Defines an activity — the only place where side effects are allowed.

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

  @impl true
  def run(order_id, amount) do
    Stripe.charge(order_id, amount)
  end
end

Activities are not subject to the determinism scanner — they can do arbitrary I/O, talk to NIFs, raise, etc. Their return value is journaled on first success and replayed on workflow resume. Results therefore must not contain PIDs, references, ports, or functions; an invalid result becomes a non-retryable activity failure with a path-aware Continuum.DurableTermError.

Execution policy is normalized by Continuum.Activity.Policy. Invalid module defaults fail compilation; invalid call-site overrides or idempotency keys fail the workflow before durable work is scheduled.

Implementing the optional idempotency_key/1 callback stores the key in the durable task payload. Once an activity result has been committed, another task for the same activity module and key reuses that committed result without running the activity body again. This suppresses duplicate execution after Continuum has recorded success; external systems should still receive the same key because a worker can crash after the side effect succeeds but before Continuum commits the result.