State machines

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A durable FSM is a module with a step/2 clause per step name. The engine's one guarantee: on step completion, the new state is committed to the database before execution proceeds. On a crash before commit, the step re-executes from scratch (at-least-once). The unit of re-execution is the whole step — make each step idempotent, and keep steps small.

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

  defmodule State do
    use GenDurable.State

    embedded_schema do
      field :order, :integer
      field :n, :integer, default: 0
    end
  end

  @impl true
  def step("start", %{state: s}), do: {:next, "ship", %{s | n: s.n + 1}}
  def step("ship", %{state: s}), do: {:done, %{"order" => s.order}}
end

{:ok, id} = GenDurable.insert(Checkout, state: %{order: 42})

The step text column maps to a function-clause head; the instance walks from clause to clause until a terminal outcome.

State

ctx.state is the instance's state, loaded before every step and committed after it.

  • Nested State schema (recommended). Declare a defmodule State with use GenDurable.State inside the FSM module; it is adopted by convention (no option needed) and ctx.state is that typed struct. Return a struct of the same type from your outcome.
  • Explicit :state. Point at a schema module elsewhere with use GenDurable.FSM, state: MyState.
  • Plain map. Omit both; ctx.state is a string-keyed map. Supported but untyped.

Outcomes

A step (and the error handler handle/2) returns one of:

OutcomeEffect
{:next, step, state}transition to step, runnable, attempt := 0
{:next, step, state, opts}…with per-transition rate_limit:/weight:
{:retry, state, delay_ms}re-run the same step after delay_ms, attempt += 1
{:await, names, next_step, state}park until a signal arrives
{:schedule_childs, next_step, children, state}fan out and wait for the children
{:done, result}terminal, done, result recorded
{:stop, reason}terminal, failed, reason recorded

:next resets the attempt counter (a fresh step); :retry keeps re-running the current step with attempt += 1 (the poll/backoff primitive) — the delay and the retry policy are yours, computed from ctx.attempt.

Errors

A raised exception routes to handle(reason, ctx), which returns any outcome above (it defaults to {:stop, reason}, and is overridable):

@impl true
def handle(reason, ctx) do
  if ctx.attempt < 5, do: {:retry, ctx.state, 1_000 * ctx.attempt}, else: {:stop, reason}
end

A worker crash (no return at all) is different: the reaper recovers it and re-runs the step from scratch — handle/2 is not called.

Options

use GenDurable.FSM, ...:

  • :name — the fsm column value (default inspect(module)).
  • :version — old versions coexist as separately-registered modules; instances finish on the version they started.
  • :queue — default queue for instances.
  • :initial — the first step (default "start").
  • :state — the GenDurable.State schema module (usually unnecessary; see above).