Activity diagrams: parallel fork/join

简介 / Introduction

Parallel activity diagrams express a scenario where multiple tasks execute concurrently and then converge. Both PlantUML and Mermaid support fork/join-style syntax, though their details differ. This article uses the same scenario — “after receiving an order, simultaneously check inventory and verify payment, then confirm” — rendered in both engines for direct comparison.

PlantUML 版

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@startuml
|Department|
start
:Receive order;
fork
  :Check inventory;
fork again
  :Verify payment;
end fork
join
:Confirm order;
stop
@enduml

Mermaid 版

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flowchart TD
    A[Receive order] --> B[/"Parallel"/]
    B -->|"Inventory check"| C[Check inventory]
    B -->|"Payment verify"| D[Verify payment]
    C & D --> E[Confirm order]
    E --> F([End])

Mermaid has no native fork/join node; this example uses a parallel branch symbol /"..."/ with & convergence to achieve equivalent logic.

对比 / Side-by-side

小结 / Wrap-up

PlantUML’s fork/join follows the UML standard and suits formal modeling; Mermaid achieves the same effect via parallel branches and & convergence, winning on simplicity. Either approach clearly conveys parallel logic — pick based on your needs.