Types Of Sequence Diagram

Posted : admin On 1/26/2022

The sequence - is used to draw a message between two participants. Participants do not have to be explicitly declared. To have a dotted arrow, you use - It is also possible to use sequence diagrams, rules are different for the other. UML Sequence Diagram. Design Elements. UML Sequence Diagram is a type of Interaction Diagrams that describes the interactions between objects and classes that are involved in the scenario and the sequence of messages exchanged between them. UML Sequence Diagram represents the processes and objects that occur at the same time as lifelines (parallel vertical lines), and the messages.

  1. Simple Sequence Diagram
  2. Types Of Objects In Sequence Diagram
Notation DescriptionVisual Representation


  • a type of role played by an entity that interacts with the subject (e.g., by exchanging signals and data)
  • external to the subject (i.e., in the sense that an instance of an actor is not a part of the instance of its corresponding subject).
  • represent roles played by human users, external hardware, or other subjects.

Note that:

  • An actor does not necessarily represent a specific physical entity but merely a particular role of some entity
  • A person may play the role of several different actors and, conversely, a given actor may be played by multiple different person.


  • A lifeline represents an individual participant in the Interaction.


  • A thin rectangle on a lifeline) 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

Call Message

  • A message defines a particular communication between Lifelines of an Interaction.
  • Call message is a kind of message that represents an invocation of operation of target lifeline.

Return Message

  • A message defines a particular communication between Lifelines of an Interaction.
  • Return message is a kind of message that represents the pass of information back to the caller of a corresponded former message.

Self Message

  • A message defines a particular communication between Lifelines of an Interaction.
  • Self message is a kind of message that represents the invocation of message of the same lifeline.

Recursive Message

  • A message defines a particular communication between Lifelines of an Interaction.
  • Recursive message is a kind of message that represents the invocation of message of the same lifeline. It's target points to an activation on top of the activation where the message was invoked from.

Create Message

  • A message defines a particular communication between Lifelines of an Interaction.
  • Create message is a kind of message that represents the instantiation of (target) lifeline.

Destroy Message

  • A message defines a particular communication between Lifelines of an Interaction.
  • Destroy message is a kind of message that represents the request of destroying the lifecycle of target lifeline.

Duration Message

  • A message defines a particular communication between Lifelines of an Interaction.
  • Duration message shows the distance between two time instants for a message invocation.


A note (comment) gives the ability to attach various remarks to elements. A comment carries no semantic force, but may contain information that is useful to a modeler.

Sequence diagram is the most common kind of interaction diagram, which focuses on the message interchange between a number of lifelines.

Sequence diagram describes an interaction by focusing on the sequence of messages that are exchanged, along with their corresponding occurrence specifications on the lifelines.

The following nodes and edges are typically drawn in a UMLsequence diagram: lifeline, execution specification, message, combined fragment, interaction use, state invariant, continuation, destruction occurrence.

Major elements of the sequence diagram are shown on the picture below.

You can find some sequence diagram examples here:


Lifeline is a named element which represents an individual participant in the interaction. While parts and structural features may have multiplicity greater than 1, lifelines represent only one interacting entity.

If the referenced connectable element is multivalued (i.e, has a multiplicity > 1), then the lifeline may have an expression (selector) that specifies which particular part is represented by this lifeline. If the selector is omitted, this means that an arbitrary representative of the multivalued connectable element is chosen.

A lifeline is shown using a symbol that consists of a rectangle forming its 'head' followed by a vertical line (which may be dashed) that represents the lifetime of the participant.

Information identifying the lifeline is displayed inside the rectangle in the following format (slightly modified from what's in UML 2.4 standard):

lifeline-ident ::= [ connectable-element-name [ '[' selector ']' ]] [ ':' class-name ] [ decomposition ] 'self'
selector ::= expression
decomposition ::= 'ref' interaction-ident [ 'strict' ]

where class-name is the type referenced by the represented connectable element. Note that although the syntax allows it, lifeline-ident cannot be empty.

The lifeline head has a shape that is based on the classifier for the part that this lifeline represents. Usually the head is a white rectangle containing name of class.

Lifeline 'data' of class Stock


Lifeline 'x' of class X is selected with selector [k]

If the name is the keyword self, then the lifeline represents the object of the classifier that encloses the Interaction that owns the Lifeline. Ports of the encloser may be shown separately even when self is included.


A gate is a message end, connection point for relating a message outside of an interaction fragment with a message inside the interaction fragment.

The purpose of gates and messages between gates is to specify the concrete sender and receiver for every message. Gates play different roles:

  • formal gates - on interactions
  • actual gates - on interaction uses
  • expression gates - on combined fragment

The gates are named implicitly or explicitly. Implicit gate name is constructed by concatenating the direction of the message ('in' or 'out') and the message name, e.g. in_search, out_read.

Gates are notated just as message connection points on the frame.

Interaction Fragment

Interaction fragment is a named element representing the most general interaction unit. Each interaction fragment is conceptually like an interaction by itself.

There is no general notation for an interaction fragment. Its subclasses define their own notation.

Examples of interaction fragments are:


Occurrence (complete UML name - occurrence specification, i.e. 'event description') is interaction fragment which represents a moment in time (event) at the beginning or end of a message or at the beginning or end of an execution.

An occurrence specification is one of the basic semantic units of interactions. The meanings of interactions are specified by sequences of occurrences described by occurrence specifications.

Each occurrence specification appears on exactly one lifeline. Occurrence specifications of a lifeline are ordered along the lifeline.

Occurrence specification has no notation and is just a point at the beginning or end of a message or at the beginning or end of an execution specification.

Examples of occurrences are:

Message Occurrence

Message occurrence (complete UML name - message occurrence specification) is an occurrence which represents such events as sending and receiving of signals or invoking or receiving of operation calls.

Destruction Occurrence

Destruction occurrence is a message occurrence which represents the destruction of the instance described by the lifeline. It may result in the subsequent destruction of other objects that this object owns by composition. No other occurrence may appear below the destruction event on a given lifeline.

Complete UML name of the occurrence is destruction occurrence specification. Until UML 2.4 it was called destruction event, and earlier - stop.

The destruction of instance is depicted by a cross in the form of an X at the bottom of a lifeline.

Execution Occurrence

Execution occurrence (complete UML name - execution occurrence specification) is an occurrence which represents moments in time at which actions or behaviors start or finish.

Execution occurrence references exactly one execution specification which describes the execution that is started or finished at this execution occurrence.

Duration of an execution is represented
by two execution occurrences - start and finish.


Execution (full name - execution specification, informally called activation) is interaction fragment which represents a period in the participant's lifetime when it is

  • executing a unit of behavior or action within the lifeline,
  • sending a signal to another participant,
  • waiting for a reply message from another participant.

Note, that the execution specification includes the cases when behavior is not active, but just waiting for reply. The duration of an execution is represented by two execution occurrences - the start occurrence and the finish occurrence.

Execution is represented as a thin grey or white rectangle on the lifeline.

Execution specification shown as grey rectangle on the Service lifeline.

Execution specification can be represented by a wider labeled rectangle, where the label usually identifies the action that was executed.

Execution Specification represented as wider rectangle labeled as action.

For execution specifications that refer to atomic actions such as reading attributes of a signal (conveyed by the message), the action symbol may be associated with the reception occurrence specification with a line in order to emphasize that the whole action is associated with only one occurrence specification (and start and finish associations refer to the same occurrence specification).

Overlapping execution specifications on the same lifeline are represented by overlapping rectangles.

Overlapping execution specifications on the same lifeline - message to self.

Overlapping execution specifications on the same lifeline - callback message.

State Invariant

A state invariant is an interaction fragment which represents a runtime constraint on the participants of the interaction. It may be used to specify different kinds of constraints, such as values of attributes or variables, internal or external states, etc.

The constraint is evaluated immediately prior to the execution of the next occurrence specification such that all actions that are not explicitly modeled have been executed. If the constraint is true, the trace is a valid trace, otherwise the trace is an invalid trace.

State invariant is usually shown as a constraint in curly braces on the lifeline.

Attribute t of Task should be equal to complete.

It could also be shown as a state symbol representing the equivalent of a constraint that checks the state of the object represented by the lifeline. This could be either the internal state of the classifier behavior of the corresponding classifier or some external state based on a 'black-box' view of the lifeline.

State invariant can optionally be shown as a note associated with an occurrence specification.

Interaction Use

Interaction use is an interaction fragment which allows to use (or call) another interaction. Large and complex sequence diagrams could be simplified with interaction uses. It is also common to reuse some interaction between several other interactions.

Referenced interaction has formal gates. Interaction use provides a set of actual gates that must match the formal gates of the interaction.

Interaction use works as:

  • copy the contents of the referred interaction to where this interaction needs to be used,
  • substitute formal parameters with arguments,
  • connect the formal gates with the actual ones.

The interaction use is shown as a combined fragment with operator ref.

Web customer and Bookshop use (reference) interaction Checkout.

The syntax of the interaction use of the ref operator is:

interaction-use ::= [ attribute-name '=' ] [ collaboration-use '.' ] interaction-name [ io-arguments ] [ ':' return-value ]
io-arguments ::= '(' io-argument [ ',' io-argument ]* ')'
io-argument ::= in-argument 'out' out-argument

Simple Sequence Diagram

The attribute-name refers to an attribute of one of the lifelines in the interaction that will receive interaction result. Note, that this restricts results of interactions to be assigned only to attributes. In real life, results of a method call could be assigned to a variable from calling method.

The collaboration-use is an identification of collaboration use that binds lifelines of a collaboration. The interaction name is in that case within that collaboration.

The io-arguments is list of in and/or out arguments of the interaction.

Use Login interaction to authenticate user and assign result back to the user attribute of Site Controller.

Types Of Objects In Sequence Diagram

One constraint imposed by UML specification that is sometimes difficult to follow is that the interaction use must cover all involved lifelines represented on the enclosing interaction. This means that all those lifelines should be somehow located near each other. If we have another interaction use on the same diagram it could be very tricky to rearrange all involved lifelines as required by UML.