Child fan-out

schedule_childs spawns a batch of child instances and parks the parent on a join barrier that releases only when every child reaches a terminal status. It is "fan work out, then wait for all of it."

def step("fan", %{state: s}) do
  children = for kid <- s["kids"], do: {LineItem, state: kid}
  {:schedule_childs, "join", children, s}
end

def step("join", %{childs: childs}) do
  {:done, %{"shipped" => Enum.count(childs, &(&1.status == "done"))}}
end

{:schedule_childs, next_step, children, state}:

• children is a list of child specs, each {FsmModule, insert_opts} (or a bare FsmModule). They are ordinary instances, stamped with a parent_id.
• The spawn and the park happen atomically — children become runnable only once the parent is parked, so a child can never finish before the barrier exists.
• The parent re-runs next_step (never the spawning step) once all children are terminal.

The join

When the parent wakes, its children are in ctx.childs:

[%{id: _, fsm: _, status: "done" | "failed", state: _, result: _, last_error: _}, ...]

The barrier waits for termination, not success — a child that ends failed still releases its slot. The parent sees the failure in ctx.childs and decides what to do (proceed, {:stop, _}, compensate). The engine does not auto-propagate child failure.

ctx.childs holds all of the instance's children still in the database — including those from earlier schedule_childs rounds (until GC prunes them). A parent that fans out repeatedly should filter by its own bookkeeping (e.g. ids or a marker in the child state) rather than assume the list is the latest batch.

Not covered

• No cancel/cascade — terminating or failing a parent does not touch its children, and vice-versa.
• No quorum — the join is all-or-nothing; build k-of-n yourself with await + per-child signals.
• No barrier deadline — a child that hangs forever hangs the parent; bound it inside the child (e.g. {:retry, ...} with a timeout).
• Nesting is free — a child may itself schedule_childs, each level an independent barrier.

Children are ordinary terminal rows after they finish; they are reaped by GC like any other terminal instance, not deleted on read.