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Conditional Block

An `alt`/`else` or `opt` block — a branch in the message flow that depends on a runtime condition.

6 examples across every diagram type

Read more in the Mermaid docs →
Sequence diagram, Zombie Mermaid alt [Valid credentials] [Invalid] [Account locked] Login 200 OK 401 Unauthorized 403 Forbidden Client Server
Sequence diagramSequence: Alt/Else Block
Sequence diagram, Zombie Mermaid opt [Cache miss] Get data Cache miss Query Results Store in cache App Cache Database
Sequence diagramSequence: Opt Block
Sequence diagram, Zombie Mermaid alt [Success] [Insufficient funds] POST /transfer BEGIN Debit account A Credit account B INSERT audit_log COMMIT 200 OK ROLLBACK 400 Bad Request Client Server Database
Sequence diagramSequence: Database Transaction
Sequence diagram, Zombie Mermaid alt [Multiple panels] [Single panel] CMD+W event.preventDefault() WINDOW_CLOSE_REQUESTED Start 3s timer closePanel(focusedId) 3s elapsed → window.destroy() closePanel(lastId) Auto-select fires → new panel created! 3s elapsed → window.destroy() Panel removed No confirmCloseWindow! Stack becomes [] Panel reopens User Main Process Renderer 3s Fallback Timer
Sequence diagramSequence: Self-Messages with Notes
Sequence diagram, Zombie Mermaid alt [credentials valid] [credentials rejected] x y A B C
Sequence diagramSequence: Alt/Else Long Label With Uninvolved Participant
Sequence diagram, Zombie Mermaid loop [every retry] opt [warm up] alt [cache hit] [cache miss] fetch rebuild waits for the upstream response done A B
Sequence diagramSequence: Nested Blocks With Notes