# OrderDocumentsProvider

## Import

```typescript
import { OrderDocumentsProvider } from "@alepha/commerce/checkout";
```

## Overview

What documents exist for an order - invoice numbers, in practice.

### Why a registry and not a substitution

The first version made this an abstract provider that `invoicing` substituted.
It broke on boot: `shipping` and `invoicing` are siblings that both import
`checkout`, so whichever registered first wired the checkout and instantiated
its default, and the second got `TooLateSubstitutionError`. Substitution is
order-dependent by nature, and two independent plugins cannot be ordered.

A collection that modules push into has no such problem - the same shape as
`ProductKindRegistry` and `RootComponentsProvider`, and for the same reason.
A deployment with no invoicing simply has no sources and gets an empty list,
which is the correct answer for a POS or a ticketing app.