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pricefeed: Update Off chain guide #559

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119 changes: 8 additions & 111 deletions pages/price-feeds/use-real-time-data/off-chain.mdx
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# How to Use Real-Time Data in Off-Chain Applications

This guide explains how to use the [typescript SDK client](https://github.com/pyth-network/pyth-crosschain/tree/main/price_service/client/js) to fetch the latest prices and subscribe to real-time price updates in off-chain applications.

## Installation

The Pyth SDK can be installed using npm or yarn:

### npm

```bash copy
npm install --save @pythnetwork/price-service-client
```

### Yarn

```bash copy
yarn add @pythnetwork/price-service-client
```

## Usage

Typical usage of the Pyth SDK involves creating a `PriceServiceConnection` instance, by passing the URL of the [Hermes](https://hermes.pyth.network/docs/#/) service to the constructor.
You can then use the `getLatestPriceFeeds` method to fetch the latest prices of the specified price feeds:

```typescript copy
// Get the Stable Hermes service URL from https://docs.pyth.network/price-feeds/api-instances-and-providers/hermes
const connection = new PriceServiceConnection("https://hermes.pyth.network");

const priceIds = [
"0xe62df6c8b4a85fe1a67db44dc12de5db330f7ac66b72dc658afedf0f4a415b43", // BTC/USD price id
"0xff61491a931112ddf1bd8147cd1b641375f79f5825126d665480874634fd0ace", // ETH/USD price id
];
import { Callout } from "nextra/components";

// Get the latest values of the price feeds as JSON objects.
const currentPrices = await connection.getLatestPriceFeeds(priceIds);
console.log(currentPrices);
```

The `getLatestPriceFeeds` method returns an array of `PriceFeed` objects. Each `PriceFeed` object contains the latest price, the Exponential Moving Average (EMA) price, and other metadata:

```bash
[
PriceFeed {
emaPrice: Price {
conf: '3450227100',
expo: -8,
price: '6610104500000',
publishTime: 1715870606
},
id: 'e62df6c8b4a85fe1a67db44dc12de5db330f7ac66b72dc658afedf0f4a415b43',
metadata: undefined,
vaa: undefined,
price: Price {
conf: '3315255511',
expo: -8,
price: '6619626406527',
publishTime: 1715870606
}
},
PriceFeed {
emaPrice: Price {
conf: '266713535',
expo: -8,
price: '297876550000',
publishTime: 1715870606
},
id: 'ff61491a931112ddf1bd8147cd1b641375f79f5825126d665480874634fd0ace',
metadata: undefined,
vaa: undefined,
price: Price {
conf: '296893358',
expo: -8,
price: '296499502000',
publishTime: 1715870606
}
}
]
```

You can also subscribe to real-time price updates over a WebSocket connection using the `subscribePriceFeedUpdates` method:

```typescript copy
// priceIds here is the one declared in the above code snippet.
const priceFeeds = await connection.getLatestPriceFeeds(priceIds);

connection.subscribePriceFeedUpdates(priceIds, (priceFeed) => {
// priceFeed here is the same as returned by getLatestPriceFeeds above.
console.log(
`Received an update for ${priceFeed.id}: ${priceFeed.getPriceNoOlderThan(
60
)}`
);
});

// When using the subscription, make sure to close the WebSocket upon termination to finish the process gracefully.
setTimeout(() => {
connection.closeWebSocket();
}, 60000);
```

## Additional Resources

You may find these additional resources helpful.

### Examples

This [example](https://github.com/pyth-network/pyth-crosschain/blob/main/price_service/client/js/src/examples/PriceServiceClient.ts) demonstrates how to use the Pyth SDK to fetch prices.
# How to Use Real-Time Data in Off-Chain Applications

### Hermes API Reference
This guide explains how to fetch the latest prices and subscribe to real-time price updates in off-chain applications.

The [Hermes API](https://hermes.pyth.network/docs/#/) reference lets you interactively explore the complete API of the Pyth Hermes service.
<Callout type="info" emoji="💡">
[`price-service-sdk`](https://github.com/pyth-network/pyth-crosschain/tree/main/price_service/client/js) was is deprecated and replaced by the [`hermes-client`](https://github.com/pyth-network/pyth-crosschain/tree/main/apps/hermes/client/js).
It can be used for fetching prices for off-chain applications as well as fetching price updates.

### Price Feed IDs
Please refer to the [fetch-price-updates](../fetch-price-updates) guide for the details.

The [Price Feed IDs](https://pyth.network/developers/price-feed-ids) lists the price feeds available on the Pyth network.
</Callout>
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