Activity diagrams: swimlanes / partitions

简介 / Introduction

Activity diagrams describe the flow of actions in a business process. Swimlanes group actions by responsible party or system partition, making the flow clear at a glance. PlantUML has native swimlane syntax; Mermaid approximates it with flowchart subgraphs.

PlantUML 版

PlantUML uses |partition name| to declare swimlanes; actions inside a partition belong to that lane.

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@startuml
|Order Processing|
:Receive customer order;
|Inventory Management|
:Check stock;
|Shipping|
:Pack items;
|Order Processing|
:Send confirmation email;
|Inventory Management|
if (Stock available?) then (Yes)
  :Update inventory records;
else (No)
  :Generate restock alert;
endif
|Shipping|
:Arrange courier;
@enduml

PlantUML swimlanes are automatically arranged vertically, with arrows connecting action nodes across partitions.

Mermaid 版

Mermaid flowchart has no native swimlane syntax; use subgraph with LR or TD direction to approximate partitions:

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flowchart LR
    subgraph "Order Processing"
        A1[Receive order] --> A2[Send confirmation]
    end
    subgraph "Inventory Management"
        B1[Check stock] --> B2{Available?}
        B2 -->|Yes| B3[Update records]
        B2 -->|No| B4[Generate restock alert]
    end
    subgraph Shipping
        C1[Pack items] --> C2[Arrange courier]
    end
    A1 --> B1
    B3 --> C1
    B4 --> C1
    A2 -->|notify| A1

Note that Mermaid subgraphs do not auto-align between groups; you must manually order nodes to simulate swimlane behavior.

对比 / Side-by-side

小结 / Wrap-up

PlantUML’s |swimlane| syntax is the standard implementation for activity diagram swimlanes, ideal for formal business process documentation. Mermaid approximates swimlanes with flowchart subgraphs — faster to set up and integrate — but requires extra care for partition alignment. For precise swimlane layout, use PlantUML; if you are already in the Mermaid ecosystem, the subgraph compromise is acceptable.