# `Rheo.Event`
[🔗](https://github.com/thanos/rheo/blob/v1.0.0/lib/rheo/event.ex#L1)

An immutable event stored in a Rheo stream.

Events are the durable system of record. Consumer progress (leases, ACKs,
retries, dead-letters) lives in separate per-group delivery state — never as a
`processed` flag on the event.

Persistence decoding belongs in backend codecs (e.g.
`Rheo.Backend.Mongo.Codec`), not on this struct.

## Fields

| Field | Type | Meaning |
|---|---|---|
| `id` | `String.t()` | Stable unique id (use for idempotency) |
| `stream` | `String.t()` | Stream name this event belongs to |
| `partition` | `non_neg_integer()` | Partition within the stream |
| `sequence` | `pos_integer()` | Monotonic sequence within `(stream, partition)` |
| `timestamp` | `DateTime.t()` | Event time (defaults to append time) |
| `key` | `String.t() \| nil` | Optional routing key (`:erlang.phash2/2`) |
| `type` | `String.t() \| nil` | Optional event type for queries |
| `metadata` | `map()` | Cross-cutting headers (correlation id, producer, …) |
| `payload` | `map()` | Domain body (currency, curve, price, …) |

## Example

    iex> event = %Rheo.Event{
    ...>   id: "evt_01",
    ...>   stream: "market-events",
    ...>   partition: 0,
    ...>   sequence: 1,
    ...>   timestamp: ~U[2026-01-15 12:00:00.000Z],
    ...>   key: "EUR-EURIBOR-6M",
    ...>   type: "curve_update",
    ...>   metadata: %{"correlation_id" => "abc"},
    ...>   payload: %{"currency" => "EUR", "price" => 2.913}
    ...> }
    iex> {event.type, event.payload["currency"]}
    {"curve_update", "EUR"}

# `t`

```elixir
@type t() :: %Rheo.Event{
  id: String.t(),
  key: String.t() | nil,
  metadata: map(),
  partition: non_neg_integer(),
  payload: map(),
  sequence: pos_integer(),
  stream: String.t(),
  timestamp: DateTime.t(),
  type: String.t() | nil
}
```

An immutable stream event.

See the module documentation for field meanings and examples.

---

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