A sequence diagram is a type of diagram used in software engineering to depict the sequence of messages passing between objects in a system.
It is a graphical tool that helps to visualize the interactions between objects in a system. A sequence diagram shows how objects communicate with each other, and it can help you to find problems in your system design. Some of the permitted types of interactions in a sequence diagram are method calls, returns and exceptions. OCL constraints can be used to define object properties at certain “Points-in-time”.
- Describe the flow of messages, events, and actions between objects
- Show concurrent processes and activations
- Show time sequences that are not easily depicted in other diagrams
- Typically used during analysis and design to document and understand the logical flow of your system
The elements in a sequence diagram are objects (instances of various classes). The messages exchanged by these elements are method invocations.
A system diagram is used to visualise a system constructed from different entities. For example, if you have a system that displays the sequence of interactions between a customer and a company employee (like an order sales process), you would draw a system diagram to show how these two entities are interconnected. In this case, both entities are considered different parts of the system but interact with each other.
Sequence diagrams are particularly useful for identifying potential dependencies between objects and troubleshooting problems. They can also be used to document a system’s design and verify that it meets requirements.
With a sequence diagram, the desired system run can be represented to understand better the system that needs to be implemented. Furthermore, a sequence diagram can define exemplary runs in tests and model test drivers. In sequence diagrams, we need to analyse and document the use cases. Each use case describes the main business task required. The system use case diagram is part of this activity.
Later, we come to design each subsystem, namely each element participating in the system use case diagram. We draw one or more class diagrams depicting the classes participating in the solution. Then we can draw a sequence diagram for the most important (or difficult to understand) methods.
System Sequence Diagram
System Sequence diagrams are graphical, visual representations of systems. They show the system’s components and how they interact with each other. System Sequence diagrams can help you understand how a system works and troubleshoot problems.
System Sequence Diagram is used to depict the activities that are performed by a system as a sequence of operations. This graphical representation of activities helps the business analyst to generate code in the programming language that is required to make the system work.
The elements participating (exchanging messages) in a system sequence diagram are Actors and Systems. The messages exchanged by these elements could be of any type depending on the systems (from web service calls to data input from a human).
System Sequence Diagrams define the input and outputs and sequence of interaction between the user and the system for a use case. They are used in conjunction with activity diagrams. In sequence diagrams, information flows in and out of a system called messages. The users are identified, and detailed messages are described.
A sequence diagram, in other words, is used to visualise the execution of a system. For example, if you are building a system where both employees and customers can submit an order, you would draw a sequence diagram of how orders are passed between the customer and the company employee. In this case, both entities are considered different parts of the same system, but they do not interact with each other.
A Sequence Diagram helps the system designer, implementer, and user understands the interactions between humans and the system. It is also known as the System Sequence Diagram. In a Sequence Diagram, the sequence of interactions between the different actors is depicted. It helps determine the sequence of interactions of the actors involved.
A System sequence diagram visualises a use case, while a sequence diagram visualises a method of a class. In a system sequence diagram, participating elements are known as ‘actors and systems’, while in a sequence diagram, elements participating are called ‘objects’.