The Ultimate Sequence Diagram Tutorial

 There are 3 types of Interaction diagrams in UML
• Sequence diagrams
• Communication diagrams
• Timing diagrams
 Interaction diagrams are used to illustrate interactions of
parts within a system.
 Out of these 3 types, sequence diagrams are preferred by
both developers and readers alike for their simplicity.

What is a Sequence Diagram?
 Sequence diagrams, commonly used by developers, model
the interactions between objects in a single use case.
 They illustrate how the different parts of a system interact
with each other to carry out a function, and the order in
which the interactions occur when a particular use case is
executed.

Drawing Sequence Diagrams with Creately
 Creately is a web based diagramming tool that can be used to draw sequence
diagrams including all other UML diagram types as well as flowcharts, Gantt charts
and many other diagram types
 It offers tools, professionally designed sequence diagram templates and resources
to help draw sequence diagrams
 It comes with real-time collaboration features that allow users to work together
and share diagrams with a team
 It has a desktop version for offline diagramming and a mobile app to manage and
browse diagrams on the go

 A sequence diagram is structured in such a way that it
represents a timeline which begins at the top and descends
gradually to mark the sequence of interactions.
 Each object has a column and the messages exchanged
between them are represented with arrows.
• Lifeline Notation
• Activation Bars
• Message Arrows
• Comment

Lifeline Notation
 A sequence diagram is made up of several of these
lifeline notations
 They should be arranged horizontally across the top of
the diagram
 No two lifeline notations should overlap each other
 They represent the different objects that interact with
each other in the system
 A lifeline notation with an actor element symbol is used
when the sequence diagram is owned by a use case

Activation Bars
 Activation bar is the box placed on the
lifeline
 It indicates that an object is active ((or
instantiated) during an interaction
between two objects
 The length of the rectangle indicates the
duration of the interaction

Message Arrows
 An arrow from the Message Caller to the Message Receiver
specifies a message
 The message can flow in any direction; from left to right, right to
left and back to the caller itself
 The description of the message should go on the arrow
 Arrow heads may change according to different message types

 Different message types;
• Synchronous message
• Asynchronous message
• Return message
• Participant creation message
• Participant destruction message
• Reflexive message
• The message signature is (all parts except the message_name is optional);
attribute = message_name (arguments): return_type

 A synchronous message is used when the sender waits for the receiver to
process the message and return before carrying on with another message
 An asynchronous message is used when the message caller does not wait
for the receiver to process the message and return before sending other
messages to other objects within the system

 A return message is used to indicate that the message receiver is
done processing the message and is returning control over to the
message caller.
Tip: You can avoid cluttering up your diagrams by minimizing the use of return
messages since the return value can be specified in the initial message arrow
itself.
 When an object sends a message to itself, it is called a reflexive
message. It is indicated with a message arrow that starts and ends
at the same lifeline

• Participant creation message; objects can be created in the middle
of a sequence. The dropped participant box notation is used when
you need to show that the particular participant did not exist until
the create call was sent.
• Participant destruction message; participants, when no longer
needed, can also be deleted from a sequence diagram. This is done
by adding an ‘X’ at the end of the lifeline of the said participant.

Comments
 UML generally permits the annotation of comments in all UML diagram
types.
 The comment object is a rectangle with a folded-over corner as shown
below. The comment can be linked to the related object with a dashed
line.

How to Draw a Sequence Diagram
 A sequence diagram represents the scenario or flow of events in
one single use case. The message flow of the sequence diagram is
based on the narrative of the particular use case.
 Before you start drawing the sequence diagram or decide what
interactions should be included in it, you need to ready a
comprehensive description of the particular use case.

Example:
Use case example - Create
new online library account

 From the use case diagram example of ‘Create New Online Library
Account’, we will focus on the use case named ‘Create New User
Account’ to draw our sequence diagram.
Step 1
Identify the objects or participants in the use case ‘Create New User
Account’
• Librarian
• Online Library Management system
• User credentials data base
• Email system

Step 2
 List down the steps involved in the execution of the use case
• The librarian requires the system to create a new online library account
• The librarian selects the library user account type
• The librarian enters the user’s details
• The user’s details are checked using the user Credentials Database
• The new library user account is created
• A summary of the new account’s details is emailed to the user

Step 3
Identify which messages should be passed between the objects we identified
earlier as the system executes these steps. Then draw the sequence diagram.

Sequence Diagram Best Practices
 Manage complex interactions with sequence fragments
• Sequence fragments are used to show complex interactions such as
alternative flows and loops in a more structured way.
• A sequence diagram is drawn as a box that frames a section of
interactions between objects in a sequence diagrams.
• On the top left corner of the fragment sits an operator. This – the
fragment operator - specifies what sort of a fragment it is.

Alternative Combination Fragment
• It is used when a choice needs to be made between two or more
message sequences. It models the “if then else” logic
• It is specified by mentioning ‘alt’ inside the frame’s name box
• To show 2 or more alternatives, the frame is divided into what is
called interaction operands using a dashed line
• Each operand has a guard to test against and it is placed at the top
left corner of the operand

Example of an alternative combination fragment

Option Combination Fragment
• The option fragment is used to indicate a sequence that will only occur
under a certain condition, otherwise the sequence won’t occur
• It models the “if then” statement
• It is represented with a rectangular frame where ‘opt’ is placed inside the
name box
• Unlike the alternative fragment, the option fragment is not divided to
operands

Example of an option combination fragment

Loop Fragment
• It is used to show a repetitive sequence
• It is drawn as a frame and specified by placing ‘loop’ in the name
box
• It can be used
– for the Boolean test
– to test minimum iterations (the loop must execute not less than the
number mentioned)
– to test maximum iterations (the loop mustn’t execute more than the
number mentioned)

Reference Fragment
• It allows you to reuse or refer to a part of one sequence diagram in
another
• It helps manage the size of large sequence diagrams
• Include ‘ref’ in the name box of the frame to specify the reference
fragment
• Mention the name of the sequence diagram being referred to
inside the frame

Example of a reference fragment

 Draw smaller sequence diagrams that capture the
essence of the use case
• Several objects and messages across a sequence diagram clutters it
up
• Convey how your system functions with a few smaller sequence
diagrams
• Make sure the diagram fits on a single page and leaves room for
additional comments
• Find out what is common about the diagrams and focus on the
commonality when drawing

Sequence Diagram Common Mistakes
• Adding too much detail that makes it difficult to read the diagram
• Obsolete sequence diagrams that are irrelevant when compared to the
interfaces, actual architectures of the system
• Leaving no blank space between the use case text and the message arrow
• Not considering the origins of message arrows carefully
See these common mistakes explained in detail here.

Templates and examples
Sequence Diagram Example of an Online Exam System

Sequence Diagram Example of a School Management System

Sequence Diagram Example of an Online Movie Ticket Booking System

The Ultimate Sequence Diagram Tutorial

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