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Visual Paradigm Community Circle UML Diagrams Sequence Diagram Object Creation A sequence diagram is an interaction diagram that details how operations are carried out - what messages are sent and when. Sequence diagrams are organized according to time. The time progresses as you go down the page.

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

The final sequence diagram looks like this. Sequence diagrams are a very powerful too you can use to model your software. Sequence diagrams are commonly used as a planning tool before the development team starts programming, or to show others how a system is designed. And they can help you to determine the functions you will need to right.
UML 2 Tutorial - Sequence Diagram Sequence Diagrams. A sequence diagram is a form of interaction diagram which shows objects as lifelines running down the page, with their interactions over time represented as messages drawn as arrows from the source lifeline to the target lifeline.

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

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.

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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.

Gate

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.

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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

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

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.

Executi

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