconsistent since the temperature sensor is not used by the pump control unit.

• The requirement “The collector tray shall have a capacity of at least 500 liters” is opaque since there are no other references to a collector tray and no rational as to why we need one.

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Examples • The requirement “The operator’s chair shall

have leather upholstery” is noise. There is no reason why such a requirement is needed for the control system.

The line between noise and opacity can some times be quite thin. In both cases, the requirement can sometimes be made into a real requirement if it is supplied with the proper rationale.

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Examples • There is a requirement stating that the

pump controller gets the water level from a water level sensor

• There is no requirement specifying that the boiler vessel shall be equipped with a water level sensor

In this case, the requirements set is incomplete.

Quality metrics on a requirements set provides useful understanding, tracking and control of requirements improvement process.

Requirements quality metrics

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Requirements quality metrics in use – 1 In order to use the quality metrics for

requirements elicitation, we will need an ontology.

An ontology is a structured description of the components that we expect to be present for system of a certain type.

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Requirements quality metrics in use – 2

……Water-level……

Boiler-tankFeeding-pumpNon-return valveTank

Max-limit

Pump

infers

has

is controlledby

Min. water levelMax. water levelWater-level indicator

OnOff

Control-system

has-state

infers

is coupledwith

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Requirements quality metrics in use – 3 Using an ontology we can check that e.g.,

• All parts of the system or subsystem are included in the requirements – completeness.

• Terms belonging to one subsystem are not used for other subsystems - consistency.

• Only terms found in the ontology are accepted – no ambiguity.

• If one part of a subsystem is included, the rest of the subsystem is also included – no opacity.

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Requirements quality metrics in use – 4 Using the quality metrics approach for requirements elicitation will in most case require a tool containing the relevant domain ontologies.

E.g., if we have a requirement about the pump but nothing about how it is controlled – control system – or that it can be turned on and off – states – we will get a warning about low completeness.