par

Attested. A fixture written in AXON is compiled and executed by a gate in the test suite, so this page describes behaviour a test exercises.

Since pre-v2.15.0; real fan-out v2.15.0 · Used inside a declaration

Grammar

par {
    <branch-steps>
}

par is concurrent fan-out: its branches execute in parallel and their results join.

What the runtime actually does (v2.15.0)

A real futures::join_all over the lowered branches (parallel::run_par) — not sequential execution wearing a concurrent keyword. Branch tool calls ride the v2.69.0 channel semaphore, so a resource { capacity: N } bound holds ACROSS a fan-out.

Honest limits

Static branches only: par over <collection> (dynamic fan-out) does not exist in the grammar today, which is also why par needs no concurrency backstop of its own — the author writes each branch.

See also

• axon://primitives/tool — calls inside branches stay governed.