![\"Illustrated](\"https:\/\/www.vpascode.com\/tw\/wp-content\/uploads\/sites\/2\/2026\/08\/plantuml-sequence-diagram-guide-hero-banner.jpg\")
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What is a PlantUML Sequence Diagram? (And Why Use Diagram-as-Code)<\/h2>\n
A PlantUML sequence diagram illustrates the step-by-step exchange of messages between participants in a system during a specific execution scenario. Unlike traditional drag-and-drop design tools, PlantUML follows the Diagram-as-Code<\/strong> paradigm, allowing software architects and engineers to write readable script files (such as PlantUML uses simple, human-readable keywords to define system entities and interaction paths.<\/p>\n You can define participants explicitly using keywords that match their role in the system architecture:<\/p>\n The direction and appearance of arrows indicate how communication flows between components:<\/p>\n <\/p>\n Copy and adapt these common architectural patterns into your technical documentation.<\/p>\n Corresponding PlantUML code:<\/p>\n .puml<\/code>) that render programmatically into diagrams.<\/p>\nKey Benefits: Version Control, Speed, and Consistency<\/h3>\n
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\nCore Syntax Basics: How to Create Your First Sequence Diagram<\/h2>\n
Declaring Participants, Actors, and Boundaries<\/h3>\n
@startuml\r\nactor User\r\nparticipant \"API Gateway\" as Gateway\r\ndatabase \"PostgreSQL\" as DB\r\nboundary \"Web App\" as UI\r\n\r\nUser -> UI: Clicks Submit\r\nUI -> Gateway: POST \/api\/submit\r\nGateway -> DB: Save Record\r\n@enduml<\/code><\/pre>\nMessaging and Arrows: Synchronous vs. Asynchronous Calls<\/h3>\n
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\n \nSyntax<\/th>\n Visual Output<\/th>\n Communication Type<\/th>\n<\/tr>\n<\/thead>\n \n A -> B<\/code><\/td>\nSolid arrow with solid head<\/td>\n Synchronous message call<\/td>\n<\/tr>\n \n A --> B<\/code><\/td>\nDotted arrow with solid head<\/td>\n Response \/ Return message<\/td>\n<\/tr>\n \n A ->> B<\/code><\/td>\nSolid arrow with open head<\/td>\n Asynchronous message call<\/td>\n<\/tr>\n \n A -\\ B<\/code><\/td>\nHalf-headed arrow<\/td>\n Unidirectional \/ Event message<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
\nReal-World PlantUML Sequence Diagram Examples<\/h2>\n
Example 1: User Authentication & JWT Token Flow<\/h3>\n
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