PlantUML Sequence Diagram: Basic Syntax

PlantUML Sequence Diagram Basics

Sequence diagrams show how objects interact over time. They are one of the most commonly used UML diagram types for documenting APIs, microservice communication, and user flows.

Declaring Participants

@startuml
participant "Web Client" as WC
participant "API Server" as API
database "PostgreSQL" as DB
@enduml

Participants appear in the order they are declared. Use the as keyword to create shorter aliases. PlantUML supports several participant types: participant, actor, database, boundary, control, entity, collections, and queue.

Arrow Types

Arrow	Meaning
->	Solid line, solid arrowhead (synchronous call)
-->	Dashed line, solid arrowhead (return/response)
->>	Solid line, open arrowhead (asynchronous)
-->	Dashed return

Adding Messages

Alice -> Bob: Hello Bob
Bob --> Alice: Hi Alice
Alice ->> Bob: Async message

The text after the colon becomes the message label on the arrow. Messages are rendered in the order they appear in the source.

Activation and Deactivation

Use activate and deactivate to show when a participant is processing:

Client -> Server: Request
activate Server
Server -> DB: Query
activate DB
DB --> Server: Results
deactivate DB
Server --> Client: Response
deactivate Server

Grouping Messages

Use alt, else, opt, loop, par, and break to group messages:

alt success
  Server --> Client: 200 OK
else failure
  Server --> Client: 500 Error
end