The first step in building a sequence diagram is to _____.

This sequence diagram tutorial is to help you understand sequence diagrams better; to explain every little thing you need to recognize, from how to draw a sequence diagram to the common mistakes you should prevent as soon as illustration one.

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Tbelow are 3 kinds of Interaction diagrams; Sequence diagrams, interaction diagrams, and also timing diagrams. These diagrams are provided to illustrate interactions in between components within a mechanism. Amongst the 3, sequence diagrams are wanted by both developers and readers achoose for their simplicity.

In this sequence diagram tutorial you will certainly learn about;

What is a Sequence Diagram?

Sequence diagrams, frequently offered by developers, design the interactions in between objects in a single usage case. They highlight just how the different parts of a mechanism interact with each other to lug out a function, and also the order in which the interactions happen once a details use instance is executed.

In simpler words, a sequence diagram reflects various parts of a mechanism work in a ‘sequence’ to obtain something done.

Sequence Diagram Notations

A sequence diagram is structured in such a method that it represents a timeline which begins at the peak and descends progressively to note the sequence of interactions. Each object has a column and the messeras exreadjusted between them are stood for by arrows.

A Fast Synopsis of the Various Parts of a Sequence Diagram

Lifeline Notation

A sequence diagram is consisted of of several of these lifeline notations that should be arranged horizontally throughout the top of the diagram. No 2 lifeline notations should overlap each various other. They represent the various objects or parts that connect via each other in the system in the time of the sequence.

A lifeline notation with an actor facet symbol is offered as soon as the specific sequence diagram is owned by a usage instance.


A lifeline with an entity aspect represents system information. For example, in a customer service application, the Customer entity would control all information concerned a customer.


A lifeline through a boundary facet shows a device boundary/ software program element in a system; for example, user interconfront displays, database gateways or menus that individuals connect with, are borders.


And a lifeline through a control facet shows a managing entity or manager. It organizes and also schedules the interactions in between the borders and also entities and also serves as the mediator between them.


Activation Bars

Activation bar is the box inserted on the lifeline. It is used to show that an item is energetic (or instantiated) in the time of an interactivity in between two objects. The length of the rectangle indicates the duration of the objects continuing to be active.

In a sequence diagram, an interactivity in between 2 objects occurs when one object sends a message to one more. The use of the activation bar on the lifelines of the Article Caller (the object that sends out the message) and the Blog post Receiver (the object that receives the message) shows that both are active/is instantiated during the exadjust of the message.


Message Arrows

An arrow from the Post Caller to the Blog post Receiver states a message in a sequence diagram. A message can circulation in any direction; from left to appropriate, best to left or back to the Article Caller itself. While you can explain the message being sent from one object to the various other on the arrowhead, via different arrowheads you deserve to indicate the type of message being sent or received.

The message arrow comes with a description, which is known as a message signature, on it. The format for this message signature is below. All parts except the message_name are optional.

attribute = message_name (arguments): return_type

Synchronous message

As displayed in the activation bars example, a synchronous message is offered when the sender waits for the receiver to process the message and also return prior to transferring on via an additional message. The arrowhead provided to show this type of message is a solid one, choose the one below.


Asynchronous message

An asynchronous message is provided as soon as the message caller does not wait for the receiver to process the message and rerevolve prior to sending various other messperiods to various other objects within the system. The arrowhead provided to display this type of message is a line arrowhead prefer shown in the instance below.


Rerotate message

A return message is used to indicate that the message receiver is done processing the message and is returning manage over to the message caller. Rerotate messeras are optional notation pieces, for an activation bar that is prompted by a synchronous message always suggests a rerotate message.

Tip: You can protect against cluttering up your diagrams by minimizing the use of rerotate messperiods considering that the rerevolve value have the right to be specified in the initial message arrowhead itself.


Participant development message

Objects execute not necessarily live for the whole duration of the sequence of events. Objects or participants can be created according to the message that is being sent out.

The dropped participant box notation can be provided when you have to show that the specific participant did not exist until the develop speak to was sent out. If the produced participant does something automatically after its production, you need to include an activation box ideal below the participant box.


Participant destruction message

Likewise, participants once no much longer needed have the right to also be deleted from a sequence diagram. This is done by including an ‘X’ at the finish of the lifeline of the said participant.


Reflexive message

When an item sends a message to itself, it is called a reflexive message. It is shown through a message arrow that starts and ends at the very same lifeline as shown in the instance below.



UML diagrams mainly permit the annotation of comments in all UML diagram types. The comment object is a rectangle via a folded-over corner as presented listed below. The comment deserve to be linked to the related object via a dashed line.


Note: View Sequence Diagram Best Practices to learn around sequence pieces.

Sequence Diagram Best Practices

Manage complex interactions through sequence pieces

A sequence fragment is represented as a box that frames a area of interactions in between objects (as displayed in the examples below) in a sequence diagram.

It is provided to display complicated interactions such as alternative flows and loops in a much more structured means. On the peak left edge of the fragment sits an operator. This – the fragment operator – mentions what sort of a fragment it is.

See more: Which Of The Following Is Not True Of Dialects? Do You Speak American


The alternate combination fragment is used as soon as a choice needs to be made between 2 or more message sequences. It models the “if then else” logic.

The alternative fragment is stood for by a big rectangle or a frame; it is specified by mentioning ‘alt’ inside the frame’s name box (a.k.a. fragment operator).

To show 2 or more choices, the bigger rectangle is then separated right into what is called interaction operands using a damelted line, as shown in the sequence diagram instance above. Each operand has actually a guard to test versus and it is inserted at the optimal left corner of the operand also.



The alternative combicountry fragment is used to suggest a sequence that will only take place under a particular problem, otherwise, the sequence won’t take place. It models the “if then” statement.  

Comparable to the alternative fragment, the option fragment is likewise stood for with a rectangular structure where ‘opt’ is put inside the name box.

Unprefer the different fragment, an option fragment is not split right into two or more operands. Option’s guard is placed at the height left edge.

(Find an example sequence diagram with an option fragment in the Sequence Diagram Templates and Examples section).


Loop fragment is provided to reexisting a recurring sequence. Place the words ‘loop’ in the name box and the guard problem near the height left corner of the frame.

In addition to the Boolean test, the guard in a loop fragment deserve to have 2 various other unique problems tested versus. These are minimum iterations (created as minint = and maximum iterations (created as maxint = ).

If it is a minimum iterations guard, the loop should execute not much less than the number mentioned, and also if it is a maximum iterations guard, the loop mustn’t execute even more than the number shown.

(Find an instance of a loop fragment listed below in the sequence diagram templates and example section)

Reference Fragment

You have the right to use the ref fragment to regulate the size of huge sequence diagrams. It enables you to reusage component of one sequence diagram in another, or in various other words, you can recommendation part of a diagram in an additional diagram using the ref fragment.

To specify the referral fragment, you have to mention ‘ref’ in the name box of the frame and the name of the sequence diagram that is being described inside the structure.


For more sequence fragments refer to Beyond the Basics of Sequence Diagrams: Part 1, Part 2 and also Part 3.

Draw smaller sequence diagrams that capture the essence of the use case

Instead of cluttering your sequence diagram via a number of objects and teams of messages that will confuse the reader, attract a few smaller sized sequence diagrams that aptly explain what your device does. Make certain that the diagram fits on a solitary web page and leaves area for explanatory notes as well.

Also rather of drawing dozens of sequence diagrams, uncover out what is widespread among the scenarios and focus on that. And if the code is expressive and also deserve to stand on its very own, there’s no should attract a sequence diagram in the initially location.

How to Draw a Sequence Diagram

A sequence diagram represents the scenario or flow of occasions in one single usage case. The message flow of the sequence diagram is based upon the narrative of the certain usage instance.

Then, prior to you start drawing the sequence diagram or decide what interactions need to be consisted of in it, you must attract the usage case diagram and also prepared a substantial description of what the specific use instance does.


From the over use situation diagram example of ‘Create New Online Library Account’, we will certainly emphasis on the usage case named ‘Create New User Account’ to draw our sequence diagram instance.

Before drawing the sequence diagram, it’s vital to identify the objects or actors that would be involved in creating a new user account. These would certainly be;

LibrarianOnline Library Management systemUser credentials databaseEmail system

Once you identify the objects, it is then essential to create a in-depth summary on what the usage case does. From this summary, you have the right to conveniently number out the interactions (that must go in the sequence diagram) that would certainly occur between the objects above, when the use situation is executed.

Here are the measures that take place in the usage case named ‘Create New Library User Account’.

The librarian request the system to produce a new digital library accountThe librarian then selects the library user account typeThe librarian enters the user’s detailsThe user’s details are checked making use of the user Credentials DatabaseThe brand-new library user account is createdA summary of the of the new account’s details are then emailed to the user

From each of these procedures, you deserve to quickly specify what messages must be exchanged between the objects in the sequence diagram. Once it’s clear, you deserve to go ahead and also start illustration the sequence diagram.

The sequence diagram below mirrors how the objects in the digital library management mechanism connect through each other to perform the function ‘Create New Library User Account’.


Sequence Diagram Usual Mistakes

When drawing sequence diagrams, developers tfinish to make these prevalent mistakes. By staying clear of these mistakes you deserve to encertain the top quality of your diagram.

Adding too much detail. This clutters up the diagram and also makes it difficult to review.Obsolete and also out of date sequence diagrams that are irappropriate when compared to the interencounters, actual architectures and so on. of the system. Don’t forobtain to rearea them or modify them.Not considering the beginnings of message arrows carefully.

See these common mistakes described in information in Sequence Diagram Guide: Common Mistakes to Avoid When Drawing Sequence Diagrams.

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Sequence Diagram Examples and Templates

Following are a few sequence diagram examples and also templates that are attracted making use of Create sequence diagrams digital utilizing’s online tool. Click on the template to open it in the editor.