# `IdempotencyKey`

A key a client sends so that a retried request isn't applied twice.

Use it on a route where a repeat must have no second effect, such as a payment. It runs to 255 printable ASCII characters. For a record's own id, use [`Uuid`](/docs/types/type-uuid.md).

## Shape

One value, held as text in one set form, with no parts: `a`.

## Example

```jadpo
IdempotencyKey("order-1042-attempt")
```

`IdempotencyKey(raw)` checks the text, and text that isn't one breaks the `format` rule.

`text.parse(raw, as: IdempotencyKey)` reads one from text that comes from outside. Text that isn't one at all fails `malformed`, such as ` key`, `key ` or `say "hi"`.

## Good to know

- `==` tells you whether two are the same. `<` and `list.sort_by` don't take it.

## In JSON

A string, in a request, a response or `json.write`:

```json
"8e03978e-40d5-43e8-bc93-6894a57f9324"
"order-1042-attempt"
"a"
```

## Details

<details>
<summary>Text form, storage, how it is shown, and operations</summary>

`IdempotencyKey` is in the group Software, and has been in Jadpo since 0.1.0.

### Text form

`text.from` writes one standard text form, the same form `text.parse` reads back: `8e03978e-40d5-43e8-bc93-6894a57f9324`, `order-1042-attempt` and `a`.

### How it is stored

It is stored in one column named for the field, of type `text`. `order-1042-attempt` is stored as `'order-1042-attempt'`.

### How it is shown to people

`text.format(value, locale:)` shows it to people in their locale.

With `locale: Locale.en_gb`:

| Value | Shown |
|---|---|
| `8e03978e-40d5-43e8-bc93-6894a57f9324` | `8e03978e-40d5-43e8-bc93-6894a57f9324` |
| `order-1042-attempt` | `order-1042-attempt` |
| `a` | `a` |

### Operations

It also takes `text.parse`, `text.from`, `text.format`, `json.read`, `json.write` and `==`. [How every standard type behaves](/docs/every-standard-type-meets-one-contract.md) says how they work.

</details>