# `Accrue.Dunning.Engine`
[🔗](https://github.com/szTheory/accrue/blob/accrue-v1.5.1/lib/accrue/dunning/engine.ex#L1)

Behaviour for dunning campaign orchestration engines (DUN-03, D-01).

Engines control **which orchestration system** is invoked at campaign
boundaries. DB state (`dunning_campaign_started_at` on
`accrue_subscriptions`) is managed by Accrue regardless of engine choice —
the engine only decides how the resulting Oban jobs or external workflows
are enqueued and cancelled.

## Built-in engine

`Accrue.Dunning.Engine.Oban` is always-on and requires no additional
configuration. It wraps the existing Oban-backed `DunningStep` worker.

## Optional Chimeway engine

`Accrue.Integrations.Chimeway` is off by default and conditionally compiled
when the optional `:chimeway` dependency is present. To opt in:

    # config/config.exs
    config :accrue, dunning: [engine: Accrue.Integrations.Chimeway]

The `:chimeway` dep must also be added to the host `mix.exs`:

    {:chimeway, "~> 1.0"}

## Dispatch

The engine is invoked directly via `Config.dunning_engine().start_campaign(...)`.
This module is behaviour-only — no facade delegation functions are defined
here, unlike `Accrue.Auth`.

# `cancel_campaign`

```elixir
@callback cancel_campaign(
  subscription :: Accrue.Billing.Subscription.t(),
  iso_anchor :: String.t(),
  opts :: keyword()
) :: :ok | {:error, term()}
```

# `start_campaign`

```elixir
@callback start_campaign(
  subscription :: Accrue.Billing.Subscription.t(),
  anchor_at :: DateTime.t(),
  opts :: keyword()
) :: :ok | {:error, term()}
```

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*Consult [api-reference.md](api-reference.md) for complete listing*