Software Engineering Design &
modelling
Activity Diagram
Interaction Overview Diagram
Introduction

This chapter looks at how we can use the UML
to model the details of the work that objects do,
both internally and in conjunction with other objects
 to model the details of computational processes
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Chapter outline: Activity diagram
 Processes and threads
 Interaction Overview
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Activity Diagrams
Topic Outline
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Activity diagram
Definition
 Properties
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Activity and action
 Transition
 Object flow
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Modelling activity diagram
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Introduction
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An activity diagram is used to model the
dynamic aspects of a system
Activity diagrams can be attached to
use cases
 communication
Activity diagrams are used to model use cases and
complex object workflows
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Activity diagrams allow the readers to see the system’s
execution, and how it changes direction based upon
different conditions and stimuli.
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In this case, activity diagrams are used to model workflow for
use cases.
For the purpose of this lecture, we will see how activity
diagrams can be used to take your use case to the next level..
In the context of use cases, activity diagrams are used
to: 


elaborate
identify the pre- and post-conditions
discover
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Definition
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An activity diagram is essentially a flowchart, showing
flow of control from activity to activity
It involves
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modelling the sequential (and possibly concurrent) steps in a
computational process
modelling the flow of an object as it moves from state to
state at different points in the flow of control
Activity diagrams emphasize the flow of control from
activity to activity

Whereas, interaction diagram emphasize the flow of control
from object to object
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Properties
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Activity diagram commonly contains: Activity states and action states
 Transitions
 Swimlanes
 Object flow
 may also contain notes and constraints
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Activity and action states
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An activity is something that an object performs on an
ongoing basis
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It is non-atomic, which means that it can be interrupted
It can also be decomposed into other activities
Activities ultimately result in some action
An action is a set of executable computations that
results in a change in the state of the system, or the
return of a value to some objects, or both.
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It is atomic, meaning it cannot be interrupted
It cannot be decomposed
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It is assumed that an activity lasts for some duration
of time, whereas an action generally takes an
insignificant amount of time
The UML notation is the same for an activity and an
action : a lozenge
Calculate shipping
cost
Update customer
preferences
shipCost = countyCode
* subTotal
Ship order
isFulfilled = True
activity
action
Example of some activities and actions associated with the Online Bookstore
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Transitions
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When the action or activity of a state completes, flow
of control passes immediately to the next action or
activity states
Transitions are used to show the path from one
action or activity state to the next action or activity
state
A flow of control has to start and end someplace

this can be specified by an initial state (a solid ball) and a
stop state ( a solid ball inside a circle)
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In the UML, a transition is presented as a simple
directed line
initial state
Activity / Action
transition
Activity / Action
stop state
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A transition between activities or actions can
be modelled: sequentially
 by using branching and merging
 by using forking and joining
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Branching and merging
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A branch is a decision point at which there are two
or more possible paths of flow of control
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Each possible path out of a branch has a guard condition,
which is a boolean expression that must be evaluated to
true in order for the branch to be taken.
Optionally, an “else” branch can also be inserted to
account for a “none of the above” situation
A guard condition is shown within a square bracket near
a path out of the branch
Similar to conditional state
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A merge is a point at which two or more branched paths
come together.
The UML represents both a branch and a merge with a
diamond shape.
Compute subtotal
[ship outside of US]
branching
Look up country code
[ship within of US]
shipCost = regionPercent
* subTotal
shipCost = countryCode *
subTotal
merge
Compute new subtotal
Example of branching and merging for the
scenario Process Customer’s Order of the
Online Bookstore
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Forking and Joining
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A fork is a splitting of a flow of control into two or
more flows of control, each of which operates
independently of, and concurrently with, the others.
A join is a synchronization of two or more flows of
control into one flow
In UML, the symbol that represents both forks and
joins is a long, thin, black rectangle called a
synchronization bar.
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Example of forking and joining associated with the Online Bookstore
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Swim-lanes
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Group of activities and actions on an activity
diagram can be grouped together with the help of
vertical lines.
In the UML, each group is called a swim-lane, which
is simply a lane down which you can track activities
and actions associated with a particular part of an
organization.
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Each swim-lane has a name unique within its diagram
The boundaries are simply conceptual and transitions
may cross lanes
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Receive order
Shipping
Retrieve books
from inventory
Marketing
Update customer
preferences
Accounting
Post General
Ledger entries
Calculate shipping
costs
swimlane
Example of swimlanes
Ship order
swimlane
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Object Flow
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
Objects may be involved in the flow of
control associated with an activity diagram.
These objects may be specified in the activity
diagram by using object flows

Object flows are simply dependencies that show
the details of how the object(s) involved in the
various activities/actions are specifically affected
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The UML provides two ways to show objects
within object flows, involving: showing the new values for one or more
attributes, using the notation (attribute = value)
 showing a “state” value in square brackets
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An example of object flows
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Modelling activity diagrams

Activity diagrams can be used in two ways: 
To model a workflow
focus on activities as viewed by the actors that
collaborate with the system
 Workflows are used to visualize, specify, construct, and
document business processes that involve the system
being developed

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To model an operation

used to model the details of a computation
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Steps:
1.
2.
Identify the activities and actions involved (to be rendered as
activity and action states in the activity diagram)
Identify activities that can be grouped together in order to
form swimlanes

3.
Render the transitions that connect these activity and action
states
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
4.
For each swimlanes identified, give an appropriate name
Start with sequential flows
Use branching and merging, or forking and joining, or both, if and
where necessary
Starting from the initial state, draw the activity diagrams until
the final state

If there are important objects that are involved, include them in the
activity diagram
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Receive order
Shipping
Marketing
Retrieve books
from inventory
Calculate shipping cost
Update customer
preferences
Accounting
Post General
Ledger entries
Compute subtotal
[ship outside of US]
Look up country
code
[ship within of US]
shipCost = regionPercent
* subTotal
shipCost = countryCode
* subTotal
Compute new subtotal
Example of an activity diagram associated with the
Process Customer’s Order scenario of the Online
Bookstore
Ship order
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A simple example

Develop an activity diagram based on the following
narrative.
The purpose of the Open Access Insurance System is to
provide automotive insurance to car owners. Initially,
prospective customers fill out an insurance application,
which provides information about the customer and his or
her vehicles. This information is sent to an agent, who
sends it to various insurance companies to get quotes for
insurance. When the responses return, the agent then
determines the best policy for the type and level of
coverage desired and gives the customer a copy of the
insurance policy proposal and quote.
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
Steps:
1.
2.
Identify the activities and actions involved (to be rendered as
activity and action states in the activity diagram)
Identify activities that can be grouped together in order to
form swimlanes

3.
Render the transitions that connect these activity and action
states


4.
For each swimlanes identified, give an appropriate name
Start with sequential flows
Use branching and merging, or forking and joining, or both, if and
where necessary
Starting from the initial state, draw the activity diagrams until
the final state

If there are important objects that are involved, include them in the
activity diagram
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
Steps: 1)
Identify the activities and actions involved
The purpose of the Open Access Insurance System is to
provide automotive insurance to car owners. Initially,
prospective customers fill out an insurance application, which
provides information about the customer and his or her
vehicles. This information is sent to an agent, who sends it to
various insurance companies to get quotes for insurance.
When the responses return, the agent then determines the best
policy for the type and level of coverage desired and gives the
customer a copy of the insurance policy proposal and quote.
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
Actions and activities identified

Prospective customers fill out an insurance application

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This information is sent to an agent…

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Agent receives the quotes
… agent then determines the best policy

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Agent receives the application
Agent sends it to various insurance companies to get quotes for
insurance
Insurance companies come up with/develop quotes
When the responses return …


The application is sent to an insurance agent
… who sends it to various insurance companies to get quotes
for insurance


Prospective customers fill out form
The agent decides best policy
… and gives the customer a copy of the insurance policy
proposal and quote.


The agent sends the customer a copy of the insurance policy proposal
and quote.
Customer receives proposal and quotes
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2)
Identify activities that can be grouped together in order to
form swimlanes. For each swimlanes identified, give an
appropriate name

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


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Here, the activities and actions can be grouped together based on who performed
those activities/actions
Prospective customers fill out application
The application is sent to an insurance agent
Agent receives the application
Agent sends it to various insurance companies to get quotes for
insurance.
Insurance companies come up with/develop quotes
Agent receives the quotes,
The agent decides best policy
The agent sends the customer a copy of the insurance policy proposal
and quote.
Customer receives proposal and quotes
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
Group of activities and actions according to swimlanes: 
Prospective customer

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

Insurance agent




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Prospective customers fill out application
The application is sent to an insurance agent
Customer receives proposal and quotes
Agent receives the application
Agent sends it to various insurance companies to get quotes for insurance.
Agent receives the quotes
The agent decides best policy
The agent sends the customer a copy of the insurance policy proposal and
quote.
Insurance company

Insurance companies come up with/develop quotes
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3)

Render the transitions that connect these activity and
action states (sequential flows)
Prospective customer
1. Fill out application
2. Send in application
9. Receives proposal and quotes

Insurance agent
3. Receives application
4. Sends to insurance companies.
6. Receives quotes
7. Decides best policy
8. Sends policy to prospective customer.

Insurance company
5. Develop quotes
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Case Study: On-Line
Bookstore
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On-line Bookstore is a web application that can be
accessed by the store’s registered customer, whereby
each customer can order books, review one or more
books sold in the book store, and sell used books to
other customers. Before performing any one of these
transactions, the customer must first log-in into the
system using their user id and password kept in their
account.
Problem: Draw the activity diagrams for the above
system
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On-line Bookstore System
Register
<<extend>>
(CustID)
Customer
Check out
Order books
<<include>>
Sell used books
<<include>>
Log-in
<<include>>
Use Case Context
Diagram
Review books
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Register
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Log-in
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Order
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Check out
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Sell used books
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Process, threads &
active objects
Introduction

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In the real-world, events may happen and things may
take place all at the same time (concurrently).
Example: 
In a high rise building, there exists quite a number of tenants,
each following their own agenda. But all must pass through a
limited set of entrances. All must share the same bank of
elevators. All must share the same electrical, water and
parking facilities. If they are to work together optimally, they
have to communicate and synchronize their interactions
properly.
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Therefore, when we model a system of the
real-world, we must take into account its
process view.
The process view encompasses the threads
and processes that form the system’s
concurrency and synchronization mechanism.
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Flow of control
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There are two types of flow: - sequential and
concurrent flow.

In a purely sequential system, there is only one flow
of control.
meaning that one thing, and one thing only, can take place
at a time
 Example: - When a sequential program starts, control is
rooted at the beginning of the program and operations are
dispatched one after another. No two operation can occur
at the same time.
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
In a concurrent system, there is more than one flow of
control
meaning that more than one thing can take place at a time
 Here, there are multiple simultaneous flow of control, each
rooted at the head of an independent process or thread
 Because more than one flow of control can take place at a
single time, one flow may interfere with another flow of
control. This may cause concurrent systems to fail.
 Therefore, there must exists a form of synchronization to
avoid this kind of situation.
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In the UML, each independent flow of control is
modelled as an active object.
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Active objects, processes and threads
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An active object is a process or thread that can
initiate control activity.

A process is a heavyweight flow that can execute
concurrently with other processes
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
meaning that it is a thing known to the operating system
and runs in an independent address space
A thread is a lightweight flow that can execute
concurrently with other threads within the same
process

meaning that it generally runs inside a process
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An active object is an instance of an active class.
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An active class, therefore, is a class whose instances are
active objects.
An active class represents an independent flow of control,
whereas a plain class embodies no such flow.
An active class shares the same properties as all other
classes
Active classes can be used to model common families of
processes and threads.
Active objects can communicate with one another by
passing messages

here, message passing must be extended to include certain
concurrency semantics in order to synchronize the
interactions among independent flows
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Both active objects and active classes is represented
in the UML using the class box, but with a thicker
border.
Active class
Active object
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In a system with both active and passive objects,
there are four possible combinations of interaction
that must be considered: 

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A message may be passed from one passive object to
another
A message may be passed (synchronously or
asynchronously) from one active object to another
A message may be passed from an active object to a
passive object
A message may be passed from a passive object to an
active object
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By modelling concurrent systems with active objects, each
independent flow of control can be given a name.
Furthermore, when an active object is created, the associated
flow of control is started; when the active object is
destroyed, the associated flow of control is terminated
We can model the collaboration of active and passive objects
by using interaction diagrams.
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When we model interactions that involve multiple flows of control,
it is important to identify the process or thread that sent a particular
message.
In the UML, we can distinguish one flow of control from another by
prefixing a message’s sequence number with the name of the process
or thread that sits at the root of the sequence.
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Example: c : CNNNewsFeed
c1 : postBreaingStory( )
s1 : postValue( )
s : StockTicker
a1 : Analyst
s2 : postAlert( )
m1 : postValue( )
m : AlertManager
t : TradingManager
i2 : postAlert( )
i : IndexWatcher
a2 : Analyst
i1 : postValue( )
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