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Black box or transparent? How do system sequence diagrams change our understanding of software?
In the field of software engineering, a system sequence diagram is a visual tool that displays process interactions in chronological order. This kind of diagram not only highlights the processes and objects involved, but also shows the sequence of messages exchanged to achieve the function. System sequence diagrams are often closely related to the implementation of use cases, especially in the 4+1 architectural view model. In a specific use case scenario, these diagrams show events generated by external actors, their sequence, and possible cross-system events.
System sequence diagrams should be compiled for the use case's primary success scenario as well as common or complex alternative scenarios.
Key elements
System sequence diagrams show multiple handlers, or objects, represented by vertical lifelines so that they can exist simultaneously. Horizontal arrows represent the messages exchanged between objects in the order of events. This graphical representation can clearly specify simple execution situations. The system sequence diagram should clearly show the following elements:
- External roles
- Messages (methods) triggered by these characters
- Return value related to the previous message (if any)
- Indication of any loop or iteration region
Interpretation of system sequence diagram
When professionals develop projects, they often use system sequence diagrams to illustrate how users and the system perform specific tasks. These tasks may include repetitive, simple or complex activities. The purpose is to present use cases in a visual format. Familiarity with Unified Modeling Language (UML) is a must for drawing system sequence diagrams.
These models show the logic between roles (people who influence the system) and the tasks the system performs.
Reading a sequence diagram usually starts at the top and works down to the details below the role or system. Under each character or system there are long dotted lines called "lifelines". The lines connecting these lifelines and actions show the interactions between characters or systems. Messages typically appear above and below system sequence diagrams detailing the action. For example, a login request for a role might be expressed as login(username, password). After each action is performed, the corresponding response or next action will be located below the previous action. Through top-down reading, you can learn in detail how specific actions are performed in the mentioned model, and the order in which they are performed.
Image construction basis
If the lifeline represents an object, it represents a character. Leave the instance name blank to represent an anonymous, unnamed instance.
Messages are shown by horizontal arrows with the name of the message, solid arrows represent synchronous calls, and open arrows represent asynchronous messages. Dotted lines represent reply messages. If the caller sends a synchronous message, it must wait until the message is completed before proceeding to the next step; if it sends an asynchronous message, it can continue its processing without waiting for a response. Asynchronous calls are common in multi-threaded applications, event-driven applications, and message-based middleware.
Advanced interactive snippets
The startup box or method call box is an opaque rectangle drawn on the lifeline, which represents the process executed according to the message. If an object is destroyed (removed from memory), an X is drawn below the lifeline, and no dotted line is drawn below it. This should be triggered by some message, either from the object itself, or a call from another object.
UML has made important improvements to the capabilities of sequence diagrams. These improvements are based on the concept of interaction fragments, which represent smaller parts within a closed interaction and can represent parallel execution, conditional branches, and optional interactions.
Understand the necessity of sequence diagrams
With the continuous advancement of information technology and the evolution of use cases, the application of system sequence diagrams has become increasingly widespread. They are not only communication tools between technicians, but also the key to understanding how the system operates, helping developers, users and management to reach a consensus. Facing increasingly complex system structures, should we rethink how to use these graphical tools to better understand and develop our software products?
