Sequence diagrams: alt / else / opt fragments

简介 / Introduction

The alt / else / opt fragments in sequence diagrams express conditional branching and optional interactions. Both PlantUML and Mermaid support these fragments, but with slightly different syntax details. This article compares both using the same scenario.

PlantUML version

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@startuml
Alice -> Bob: operation request
alt authorization succeeded
    Bob -> Carol: forward request
    Carol --> Bob: response data
else authorization failed
    Bob -> Dave: log error
    Dave --> Bob: error acknowledgment
end
Bob --> Alice: operation result
@enduml

PlantUML uses alt / else / end to form a complete conditional block. Multiple else branches are supported, with guards written on the same line as alt.

Mermaid version

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sequenceDiagram
    Alice->>+Bob: operation request
    alt authorization succeeded
        Bob->>+Carol: forward request
        Carol-->>-Bob: response data
    else authorization failed
        Bob->>+Dave: log error
        Dave-->>-Bob: error acknowledgment
    end
    Bob-->>-Alice: operation result

Mermaid’s alt / else structure mirrors PlantUML closely. Indentation marks nesting and end closes the block.

对比 / Side-by-side

小结 / Wrap-up

Both engines share nearly identical fragment syntax for alt / else / opt / loop. PlantUML is more flexible with nested conditions and guard expressions; Mermaid is more concise and readable. Choose based on your team’s toolchain and editor support.