Free Online Sequence Diagram Maker

The main elements of a sequence diagram include six types: roles, objects, lifelines, control focus, messages, and combined fragments.

ProcessOn includes a comprehensive library of UML 2.5 sequence diagram symbols, including roles, objects, entities, lifelines, control focus, and combined segments. Drag-and-drop operation is simple and efficient, with intelligent line snapping, one-click alignment, and batch spacing adjustment, allowing for quick layout even with complex UML sequence diagrams.

Objects are represented at the top of a sequence diagram as a rectangle. There are generally three methods of naming objects:

1) Display both object name and class name, for example: “iPhone:Phone”
2) Display only the object name, not the class name, for example: “iPhone:”
3) Display only the class name, not the object name, for example: “:Phone”

Messages are generally divided into synchronous messages, asynchronous messages, return messages, and self-associated messages.

Synchronous Message: After sending a message, the object waits for the receiving object to respond and return the message before proceeding with other tasks, represented by a “solid line arrow”;

Asynchronous Message: After sending a message, the object does not need to wait for the receiving object's return message and can continue with other tasks, represented by a “solid line with greater than sign”;

Return Message: Returns from a process call, represented by a “dashed line arrow”;

Self-Associated Message: Represents a method's self-call, or a method within an object calling another method, represented by a “semi-closed rectangle with a solid line arrow below”.

ProcessOn supports setting various line styles to represent sequence diagram message types, including synchronous messages, asynchronous messages, return messages, and self-related messages.

A combined fragment is a logical grouping represented by a rectangle, which includes conditional structures that affect the message flow. It is used to define special conditions and subprocesses for any part of any lifeline by specifying conditions or application areas of subprocesses.

There are 13 types of combined fragments, which can be used to describe various control and logical structures in a compact and concise manner.

Sequence diagrams can be used at different stages of the development process to describe the interaction between objects in a system.

Analysis: In the analysis phase, sequence diagrams can help identify the classes needed by the system and the operations objects perform in interactions.

Design: In the design phase, sequence diagrams illustrate how the system works to complete interactions.

Construction: During the construction of the system architecture, sequence diagrams can be used to illustrate the behavior of design patterns and the mechanisms used by the system.

Sequence diagrams and collaboration diagrams are both UML interaction diagrams that can be converted into each other, have similar responsibilities, are message-driven, and have sequentiality.
However, there are also significant differences. Sequence diagrams express the chronological order of messages generated between objects, while collaboration diagrams express the cooperative relationships between objects.