# Crypto Top-of-Book Streams
Source: https://docs.chain.link/data-streams/concepts/crypto-top-of-book-and-mark-price

> For the complete documentation index, see [llms.txt](/llms.txt).

In addition to Chainlink's standard liquidity-weighted crypto price streams, Data Streams offers **Top-of-Book** streams for perpetual futures markets. These are sourced directly from a single centralized exchange's public order book, rather than aggregated across venues.

## Top-of-Book

Top-of-Book streams report the best bid and best ask currently resting on a specific exchange's order book for a given instrument. These streams are useful for protocols that need to reflect a specific venue's live order book conditions, such as perpetual futures for tokenized equities and commodities (RWA perps) traded on Binance and Hyperliquid.

> **NOTE: Single-source data**
>
> Top-of-Book streams are sourced from a single exchange's order book, not a liquidity-weighted composite across
> venues. There is no redundant cross-validation between exchanges for a given Top-of-Book feed.

### Data sources

| Exchange    | Public channel                                               | Fields consumed                                |
| ----------- | ------------------------------------------------------------ | ---------------------------------------------- |
| Binance     | `<symbol>@bookTicker` (Individual Symbol Book Ticker Stream) | `b` (best bid price), `a` (best ask price)     |
| Hyperliquid | `l2Book`                                                     | Best bid/ask `px` from `levels[0]`/`levels[1]` |

Additional exchanges may be added over time as new streams are onboarded.

### Schema

Top-of-Book streams use the [RWA Advanced (v11) schema](/data-streams/reference/report-schema-v11):

| Schema field   | Populated from                                                                                                                                                                                                              |
| -------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `bid`          | Exchange's best bid price                                                                                                                                                                                                   |
| `bidVolume`    | Size at the best bid, when provided by the exchange                                                                                                                                                                         |
| `ask`          | Exchange's best ask price                                                                                                                                                                                                   |
| `askVolume`    | Size at the best ask, when provided by the exchange                                                                                                                                                                         |
| `mid`          | Calculated by the Chainlink DON — **not** provided by the exchange                                                                                                                                                          |
| `marketStatus` | Reflects the venue's availability. Crypto perpetual markets generally trade continuously; see [Market Status Values](/data-streams/reference/report-schema-v11#market-status-values) for how this maps for a specific feed. |

> **CAUTION: Mid price is calculated, not sourced**
>
> `mid` is calculated by the Chainlink DON from the reported bid and ask. It is not a field provided directly by the
> underlying exchange.

## Data freshness and update frequency

Top-of-Book streams are **event-driven**: a new observation is only published when the underlying exchange emits a new datapoint on its public channel. There is no fixed polling interval.

End-to-end latency from the underlying exchange to node operators is sub-second under normal conditions. Actual latency can vary based on network conditions and exchange-side publishing behavior.

## Risk considerations

- **Single-source dependency**: Each Top-of-Book feed is sourced from one exchange. If that exchange stops publishing updates, there is no automatic fallback to another venue for that feed.
- **Venue-specific pricing**: Top-of-Book reflects one exchange's order book, which can diverge from prices on other venues, especially during periods of low liquidity or high volatility on the source exchange.
- **Review [Developer Responsibilities](/data-streams/developer-responsibilities)** before integrating Top-of-Book streams into a production application.