State diagrams: choice and fork pseudostates

简介 / Introduction

State machines describe how an object transitions through a set of discrete states during its lifecycle. Two important pseudostates power most real-world models:

• choice — conditional branching, like if/else, selecting the next state based on guard conditions.
• fork — parallel branching, splitting a single flow into multiple concurrent paths.

This post uses the same scenario (order processing) to demonstrate both PlantUML and Mermaid, then compares them head-to-head.

PlantUML 版

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@startuml
hide empty description

state "新订单" as New
state "已支付" as Paid
state "已发货" as Shipped
state "已签收" as Received
state "已取消" as Cancelled
state "退款中" as Refunding

[*] --> New
New --> Paid : paymentOk
New --> Cancelled : paymentFail
Paid --> Shipped : ship
Paid --> Refunding : refundRequest

state fork_state <<fork>>
Paid --> fork_state
fork_state --> Shipped
fork_state --> Refunding

state join_state <<join>>
Shipped --> join_state
Refunding --> join_state
join_state --> Received

Received --> [*]
Cancelled --> [*]
@enduml

PlantUML uses <<fork>> and <<join>> stereotypes to mark fork and join pseudostates on the diagram; multiple arrows then branch out from the fork.

Mermaid 版

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stateDiagram-v2
    [*] --> New
    New --> Paid : paymentOk
    New --> Cancelled : paymentFail
    Paid --> Shipped : ship
    Paid --> Refunding : refundRequest

    Paid --> Fork : ship
    Fork --> Shipped
    Fork --> Refunding

    Shipped --> Received
    Refunding --> Received
    Received --> [*]
    Cancelled --> [*]

Mermaid uses Fork and Join as special state nodes to achieve the same effect, with guards written inline on the transition lines.

对比 / Side-by-side

小结 / Wrap-up

choice and fork are the two core pseudostates in state machine modeling. PlantUML implements them via <<choice>>, <<fork>>, and <<join>> stereotypes — semantically precise but more verbose. Mermaid uses built-in Fork/Join nodes — much shorter but lacks an explicit choice pseudostate (guards are inline only). Pick based on whether you need semantic precision or syntactic brevity.