Activity diagrams: decision and merge nodes

简介 / Introduction

Activity diagrams model business processes in a system. The core elements are decision nodes (branching logic) and merge nodes (converging paths). Both are drawn as diamonds: a decision node picks a branch based on a condition, while a merge node brings multiple paths back together. This article uses the same login flow example in both PlantUML and Mermaid to compare syntax and rendering.

PlantUML 版

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@startuml
skinparam activity {
  BackgroundColor #Fog
  DiamondBackgroundColor #LightYellow
  DiamondBorderColor #Orange
}
|Client|
start
:输入用户名和密码;
:提交登录请求;
if (凭证有效？) then (是)
  :显示仪表盘;
  stop
else (否)
  :显示错误提示;
  :返回登录页;
endif
stop
@enduml

PlantUML uses if (...) then (...) else (...) endif to express decision nodes; merge nodes are implied by endif. The |Client| swimlane syntax tracks the actor. Branch labels like (是) / (否) are written directly after then / else.

Mermaid 版

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flowchart TD
    A[输入用户名和密码] --> B[提交登录请求]
    B --> C{凭证有效？}
    C -->|是| D[显示仪表盘]
    C -->|否| E[显示错误提示]
    E --> F[返回登录页]
    D --> G[结束]
    F --> G

Mermaid draws decision nodes with { } (diamond shape). Branch labels are placed on edges using |是| / |否| syntax. A merge node is simply two edges converging on the same target node — no explicit merge syntax required. Mermaid does not support swimlanes natively, but subgraph can approximate the effect.

对比 / Side-by-side

小结 / Wrap-up

Decision and merge nodes are the basic building blocks of activity diagrams. PlantUML’s if/endif structure reads like code and scales well for deeply nested conditions. Mermaid’s { } nodes with edge labels are more visual and require less ceremony for straightforward flows. Both clearly express branching and convergence; pick based on your team’s toolchain and the diagram’s complexity.