# `SpanId`

One step's id in a trace: 16 lower-case hexadecimal digits, not all zero.

Use it beside a [`TraceId`](/docs/types/type-trace-id.md). For the header that carries both, use [`TraceParent`](/docs/types/type-trace-parent.md).

## Shape

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

## Example

```jadpo
SpanId("00f067aa0ba902b7")
```

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

`text.parse(raw, as: SpanId)` reads one from text that comes from outside. Text that isn't one at all fails `malformed`, such as `0000000000000000`, `00F067AA0BA902B7` or `00f067aa0ba902b`.

## 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
"00f067aa0ba902b7"
"ffffffffffffffff"
```

## Details

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

`SpanId` 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: `00f067aa0ba902b7` and `ffffffffffffffff`.

### How it is stored

It is stored in one column named for the field, of type `text`. `ffffffffffffffff` is stored as `'ffffffffffffffff'`.

### How it is shown to people

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

With `locale: Locale.en_gb`:

| Value | Shown |
|---|---|
| `00f067aa0ba902b7` | `00f067aa0ba902b7` |
| `ffffffffffffffff` | `ffffffffffffffff` |

### 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>