Visual Paradigm Handbook

Sequence Diagram

Sequence Diagram
Written Date : September 02, 2016

Both sequence diagrams and collaboration diagrams are kinds of interaction diagrams. Interaction
diagrams address the dynamic view of a system. A sequence diagram is an interaction diagram that
emphasizes the time-ordering of messages. It depicts the objects and classes involved in the scenario
and the sequence of messages exchanged between the objects needed to carry out the functionality
of the scenario. Typically, you'll use one sequence diagram to specify a use case's main flow, and
variations of that diagram to specify a use case's exceptional flows.

Object
In the UML, an object in a sequence diagram is drawn as a rectangle containing the name of the
object, underlined. An object can be named in one of three ways: the object name, the object name
and its class, or just the class name (anonymous object). The three ways of naming an object are
shown in Figure below.

Lifeline
Entities of participants in a collaboration (scenario) are written horizontally across the top of the
diagram. A lifeline is represented by dashed vertical line drawn below each object. These indicate the
existence of the object.

Object names can be specific (e.g., myAccount) or they can be general (e.g., myAccount :Account).
Often, an anonymous object (:Account) may be used to represent any object in the class. Each object
also has its timeline represented by a dashed line below the object. Messages between objects are
represented by arrows that point from sender object to the receiver object.
Everything in an object-oriented system is accomplished by objects. Objects take on the responsibility
for things like managing data, moving data around in the system, responding to inquiries, and
protecting the system. Objects work together by communicating or interacting with one another.

Message
Messages depict the invocation of operations are shown horizontally. They are drawn from the sender
to the receiver. Ordering is indicated by vertical position, with the first message shown at the top of
the diagram, and the last message shown at the bottom. As a result, sequence numbers is optional.

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The line type and arrowhead type indicates the type of message being used:
1. A synchronous message (typically an operation call) is shown as a solid line with a filled
arrowhead. It is a regular message call used for normal communication between sender and
receiver.

2. A return message uses a dashed line with an open arrowhead.

3. An asynchronous message has a solid line with an open arrowhead. A signal is an

Creation and Destruction Messages

Participants do not necessarily live for the entire duration of a sequence diagram's interaction.
Participants can be created and destroyed according to the messages that are being passed.
A constructor message creates its receiver. The sender that already exist at the start of the
interaction are placed at the top of the diagram. Targets that are created during the interaction by a
constructor call are automatically placed further down the diagram.

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A destructor message destroys its receiver. There are other ways to indicate that a target is
destroyed during an interaction. Only when a target's destruction is set to 'after destructor' do you
have to use a destructor.

Non instantaneous message

Messages are often considered to be instantaneous, thus, the time it takes to arrive at the receiver
is negligible. The messages are drawn as a horizontal arrow. To indicate that it takes a certain while
before the receiver actually receives a message, a slanted arrow is used.

Focus of Control
Focus of Control represents the period during which an element is performing an operation. The
top and the bottom of the of the rectangle are aligned with the initiation and the completion time
respectively

Iteration notation
Iteration notation represents a message is sent many times to multiple receiver objects, as would
happen when you are iterating over a collection. You can show the basis of the iteration within
brackets, such as *[for all order lines].

Example: Place Order

2. Step 3: Customer add items to the order.

3. Step 4, 5: Each item is checked for availability in inventory.

4. Step 6, 7, 8 : If the product is available, it is added to the order.

5. Step 9 return

6. Step 10, 11: save and destroy order

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Sequence Fragments
In a UML sequence diagram, combined fragments let you show loops, branches, and other
alternatives. A combined fragment consists of one or more interaction operands, and each of these
encloses one or more messages, interaction uses, or combined fragments.
A sequence fragment is represented as a box called a combined fragment, which encloses a portion
of the interactions within a sequence diagram. The fragment operator (in the top left cornet) indicates
the type of fragment. Fragment types include ref, assert, loop, break, alt, opt and neg, ref, sd.
Operator Meaning
alt Alternative multiple fragments: only the one whose condition is true
will execute.
opt Optional: the fragment executes only if the supplied condition is
true. Equivalent to an alt only with one trace.
par Parallel: each fragment is run in parallel.
loop Loop: the fragment may execute multiple times, and the guard
indicates the basis of iteration.
critical Critical region: the fragment can have only one thread executing it
at once.
neg Negative: the fragment shows an invalid interaction.
ref Reference: refers to an interaction defined on another diagram.
The frame is drawn to cover the lifelines involved in the interaction.
You can define parameters and a return value.
sd Sequence diagram: used to surround an entire sequence diagram.

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Example - Place Order Scenario

A member of a ship who would like to place an order online. The item ordered will be sent to the
member either send by courier or by ordinary mail depending on she member status (VIP, Ordinary
membership). Optionally, the shop will send the member a confirmation notification if the member
opted for the notification option in the order.

Visual Paradigm supports sequence diagram and other UML diagram types. You can find all the tools
you need in modeling the dynamic behaviors of a system using sequence diagram.
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References
1. How to draw a Sequence Diagram in UML - Visual Paradigm User's Guide

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