Behaviour for a durable FSM definition.

defmodule Checkout do
  use GenDurable.FSM, version: 1, queue: "checkout", state: Checkout.State

  @impl true
  def step("start", %{state: s}), do: {:next, "await_pay", %{s | n: s.n + 1}}
  def step("await_pay", ctx) do
    case Enum.find(ctx.signals, & &1.name == "payment_confirmed") do
      nil -> {:await, "payment_confirmed", ctx.state}
      sig -> {:next, "ship", apply_payment(ctx.state, sig)}
    end
  end
  def step("ship", _ctx), do: {:done, %{"shipped" => true}}

  @impl true
  def handle(reason, ctx) do
    if ctx.attempt < 5, do: {:replay, ctx.state, backoff(ctx.attempt)}, else: {:stop, reason}
  end
end

use options

• :name — FSM name stored in the fsm column (default: inspect(module)).
• :version — fsm_version (default 1). Old versions coexist as separate registered modules; see GenDurable.Registry.
• :queue — default queue for instances (default "default").
• :state — the GenDurable.State embedded-schema module (default nil, i.e. plain-map state).
• :initial — initial step for GenDurable.insert/2 (default "start").

handle/2 defaults to {:stop, reason} and is overridable.