This presentation is about a lecture I gave within the "Software Modeling" course of the Computer Science bachelor program, of the Vrije Universiteit Amsterdam. http://www.ivanomalavolta.com

1. Software and Services research group (S2)
Department of Computer Science, Faculty of Sciences
Vrije Universiteit Amsterdam
VRIJE
UNIVERSITEIT
AMSTERDAM
Requirements engineering with UML
Software modeling (401016) – 2016/2017
Ivano Malavolta
i.malavolta@vu.nl

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Announcement
The template for Deliverable 1 will be available today on
BlackBoard!
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Roadmap
• Introduction to UML
• What is UML?
• Main characteristics of UML
• UML diagrams
• Requirement engineering
• Use case diagrams
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What is UML?
• In the 80s there were multiple OO approaches
• each approach had its own notation
• then Rational Inc. (now IBM)
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Booch notation Jacobson‘s OOSE Rumbaugh's Technique

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What is UML?
• UML = Unified Modeling Language
• De facto standard software design language
• Developed by OMG
• A “Swiss Army Knife” of notations
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Why UML in this course?
The most used language for modeling software
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34 job postings requiring UML in Amsterdam
(as of last week)

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Who uses UML?
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Main characteristics of UML
• It is not tied to any development process
• à waterfall, agile, whatever…
• Can be used across the whole life cycle
• promotes iterative refinement of models
• General purpose
• it can be used for modeling a mobile app, but also a satellite
• It has different representations:
• graphical
• textual
• others…
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Main characteristics of UML
• It is comprehensive
• all parts of a system can be described using UML
• It is scalable
• you can zoom in with additional details when needed
• Originally intended for descriptive models
• Now it also supports prescriptive models
• models execution
• code generation
• but more importantly…
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Main characteristics of UML
UML is a formal modeling language
à all its concepts have a well-defined meaning
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Modeling with code Informal model
UML model

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Where are the “meanings” of UML concepts?
The UML superstructure
640 pages like this! à
Don’t read it, use it only as a
manual in case of doubts
http://www.omg.org/spec
/UML/2.5/
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UML diagrams
A UML model is represented graphically by diagrams
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UML structure diagrams
• Emphasize the static
description of the elements
of the system being modeled
• ex: student submission
system à
• Structural elements may
have an associated behavior
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UML behavioural diagrams
• Behavior = the direct
consequences of an action of at
least one object
• It affects how the states of
objects change over time
• Behavior can either be
• specified through the actions
of a single object
•
• result from interactions
between multiple objects à
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Submission

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Which diagrams you will see in this course
• Use case diagram
• to specify the basic functionality of a software system
• aka requirements
• Class diagram
• to define which objects or which classes are involved in the
realization of this functionality
• State machine diagram
• to define the intra-object behavior
• Sequence diagram
• specifies the inter-object behavior and communication
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In your project you can use additional UML diagrams
à BONUS in the final grade

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Models != diagrams
• A UML model contains everything related to your system
• it is complete
• Diagrams are just “windows” on your model
• technically they can be considered as projections of the
same model
• a particular diagram will show some parts of your model but
not necessarily everything (recall abstraction?)
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represented
by
System Model
Class diagram
Sequence
diagram
State machine
diagram

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Models and diagrams in Papyrus
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Diagram creation

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Models and diagrams in Papyrus
18The model
The diagrams

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Requirements engineering
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Requirements engineering
• The process of establishing
• the services that a customer requires from a system
• the constraints under which it operates and is developed
• A requirement may range between
• a high-level abstract statement of a service
• Example: all the robots must avoid obstacles autonomously
• a detailed mathematical functional specification
• Example: each robot must communicate its position to the
central station every 1 second
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Functional and non-functional requirements
Functional requirements
a. Services the system should provide
b. How the system should react to particular inputs
c. How the system should behave in particular situations
d. May state what the system should not do
Non-functional requirements
a. Constraints on the services or functions offered by the system
I. example: timing constraints, constraints on the development
process, standards, etc.
b. Often apply to the system as a whole rather than individual
features or services
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Functional requirements
• Precise
• Ambiguous requirements may be interpreted in different
ways by developers and users à problems
• Complete
• They should include descriptions of ALL facilities required
• Consistent
• There should be no conflicts or contradictions in the
descriptions of the system facilities
• In UML they are represented using Use case diagrams
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Non-functional requirements
• System properties and constraints
• e.g. reliability, response time and storage requirements
• Constraints are I/O device capability, system
representations, etc.
• Non-functional requirements may be more critical than
functional requirements
• e.g., safety requirements
• 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 your system to minimize
communications between robots
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Types of non-functional requirements
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Performance
requirements
Space
requirements
Usability
requirements
Efficiency
requirements
Dependability
requirements
Security
requirements
Regulatory
requirements
Ethical
requirements
Legislative
requirements
Operational
requirements
Development
requirements
Environmental
requirements
Safety/security
requirements
Accounting
requirements
Product
requirements
Organizational
requirements
External
requirements
Non-functional
requirements

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Robotic systems MUST be dependable
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Ways of writing requirements specifications
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Notation Description
Natural language The requirements are written using numbered sentences in natural language.
Each sentence should express one requirement.
Structured natural
language
The requirements are written in natural language on a standard form or
template. Each field provides information about an aspect of the requirement.
Design description
languages
This approach uses a language like a programming language, but with more
abstract features to specify the requirements by defining an operational model
of the system. This approach is now rarely used although it can be useful for
interface specifications.
Graphical
notations
Graphical models, supplemented by text annotations, are used to define the
functional requirements for the system; UML use case and sequence diagrams
are commonly used.
Mathematical
specifications
These notations are based on mathematical concepts such as finite-state
machines or sets. Although these unambiguous specifications can reduce the
ambiguity in a requirements document, most customers don’t understand a
formal specification. They cannot check that it represents what they want and
are reluctant to accept it as a system contract

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Natural language specification
• Requirements are written as natural language sentences
• Used for writing requirements because it is expressive,
intuitive and universal.
• These requirements can be understood by users and
customers
• Guidelines:
• Invent a standard format and use it for all requirements
• Use language in a consistent way
• Use “shall” for mandatory requirements, “should” for desirable
requirements
• Use text highlighting to identify key parts of the requirement
• Avoid the use of computer jargon
• Include an explanation (rationale) of why a requirement is
necessary
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Example
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R1. The system shall measure the blood sugar and deliver insulin, if required,
every 10 minutes. (Changes in blood sugar are relatively slow so more
frequent measurement is unnecessary; less frequent measurement could lead
to unnecessarily high sugar levels.)
R2. The system shall run a self-test routine every minute with the conditions to
be tested and the associated actions defined in Table 1. (A self-test routine can
discover hardware and software problems and alert the user to the fact the
normal operation may be impossible.)

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Requirement validation checklist
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• Validity
• Does the system provide the functions which best support
the customer’s needs?
• Consistency
• Are there any requirements conflicts?
• Completeness
• Are all functions required by the customer included?
• Realism
• Can the requirements be implemented given available
budget and technology
• Verifiability
• Can the requirements be checked?
I will use it when grading your project

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Use case diagrams
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Contents
• Introduction
• Use cases
• Actors
• Relationships between use cases and actors
• Relationships between use cases
• Relationships between actors
• Description of use cases
• Best practices
• Typical errors
• Notation elements
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Introduction
Use case diagrams express the expectations of the
customers/stakeholders
§ essential for a detailed design
The use case diagram is used during the entire analysis and
design process
We can use a use case diagram to answer the following
questions:
§ What is being described? (The system)
§ Who interacts with the system? (The actors)
§ What can the actors do? (The use cases)
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Example: Student Administration System
System
(what is being described?)
§ Student administration system
Actors
(who interacts with the system?)
§ Professor
Use cases
(what can the actors do?)
§ Query student data
§ Issue certificate
§ Announce exam

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Use Case
• Describes functionality expected from the system under
development
• Provides tangible benefit for one or more actors that
communicate with this use case
• Set of all use cases describes the functionality that a system
shall provide
• Alternative notations:

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Actor (1/3)
Actors interact with the system
§ by using use cases,
i.e., the actors initiate the execution of use cases
§ by being used by use cases,
i.e., the actors provide functionality for the execution of use
cases.
Actors represent roles that users adopt
§ Specific users can adopt and set aside multiple roles
simultaneously
Actors are not part of the system, i.e., they are outside of the
system boundaries
Alternative notations:

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Actor (2/3)
Usually user data is also administered within the system. This
data is modeled within the system in the form of objects and
classes.
Example: actor Assistant
§ The actor Assistant interacts with the system Laboratory
Assignment by using it
§ The class Assistant describes objects representing user data
(e.g., name, ssNr, …).

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Actor (3/3)
Human
§ E.g., Student, Professor
Non-human
§ E.g., E-Mail Server
Primary: has the main benefit of the execution of the use case
Secondary: receives no direct benefit
Active: initiates the execution of the use case
Passive: provides functionality for the execution of the use case
Examples:
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Non-human
Secondary
Passive
Human
Primary
Active
Human
Primary
Active
Human
Secondary
Active

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Relationships between Use Cases and Actors
• Actors are connected with use cases via solid lines
(associations)
• Every actor must communicate with at least one use case
• An association is always binary
• Multiplicities may be specified

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The behavior of one use case (included use case) is ALWAYS
integrated in the behavior of another use case (base use case)
Example:
Relationships between Use Cases
«include» - Relationship
Base use case
requires the behavior of the included use
case to be able to offer its functionality
Included use case
may be executed on its own

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Relationships between Use Cases
«extend» - Relationship
• The behavior of one use case (extending use case) may be
integrated in the behavior of another use case (base use case)
but does not have to
• Both use cases may also be executed independently of each
other
• A decides if B is executed
• Extension points define at which point the behavior is
integrated
• Conditions define under which circumstances the behavior is
integrated
Base use case
Extending use case

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Relationships between Use Cases
«extend» - Relationship: Extension Points
• Extension points are written directly within the use case
• Specification of multiple extension points is possible
• Example:

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Relationships between Use Cases
Generalization of Use Cases
Use case A generalizes use case B.
B inherits the behavior of A and may
either extend or overwrite it.
B also inherits all relationships from A.
B adopts the basic functionality of A but
decides itself what part of A is executed or changed.
A may be labeled {abstract}
§ Cannot be executed directly
§ Only B is executable
Example:
Base use case
Sub use case

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Relationships between Actors
Generalization of Actors
Actor A inherits from actor B
A can communicate with X and Y
B can only communicate with Y
Multiple inheritance is permitted
Abstract actors are possible
Example:
Super-actor
Sub-actor
Professor AND Assistant needed
for executing Query student data
Professor OR Assistant needed
for executing Query student data

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Description of Use Cases
Structured approach
§ Name
§ Short description
§ Precondition: prerequisite for successful execution
§ Postcondition: system state after successful execution
§ Error situations: errors relevant to the problem domain
§ System state on the occurrence of an error
§ Actors that communicate with the use case
§ Trigger: events which initiate/start the use case
§ Standard process: individual steps to be taken
§ Alternative processes: deviations from the standard process
[A. Cockburn: Writing Effective Use Cases, Addison Wesley,
2000]

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Description of Use Cases - Example
Name: Reserve lecture hall
Short description: An employee reserves a lecture hall at the university for an event.
Precondition: The employee is authorized to reserve lecture halls.
Postcondition: A lecture hall is reserved.
Error situations: There is no free lecture hall.
System state in the event of an error: The employee has not reserved a lecture hall.
Actors: Employee
Trigger: Employee requires a lecture hall.
Standard process: (1) Employee logs in to the system.
(2) Employee selects the lecture hall.
(3) Employee selects the date.
(4) System confirms that the lecture hall is free.
(5) Employee confirms the reservation.
Alternative processes: (4’) Lecture hall is not free.
(5’) System proposes an alternative lecture hall.
(6’) Employee selects alternative lecture hall and confirms the
reservation.

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Best Practices
Identifying Actors
• Who uses the main use cases?
• Who needs support for their daily work?
• Who is responsible for system administration?
• What are the external devices/(software) systems with which
the system must communicate?
• Who is interested in the results of the system?

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Best Practices
Identifying Use Cases
• What are the main tasks that an actor must perform?
• Does an actor want to query or even modify information
contained in the system?
• Does an actor want to inform the system about changes in
other systems?
• Should an actor be informed about unexpected events within
the system?

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Best Practices
Typical Errors To Avoid (1/5)
Use case diagrams do not model processes/workflows!

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Best Practices
Typical Errors To Avoid (2/5)
Actors are not part of the system, hence, they are positioned
outside the system boundaries!

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Best Practices
Typical Errors To Avoid (3/5)
§ Use case Issue information needs EITHER one actor
Assistant OR one actor Professor for execution
ü

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Best Practices
Typical Errors To Avoid (4/5)
Many small use cases that have the same objective may be
grouped to form one use case
ü

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Best Practices
Typical Errors To Avoid (5/5)
The various steps are part of the use cases, not separate use
cases themselves! -> NO functional decomposition
ü

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Name Notation Description
System
Boundaries between the system
and the users of the system
Use case Unit of functionality of the system
Actor Role of the users of the system
Notation Elements (1/2)

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Name Notation Description
Association
Relationship between use cases
and actors
Generalization
Inheritance relationship between
actors or use cases
Extend
relationship
B extends A: optional use of use
case B by use case A
Include
relationship
A includes B: required use of use
case B by use case A
Notation Elements (2/2)

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Exercise
Problem: flight booking system (FBS)
A travel agency needs to manage flight bookings for its customers:
1) Airline companies offer various flights. It is the airline itself that decides to
open and close the bookings for each flight, and that communicates it to the
travel agencies.
2) A customer can book multiple flights and for different passengers.
3) A booking concerns a single flight and a single passenger. A booking can be
opened, and then cancelled or confirmed.
4) When confirmed, the tickets are issued and delivered to the customer.
5) A flight has a departure airport and an arrival airport. A flight has a departure
day and time, and an arrival day and time.
6) A flight may have stopovers in airports; a stopover has an arrival time and a
departure time.
7) Each airport serves one or more cities.
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Legenda
Red: candidate actors
Blue: candidate use cases

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What this lecture means to you?
• UML = general purpose modeling language
• tailored to object-oriented software systems
• 1 UML model, many diagrams
• Requirements
• functional vs non-functional
• Functional = the WHAT
• text + use case diagrams + use case descriptions
• Non-functional = the HOW
• text + rationale
• Use case diagrams rules and best practices
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Readings
• UML@Classroom: An Introduction to Object-Oriented
Modeling” – chapters 2 and 3
• Learning UML 2.0 – chapters 1 and 2
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