# Connect Reverse ETL Source to Customer.io Audience

This guide takes you through the steps to connect a Reverse ETL source to the Customer.io Audience destination. You can create a new segment or use an existing segment to sync the data.

The below steps assume that you have already [set up a Reverse ETL source]({{< ref "sources/reverse-etl/_index.md" >}}) and [configured the connection settings]({{< ref "destinations/reverse-etl-destinations/customerio-audience/setup-guide.md#connection-settings" >}}) for the connected Customer.io Audience destination.

{{< success >}}
You can connect multiple Reverse ETL sources to the Customer.io Audience destination.
{{< /success >}}

## Customer.io segment settings

| Setting | Description |
| :----| :----- |
| Choose whether to create a new Customer.io Segment | Specify whether you want to sync data to a new or existing Customer.io segment. <br /><br /><ul><li>If you select **Create a new Customer.io Manual Segment**, then specify the name and description (optional) for the new segment.</li><li>Select **Use an existing Customer.io Manual Segment** if you have an existing Customer.io segment. Then, choose the list automatically prepopulated by RudderStack based on your specified [connection settings]({{< ref "destinations/reverse-etl-destinations/customerio-audience/setup-guide.md#connection-settings" >}}). |
| Sync mode |  RudderStack supports only [Mirror mode]({{< ref "data-pipelines/reverse-etl/developer-guides/sync-modes.md#mirror-mode" >}}) for this integration. |

### Mapping settings

{{< warning >}}
Make sure that the users exist in Customer.io before adding them to any segment. Otherwise, such records will be ignored.

See the [Customer.io Troubleshooting](https://docs.customer.io/journeys/segments/#troubleshooting-segments) guide for more information. 
{{< /warning >}}

Use this section to map the identifier warehouse column (that RudderStack uses to identify your records) to specific Customer.io segment fields. Note that you can map this warehouse column to either of these Customer.io segment fields: **ID**, **Email**, or **CIO_ID**.

{{< image src="images/event-stream-destinations/customerio-audience-mappings.webp" >}}

### Schedule settings

{{< customreadfile "/includes/retl/schedule-syncs.md" >}}

### Sync observability settings

{{< customreadfile "/includes/retl/sync-observability-settings.md" >}}

## `record` event structure

The Customer.io Audience destination supports only `record` events.

A sample `record` event that RudderStack sends to Customer.io segments is shown:

```json
{
  "action": "insert",
  "channel": "sources",
  "context": {
    "sources": {
      "job_id": "<2s4ZjYKSKztME0SmfBUB5t8KA3j>",
      "job_run_id": "cu9na4c7miqjfjeogkv0",
      "task_run_id": "cu9na4s7miqjfjeogkvg",
      "version": "local"
    }
  },
  "fields": {},
  "identifiers": {
    "id": “30”
  },
  "messageId": "57f32b79-f030-4bdc-8f24-4f573f4acf99",
  "receivedAt": "2025-01-24T11:10:58.799Z",
  "recordId": "4",
  "request_ip": "[::1]",
  "rudderId": "0afd110a-4d8a-42c5-a959-05d3ef3b4e87",
  "type": "record",
  "userId": "36b9732a-cd10-4aec-b03e-36969a9b14a2"
}
```

## FAQ

#### Why can't I add a Reverse ETL source from the destination page?

The Reverse ETL feature supports only source-driven pipeline configuration. It means that you must configure a Reverse ETL source in RudderStack and then connect it to a new or existing destination. Note that this destination should not be connected to any other source.

See [Reverse ETL FAQ]({{< ref "data-pipelines/reverse-etl/developer-guides/faq.md" >}}) for more information.