What is UML Sequence Diagram: Guide with Examples

Here are the symbols and notations used to design a Sequence Diagram. They were given meaning according to their role in the system’s sequence diagram illustration.

1. Actor

   
   
   
   Actors are entities that interact with the system while remaining external to it. They were represented by stick figures which serves as a symbol of a user in the UML sequence diagram. The system events that the actors generate in the sequence are referred to as events.
   
   Example of these actors are the users of a certain proposed project. The successive events that occur to an object during the charted process are shown by this dashed vertical line. A named rectangular shape or an actor UML sequence diagram symbol is used to start a lifeline.

2. Lifeline

   
   
   A lifeline is a dashed line that indicates events in Sequence Diagram. A named or labeled rectangular shape symbol may be used to start a lifeline.
   
   In UML sequence diagram, a lifeline symbol is plotted to let the messages flow through it. This symbol represents an object’s existence at a specific point in time in the modeled UML sequence diagram. As long as the messages passes, then the lifeline of the object still goes on.

3. Objects

   
   An object in UML sequence diagram is represented by a rectangle with the symbol name or label. The object name or the class name can be used to name an object (anonymous object). It depicts how an object will interact with the rest of the system. This shape should not include any class attributes.
   
   The objects that the system requires or need are all included in designing a UML Sequence diagram are displayed by this symbol. They will also have their lifelines according to how long their functions would last. As long as they were needed for the specific process, then they will be present in the designed diagram.

4. Package

   
   Entities that interact with or external to the system are displayed here. In UML Sequence diagram 2.0 notation, this symbol is used to include interactive sequence diagram elements. This rectangular form, often known as a frame, features a small inner rectangle for labeling the diagram.
   
   The interactive aspects of the diagram are represented by a package symbol. The Package, like items, is displayed in a rectangular shape, but it has an inner rectangle that is used to name the diagram.
   
   This symbol is used to determine the entities interacts with system. The readers will then understand that there are entities that may interact with the system externally. Not really included but has a part.

5. Option Loop

   
   
   The UML Sequence Diagram Option Loop Symbol is used for Modeling if/then situations. A situation that will only happen if specific circumstances are met.
   
   It signifies that there’s a certain process in a system that needs to be repeated until the desired outcome is met. By using this symbol in designing a sequence diagram, reader will be informed that the system includes a loop function in the system.
   
   The word opt is used to describe a workflow phase that is optional. For example, in a designed UML sequence diagram, you could use this symbol to emphasize a consequence once a certain option is chosen.

6. Alternative

   
   
   To indicate conditional flow in UML sequence diagram, an alternate combined fragment is utilized. It is used to specify an area of a set of lifelines or actors. It also models the if-then-else or conditional logic.
   
   It symbolizes a decision between two or more message sequences (which are frequently mutually exclusive). You can use the designated rectangle shape with a dashed line within to represent options. It will be used as a symbol that there will be consequences between alternative decisions. The decisions may be based on the user or the system.
   
   Readers and programmers will be guided that there should be an alternative functions in the system using this UML sequence diagram symbol.

7. Delete message symbol

   
   
   To delete an object, use the Delete Message symbol. An “X” is added to the end of an arrow to symbolize this. This message causes an object to be destroyed.
   
   This symbol indicates that the role of an actor or an object has ended in the whole process. Through this symbol, programmers will know the limitations or how long the included participants will only be usable. They will then apply the idea in the system development.

8. Messages

   
   
   Messages sent from element to element are used to depict workflow or activity across time in UML sequence diagrams. These Messages are related to the operations and behavior of the Class or objects.
   
   Messages are represented as labeled arrows in UML diagrams, with the arrowhead identifying the call’s direction. The name of the method being called on the receiving object is specified in the text associated with a message when it is sent to an object.
   
   In a UML sequence diagram, a message symbol identifies the type of communication as well as the sender and receiver. Messages can be given any name they like to represent the interaction. A message can be classified as synchronous call, asynchronous call, asynchronous signal, or reply, depending on the type of action it represents.