Activities, Retries, And Idempotency

Activities are where side effects belong. Workflow code decides what should happen; activity code talks to the outside world.

defmodule MyApp.Activities.ChargeCard do
  use Continuum.Activity,
    retry: [max_attempts: 5, backoff: :exponential, base_ms: 500],
    timeout: {:seconds, 30}

  @impl true
  def run(%{order_id: order_id, amount: amount}) do
    MyApp.Payments.charge(order_id, amount)
  end

  @impl true
  def idempotency_key([%{order_id: order_id}]) do
    "charge:#{order_id}"
  end
end

Call an activity from a workflow with the activity macro:

{:ok, charge} =
  activity MyApp.Activities.ChargeCard.run(%{order_id: order_id, amount: total}),
    retry: [max_attempts: 5, backoff: :exponential, base_ms: 500],
    idempotency_key: "charge:#{order_id}"

The Postgres runtime inserts a row in continuum_activity_tasks. The activity dispatcher leases available tasks with FOR UPDATE SKIP LOCKED, starts a worker, and the worker journals either activity_completed or activity_failed.

Retry policy is resolved in this order:

1. The activity ... retry: ... option at the call site.
2. The use Continuum.Activity, retry: ... module option.
3. A single attempt.

backoff: :exponential uses base_ms * 2 ^ (attempt - 1). Any other backoff value uses constant delay.

Idempotency keys are enforced by the Postgres runtime. Once an activity result is committed for an activity module and key, another task with the same module and key journals that committed result without running the activity body again.

This guarantee starts after Continuum commits success. Activities that perform externally visible writes, such as payments, emails, or third-party API mutations, should still pass their own idempotency key to the external system. That closes the remaining window where a worker can crash after the external write succeeds but before Continuum commits the result.

See guides/idempotency.md for the exact scope.