Combined fragment

A frame around messages with the operator in a pentagon at its top left. Each operand has a [guard] above its first event, and dashed lines divide operands.

diagram sequence "Alternatives"

participant Client
participant Server

alt "ok" {
  Client -> Server : get
} else "retry" {
  loop "(1,3)" {
    Client ->> Server : retry
  }
}

Operators: alt with else, opt, loop, break, critical, neg, and par with and. loop "(1,3)" becomes loop(1,3), which is the spec’s textual form.

Syntax

alt ["guard"] { ... } [else ["guard"] { ... }]...
par ["guard"] { ... } [and ["guard"] { ... }]...
opt|loop|break|critical|neg ["guard"] { ... }

Specification

UML 2.5.1. Operands, constraints and frames: 17.6.4.1, 17.6.4.2, 17.6.4.3 (p. 585). Loop: 17.6.4.9 (p. 586). Coregions are an optional shorthand; par covers them.

Used in

Raft log replication, RPC with retry and exponential backoff, TLS 1.3 handshake.