System Sequence Diagram
Encyclopedia
In software engineering
, a system sequence diagram (SSD) is a sequence diagram
that shows, for a particular scenario of a use case
, the events that external actors generate, their order, and possible inter-system events.
System sequence diagrams are visual summaries of the individual use cases.
All systems
are treated as a black box
; the diagram places emphasis on events that cross the system boundary from actors to systems. A system sequence diagram should be done for the main success scenario of the use case
, and frequent or complex alternative scenarios.
A system sequence diagram should specify and show the following:
in a visual format. In order to construct a system sequence diagram, you need to be familiar with the unified modeling language (UML). These models show the logic behind the actors (people who affect the system) and the system in performing the task. Reading a sequence diagram begins at the top with the actor(s) or the system(s) (which is located at the top of the page). Under each actor or system there are long dotted line called lifelines, which are attached to them. Actions are performed with lines that extend between these lifelines. When an action line is connected to a lifeline it shows the interaction between the actor or system. Messages will often appear at the top or bottom of a system sequence diagram to illustrate the action in detail. For example, the actor could request to login, this would be represented by login (username, password). After each action is performed, the response or next action is located under the previous one. As you read down the lines you will see in detail how certain actions are performed in the system.
Software engineering
Software Engineering is the application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software, and the study of these approaches; that is, the application of engineering to software...
, a system sequence diagram (SSD) is a sequence diagram
Sequence diagram
A sequence diagram in Unified Modeling Language is a kind of interaction diagram that shows how processes operate with one another and in what order. It is a construct of a Message Sequence Chart....
that shows, for a particular scenario of a use case
Use case
In software engineering and systems engineering, a use case is a description of steps or actions between a user and a software system which leads the user towards something useful...
, the events that external actors generate, their order, and possible inter-system events.
System sequence diagrams are visual summaries of the individual use cases.
All systems
Software system
A software system is a system based on software forming part of a computer system . The term "software system" is often used as a synonym of computer program or software; is related to the application of systems theory approaches in software engineering context and are used to study large and...
are treated as a black box
Black box
A black box is a device, object, or system whose inner workings are unknown; only the input, transfer, and output are known characteristics.The term black box can also refer to:-In science and technology:*Black box theory, a philosophical theory...
; the diagram places emphasis on events that cross the system boundary from actors to systems. A system sequence diagram should be done for the main success scenario of the use case
Use case
In software engineering and systems engineering, a use case is a description of steps or actions between a user and a software system which leads the user towards something useful...
, and frequent or complex alternative scenarios.
A system sequence diagram should specify and show the following:
- External actors
- Messages (methods) invoked by these actors
- Return values (if any) associated with previous messages
- Indication of any loops or iteration area
Reading a system sequence diagram
Professionals, in developing a project, often use system sequence diagrams to illustrate how certain tasks are done between users and the system. These tasks may include repetitive, simple, or complex tasks. The purpose is to illustrate the use caseUse case
In software engineering and systems engineering, a use case is a description of steps or actions between a user and a software system which leads the user towards something useful...
in a visual format. In order to construct a system sequence diagram, you need to be familiar with the unified modeling language (UML). These models show the logic behind the actors (people who affect the system) and the system in performing the task. Reading a sequence diagram begins at the top with the actor(s) or the system(s) (which is located at the top of the page). Under each actor or system there are long dotted line called lifelines, which are attached to them. Actions are performed with lines that extend between these lifelines. When an action line is connected to a lifeline it shows the interaction between the actor or system. Messages will often appear at the top or bottom of a system sequence diagram to illustrate the action in detail. For example, the actor could request to login, this would be represented by login (username, password). After each action is performed, the response or next action is located under the previous one. As you read down the lines you will see in detail how certain actions are performed in the system.