# Subscriptions

Subscriptions (also known as webhooks or callbacks) are a means to provide asynchronous responses to you. Subscriptions are event based. To receive an event you must register a subscription for the events of interest to you.

## Register for Subscriptions

Dynamic registration and callback management are performed through the [Subscriptions](/api-docs/openapi/rail-spec#tag/subscriptions-(webhooks)) api which are documented in the api-docs. You can create, delete and list your events.

You can view a list of historical events as well delivery status via the Management UI.

Whitelisting Required for Production
Production Webhook URLs need to be explicitly whitelisted by Rail to receive callbacks. Please reach out to your dedicated implementations engineer, or account manager to whitelist a Webhook URL.

## Subscription Structure

Our subscriptions share the same basic structure.

- `event_id` : a unique idempotent UUID. In the event of an event replay, this ID will remain the same.
- `event_date`: the date time the event was generated
- `event_type` : the type of event with subsequent details specific to the event
- `event_data` : business data specific to the event being generated


Our events are purposefully lightweight. They contain only critical information for decision making. Should you need an extended information set, perform an enrichment by calling the appropriate `GET` endpoint using the IDs provided in the events.


```json
{
  "event_id": "<unique_uuid>",
  "event_date": "<datetime>",
  "event_type": "<event_type>",
  "event_data":{
    <event_data_here>
  }
}
```

## Available Subscription Events

Refer to the [Subscriptions Events](/guides/subscriptions_events) page to review all available subscription events.