Sequence diagrams map out complex architecture, but they can be surprisingly easy to create. Sequence diagrams are a popular subset to UML, so if you’re new to the topic, use our sequence diagram tutorial to familiarize yourself and find different ways you can use sequence diagrams to map out your technical process and infrastructure. Sequence Diagram A Sequence diagram is a structured representation of behavior as a series of sequential steps over time. You can use it to: Depict workflow, Message passing and how elements in general cooperate over time to achieve a result.

The sequence diagram represents the flow of messages in the system and is also termed as an event diagram. It helps in envisioning several dynamic scenarios. It portrays the communication between any two lifelines as a time-ordered sequence of events, such that these lifelines took part at the run time. In UML, the lifeline is represented by a vertical bar, whereas the message flow is represented by a vertical dotted line that extends across the bottom of the page. It incorporates the iterations as well as branching.

Purpose of a Sequence Diagram

To model high-level interaction among active objects within a system.
To model interaction among objects inside a collaboration realizing a use case.
It either models generic interactions or some certain instances of interaction.

Notations of a Sequence Diagram

Sequence Diagram App

Lifeline

An individual participant in the sequence diagram is represented by a lifeline. It is positioned at the top of the diagram.

Actor

A role played by an entity that interacts with the subject is called as an actor. It is out of the scope of the system. It represents the role, which involves human users and external hardware or subjects. An actor may or may not represent a physical entity, but it purely depicts the role of an entity. Several distinct roles can be played by an actor or vice versa.

Activation

It is represented by a thin rectangle on the lifeline. It describes that time period in which an operation is performed by an element, such that the top and the bottom of the rectangle is associated with the initiation and the completion time, each respectively.

Messages

The messages depict the interaction between the objects and are represented by arrows. They are in the sequential order on the lifeline. The core of the sequence diagram is formed by messages and lifelines.

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Following are types of messages enlisted below:

Call Message: It defines a particular communication between the lifelines of an interaction, which represents that the target lifeline has invoked an operation.
Return Message: It defines a particular communication between the lifelines of interaction that represent the flow of information from the receiver of the corresponding caller message.
Self Message: It describes a communication, particularly between the lifelines of an interaction that represents a message of the same lifeline, has been invoked.
Recursive Message: A self message sent for recursive purpose is called a recursive message. In other words, it can be said that the recursive message is a special case of the self message as it represents the recursive calls.
Create Message: It describes a communication, particularly between the lifelines of an interaction describing that the target (lifeline) has been instantiated.
Destroy Message: It describes a communication, particularly between the lifelines of an interaction that depicts a request to destroy the lifecycle of the target.
Duration Message: It describes a communication particularly between the lifelines of an interaction, which portrays the time passage of the message while modeling a system.

Note

A note is the capability of attaching several remarks to the element. It basically carries useful information for the modelers.

Sequence Fragments

Sequence fragments have been introduced by UML 2.0, which makes it quite easy for the creation and maintenance of an accurate sequence diagram.
It is represented by a box called a combined fragment, encloses a part of interaction inside a sequence diagram.
The type of fragment is shown by a fragment operator.

Sequence Diagram Generator

Types of fragments

Following are the types of fragments enlisted below;

Operator	Fragment Type
alt	Alternative multiple fragments: The only fragment for which the condition is true, will execute.
opt	Optional: If the supplied condition is true, only then the fragments will execute. It is similar to alt with only one trace.
par	Parallel: Parallel executes fragments.
loop	Loop: Fragments are run multiple times, and the basis of interaction is shown by the guard.
region	Critical region: Only one thread can execute a fragment at once.
neg	Negative: A worthless communication is shown by the fragment.
ref	Reference: An interaction portrayed in another diagram. In this, a frame is drawn so as to cover the lifelines involved in the communication. The parameter and return value can be explained.
sd	Sequence Diagram: It is used to surround the whole sequence diagram.

Example of a Sequence Diagram

An example of a high-level sequence diagram for online bookshop is given below. Hotkey program for mac os.

Any online customer can search for a book catalog, view a description of a particular book, add a book to its shopping cart, and do checkout.

Benefits of a Sequence Diagram

It explores the real-time application.
It depicts the message flow between the different objects.
It has easy maintenance.
It is easy to generate.
Implement both forward and reverse engineering.
It can easily update as per the new change in the system.

The drawback of a Sequence Diagram

In the case of too many lifelines, the sequence diagram can get more complex.
The incorrect result may be produced, if the order of the flow of messages changes.
Since each sequence needs distinct notations for its representation, it may make the diagram more complex.
The type of sequence is decided by the type of message.

Next TopicUML Collaboration Diagram

Sequence Diagram Editor

Download Sample.zip of this tutorial and extract the zip file to any directory.
Study the source code. Read the register method in RegisterController.java to see how it works.
Create a new project by selecting Project > New from the application toolbar. In the New Project window, enter Account Registration as project name and click Create Blank Project.
Select Tools > Code > Instant Reverse Java to Sequence Diagram.. from the toolbar.
In the Instant Reverse Java to Sequence Diagram window, click on Add Source Folder.. button.
Select the extracted source folder src. Click Next button.
Select the method to visualize. Select src > RegisterController.java > register (String,int). Click the Next button.
You need to select a diagram to visualize the interaction. The Create new sequence diagram option is selected and diagram name is entered by default. Click Finish button.
As a result, a UML sequence diagram is formed. Let's study the diagram.
When a person invokes RegisterController's register method (message: 1), it creates an account object

Sequence Diagram Application