#Sequence diagram examples series#

Strict sequencing fragment (denoted “strict”) encloses a series of messages which must be processed in the given order.Weak sequencing fragment (denoted “seq”) encloses a number of sequences for which all the messages must be processed in a preceding segment before the following segment can start, but which does not impose any sequencing within a segment on messages that don’t share a lifeline.Parallel fragment (denoted “par”) models concurrent processing.Break fragment models an alternative sequence of events that is processed instead of the whole of the rest of the diagram.Option fragment (denoted “opt”) models switch constructs.Alternative fragment (denoted “alt”) models if…then…else constructs.A combined fragment is one or more processing sequence enclosed in a frame and executed under specific named circumstances. While this is the case, there are a number of mechanisms that do allow for adding a degree of procedural logic to diagrams and which come under the heading of combined fragments. By setting a duration constraint for a message, the message will be shown as a sloping line.Ĭombined Fragments It was stated earlier that sequence diagrams are not intended for showing complex procedural logic. Since the lifeline represents the passage of time down the screen, when modelling a real-time system, or even a time-bound business process, it can be important to consider the length of time it takes to perform actions. The following diagram shows an object being created and destroyed.īy default, a message is shown as a horizontal line. In the former case, the symbol at the head of the lifeline is shown at a lower level down the page than the symbol of the object that caused the creation. In the latter case, the lifeline is terminated by a stop symbol, represented as a cross. They are denoted going to or coming from an endpoint element.Ī lifeline may be created or destroyed during the timescale represented by a sequence diagram. Found messages are those that arrive from an unknown sender, or from a sender not shown on the current diagram. Lost messages are those that are either sent but do not arrive at the intended recipient, or which go to a recipient not shown on the current diagram. It is shown as creating a nested focus of control in the lifeline’s execution occurrence. The first is the source object sending two messages and receiving two replies the second is the target object receiving a synchronous message and returning a reply and the third is the target object receiving an asynchronous message and returning a reply.Ī self message can represent a recursive call of an operation, or one method calling another method belonging to the same object. In the previous diagram, there are three execution occurrences. In the following diagram, the first message is a synchronous message (denoted by the solid arrowhead) complete with an implicit return message the second message is asynchronous (denoted by line arrowhead), and the third is the asynchronous return message (denoted by the dashed line).Ī thin rectangle running down the lifeline denotes the execution occurrence, or activation of a focus of control. Messages can be complete, lost or found synchronous or asynchronous call or signal. Boundary, control and entity elements from robustness diagrams can also own lifelines. This will usually be the case if the sequence diagram is owned by a use case. Sometimes a sequence diagram will have a lifeline with an actor element symbol at its head. If its name is 'self', that indicates that the lifeline represents the classifier which owns the sequence diagram. A lifeline will usually have a rectangle containing its object name. Sequence diagrams are not intended for showing complex procedural logic.Ī lifeline represents an individual participant in a sequence diagram. Sequence diagrams are good at showing which objects communicate with which other objects and what messages trigger those communications. UML 2 Tutorial - Sequence Diagram Sequence DiagramsĪ 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 examples