Sequence Diagram
Understand how sequence diagrams represent interactions between actors and objects.
A sequence diagram is a form of communication diagram that illustrates how different actors and objects interact with each other or between themselves. The diagram represents these interactions as an exchange of messages between various entities and the type of exchange. Sequence diagrams also demonstrate the sequence of events that occur in a specific use case and the logic behind different operations and functions.
Elements of a sequence diagram
Various elements make up a sequence diagram. Let's discuss some of the essential elements of this diagram that appear most often.
Lifeline
In sequence diagrams, we write all entities horizontally. Each entity has a lifeline that represents its existence, i.e., when the entity is activated or deactivated. We illustrate this using a dotted line below the entity. We represent different objects, actors, entities, or boundaries in a system using lifelines, and they never overlap each other. The illustration below shows how to visualize a lifeline in UML. The horizontal boxes are used to denote the objects involved in the interaction.
Activation bars
Activation bars indicate the active period of an object, that is, the time when an object sends or receives messages. We draw these using simple vertical boxes on the lifeline. Here's an example of an activation bar:
Messages
In sequence diagrams, a message is an interaction between two objects. It can be in the form of sending and receiving messages, invoking an operation, or creating a new object or entity. Messages are drawn horizontally in any ...