Sequence diagrams: grouping with divider/box

简介 / Introduction

When a sequence diagram has many messages it gets messy. Dividers / Groups / Boxes let you visually containerize related messages. PlantUML uses group and so does Mermaid — the syntax is nearly identical but the details differ.

PlantUML 版

@startuml
skinparam backgroundColor #FEFEFE
skinparam groupBackgroundColor #E8F4FD
skinparam groupBorderColor #1E90FF

participant Client
participant "Auth Service" AS Auth
participant "API Gateway" AS GW

group 认证阶段
    Client -> Auth : 登录请求
    Auth -> Auth : 验证凭证
    Auth --> Client : JWT Token
end

group 业务请求
    Client -> GW : 请求 + JWT
    GW -> GW : 校验 Token
    GW --> Client : 业务数据
end
@enduml

Mermaid 版

sequenceDiagram
    participant C as Client
    participant A as Auth Service
    participant G as API Gateway

    group 认证阶段
        C->>A: 登录请求
        A->>A: 验证凭证
        A-->>C: JWT Token
    end

    group 业务请求
        C->>G: 请求 + JWT
        G->>G: 校验 Token
        G-->>C: 业务数据
    end

对比 / Side-by-side

Feature	PlantUML	Mermaid
Keyword	`group <label>`	`group <label>`
End marker	`end`	`end`
Nesting	Yes, multi-level	Yes
Styling	`skinparam groupBackgroundColor`	CSS / theme override
Label	Single line, right after `group`	Same as PlantUML
Arrow style	`->` / `-->`	`->>` / `-->>`

小结 / Wrap-up

PlantUML and Mermaid share nearly identical group syntax, so the learning curve is minimal. PlantUML lets you deeply customize colors and borders via skinparam; Mermaid currently relies more on CSS themes. For quick small diagrams Mermaid wins; when you need fine-grained styling go with PlantUML.