UML Sequence Diagram Summary

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? Sequence diagrams are primarily used to show the order in which objects interact with each other.

A sequence diagram represents an interaction as a two-dimensional graph. The vertical is the timeline, and the time extends downward along the vertical bar. The horizontal axis represents the meta roles of individual objects in collaboration. The class meta role is represented by a lifeline. When an object is present, the character is represented by a dashed line, and the lifeline is a double-lane when the object's procedure is active. Messages are represented by arrows from the lifeline of one object to the lifeline of another object. The arrows are arranged in chronological order from top to bottom in the diagram.

• Elements involved in a sequence diagram:

① Lifeline:
The lifeline name can be underlined. When an underscore is used, it means that the lifeline in the sequence diagram represents a specific instance of a class.

② Synchronizing messages
The sender waits for a synchronous message response before it continues.

③ Asynchronous messages
There is no need to wait for a response message until the sender continues.

④ notes

⑤ constraints
The symbol for the constraint is simple; the format is: [Boolean Test]

⑥ Combined Fragment
Combined fragments are used to solve the conditions and methods of interactive execution. It allows a logical component to be represented directly in a sequence diagram to define special conditions and sub-processes for any part of any lifeline by specifying the conditions or the area of application of the child process.

• The commonly used combination fragments are:

① Choice (ALT)
The choice used to indicate a mutually exclusive selection between two or more message sequences, equivalent to the classic if. else ...
The choice takes place only one sequence in any situation. You can set a threshold in each fragment to indicate the conditions that the fragment can run. Else the critical indicates that any other critical is not true when the fragment should run. If all the thresholds are False and there is no else, no fragments are executed.

② option (OPT)
Contains a sequence that can or does not occur

③ Loop (Loop)
The fragment repeats a certain number of times. You can indicate the condition of the fragment repetition in the threshold.

④ Parallel (Par)

• The following table lists the commonly used combined fragments:

• about how to interpret sequences of fragments
By default, a sequence diagram indicates a series of messages that may occur. On a running system, you may see other messages that you have not selected to display on the diagram. The following fragment types can be used to change this interpretation:

UML Sequence Diagram Summary