> ## Documentation Index
> Fetch the complete documentation index at: https://docs.paxos.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Smart Order Routing Top of Book

> Real-time best bid and ask updates for Smart Order Routing platform.

The Top of Book channel provides real-time updates of the best bid and best ask prices for authorized Smart Order Routing customers. This lightweight channel is ideal when you only need the best available prices without the full order book depth.

## Subscribe to Top of Book

### Request

```json theme={null}
{
  "type": "subscribe",
  "channels": [
    {
      "type": "market_data.top_of_book",
      "params": {
        "market": "BTCUSD"
      }
    }
  ]
}
```

### Response

```json theme={null}
{
  "type": "subscribe",
  "channels": [
    {
      "type": "market_data.top_of_book",
      "params": { "market": "BTCUSD" },
      "success": true
    }
  ]
}
```

The response echoes back the request parameters, allowing you to correlate responses when subscribing to multiple channels.

### Error Response

```json theme={null}
{
  "type": "subscribe",
  "channels": [
    {
      "type": "market_data.top_of_book",
      "params": { "market": "BADBAD" },
      "success": false,
      "error": "UNKNOWN_CHANNEL_PARAMS_CONFIGURATION"
    }
  ]
}
```

## Top of Book Message Format

Real-time updates of the best bid and ask prices:

```json theme={null}
{
  "channel": "market_data.top_of_book",
  "payload": {
    "market": "BTCUSD",
    "type": "TOP_OF_BOOK",
    "timestamp": "2024-01-15T10:30:00.123456789Z",
    "bids": [
      {
        "price": "50000.00",
        "amount": "1.5"
      }
    ],
    "asks": [
      {
        "price": "50100.00",
        "amount": "2.0"
      }
    ]
  }
}
```

### Message Fields

| Field       | Type   | Description                             |
| ----------- | ------ | --------------------------------------- |
| `market`    | string | The market pair (e.g., "BTCUSD")        |
| `type`      | string | Always `"TOP_OF_BOOK"` for this channel |
| `timestamp` | string | Server timestamp in RFC3339Nano format  |
| `bids`      | array  | Array with best bid price and amount    |
| `asks`      | array  | Array with best ask price and amount    |

### Price Level Fields

| Field    | Type   | Description                                              |
| -------- | ------ | -------------------------------------------------------- |
| `price`  | string | Price level as a decimal string                          |
| `amount` | string | Available amount at this price level as a decimal string |

## Unsubscribe

To stop receiving top of book updates:

```json theme={null}
{
  "type": "unsubscribe",
  "channels": [
    {
      "type": "market_data.top_of_book",
      "params": {
        "market": "BTCUSD"
      }
    }
  ]
}
```

## List Active Subscriptions

Get all active subscriptions including top of book:

```json theme={null}
{
  "type": "subscription_list"
}
```

### Response

```json theme={null}
{
  "type": "subscription_list",
  "channels": [
    {
      "type": "market_data.top_of_book",
      "params": {
        "market": "BTCUSD"
      }
    },
    {
      "type": "market_data",
      "params": {
        "market": "ETHUSD"
      }
    }
  ]
}
```

> Questions? Contact [Support](https://support.paxos.com).