Systems Analysis Chapter 7 Sequence Diagrams

Sequence diagram

Depicts the interactions among objects during a certain period of time. May be presented either in a generic form or in an instance form.

Frame

A box can enclose one or more messages to divide off a fragment of the sequence.

System sequence diagram

A diagram that depicts the interaction between an actor and the system for a use case scenario. Helps identify high-level messages that enter and exit the system.

Asynchronous message

A message in which the sender does not have to wait for the recipient to handle the message.

Simple message

A message that transfers control from the sender to the recipient without describing the details of the communication.

Synchronous message

A type of message in which the caller has to wait for the receiving object to finish executing the called operation before it can resume execution itself.

Input messages

Horizontal arrows from actor to system indicate the message inputs. UML convention for messages is to begin the first word with a lowercase letter and no space.

Output messages

Horizontal arrows from system to actor shown as dashed lines. These messages do not need to use the standard notation.

Guidelines for constructing system sequence diagrams

Identify which scenario of use you will depict. Draw a rectangle representing the system as a whole and extend a lifeline under it. Identify each actor who directly provides an input or receive output from the system. Narrative identifies system inputs and outputs. Add frames to indicate optional messages. Messages are shown in proper sequence.

Actor

Initiating actor of the use case is shown with the use case actor symbol.

Messages

Means by which objects communicate with each other.

Receiver actor

Other actors or external systems that receive messages from the system can be included.

Objects

Represented by boxes at top of diagram.

Generic form

Shows all possible sequences of interactions- sequences corresponding to all the scenarios of a use case.

Instance form

Shows the sequence for only one scenario.

Activation bars

The bars set over the lifelines indicate period of time when participant is active in the interaction.

System

The box indicates the system as a 'black box' or as a whole. The colon is standard sequence diagram notation to indicate a running diagram notation to indicate a running 'instance' of the system.

Lifelines

The dashed vertical lines extending downward from the actor and system symbols, which indicate the life of the sequence.

Lifeline

The time during which an object exists.

Activation

The time period during which an object performs an operation.