Sequence diagrams: lifeline control

简介 / Introduction

In sequence diagrams, a participant’s lifeline becomes active at different stages and can be destroyed when its role ends. Both PlantUML and Mermaid provide clean syntax to express these semantics — this article demonstrates the same scenario in both engines.

PlantUML 版

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@startuml
participant Client
participant "Order Service" as Order
participant "Payment Service" as Pay

Client -> Order: Place order
activate Order
Order -> Pay: Check balance
activate Pay
Pay --> Order: Balance OK
deactivate Pay
Order -> Order: Create order
destroy Order
Client <-- Order: Order placed
@enduml

PlantUML uses activate to activate a lifeline, deactivate to release activation, and destroy to terminate it with an × marker. Nested activations can be expressed with color blocks or nested activate blocks.

Mermaid 版

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sequenceDiagram
    participant Client
    participant Order as "Order Service"
    participant Pay as "Payment Service"

    Client->>+Order: Place order
    Order->>+Pay: Check balance
    Pay-->>-Order: Balance OK
    Order->>+Order: Create order
    Order-->>-Client: Order placed
    destroy Order

Mermaid uses the + prefix on an arrow to activate a lifeline and - to release it. The destroy keyword also terminates a lifeline with ×. Color-based activation via activate(name) / deactivate(name) requires plugins; the core syntax relies on +/- arrow modifiers.

对比 / Side-by-side

小结 / Wrap-up

Both engines share the same conceptual model: activate with +/activate, release with -/deactivate, and destroy with destroy. PlantUML offers richer visuals (color blocks, nested levels) while Mermaid’s +/- prefix notation is more compact. Choose based on your team’s toolchain and diagram complexity.