Overview

When you connect a smart contract to real-world services or off-chain data, you create a hybrid smart contract. For example, you can use Chainlink Data Feeds to connect your smart contracts to asset pricing data like the ETH / USD feed. These data feeds use the data aggregated from many independent Chainlink node operators. Each price feed has an on-chain address and functions that enable contracts to read pricing data from that address.

This guide shows you how to write, deploy, and run a smart contract that consumes data from a price data feed.

Topics

• Overview
• Examine the sample contract
• Compile, deploy, and run the contract

Examine the sample contract

The following code describes a contract that obtains the latest ETH / USD price using the Goerli testnet.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

import "@chainlink/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol";

contract PriceConsumerV3 {

    AggregatorV3Interface internal priceFeed;

    /**
     * Network: Goerli
     * Aggregator: ETH/USD
     * Address: 0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e
     */
    constructor() {
        priceFeed = AggregatorV3Interface(0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e);
    }

    /**
     * Returns the latest price
     */
    function getLatestPrice() public view returns (int) {
        (
            /*uint80 roundID*/,
            int price,
            /*uint startedAt*/,
            /*uint timeStamp*/,
            /*uint80 answeredInRound*/
        ) = priceFeed.latestRoundData();
        return price;
    }
}

The contract has the following components:

• The import line imports an interface named AggregatorV3Interface. Interfaces define functions without their implementation, which leaves inheriting contracts to define the actual implementation themselves. In this case, AggregatorV3Interface defines that all v3 Aggregators have the function latestRoundData. You can see the complete code for the AggregatorV3Interface on GitHub.

• The constructor() {} initializes an interface object named priceFeed that uses AggregatorV3Interface and connects specifically to a proxy aggregator contract that is already deployed at 0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e. The interface allows your contract to run functions on that deployed aggregator contract.

• The getLatestPrice() function calls your priceFeed object and runs the latestRoundData() function. When you deploy the contract, it initializes the priceFeed object to point to the aggregator at 0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e, which is the proxy address for the Goerli ETH / USD data feed. Your contract connects to that address and executes the function. The aggregator connects with several oracle nodes and aggregates the pricing data from those nodes. The response from the aggregator includes several variables, but getLatestPrice() returns only the price variable.

Compile, deploy, and run the contract

Deploy the PriceConsumerV3 smart contract on the Goerli testnet.

1. Open the example contract in Remix. Remix opens and shows the contents of the smart contract.

   Open the contract in Remix

2. Because the code is already written, you can start the compile step. On the left side of Remix, click the Solidity Compiler tab to view the compiler settings.
   

3. Use the default compiler settings. Click the Compile PriceConsumerV3.sol button to compile the contract. Remix automatically detects the correct compiler version depending on the pragma that you specify in the contract. You can ignore warnings about unused local variables in this example.
   

4. On the Deploy tab, select the Injected Provider environment. This contract specifically requires Web3 because it connects with another contract on the blockchain. Running in a JavaScript VM will not work.
   

5. Because the example contract has several imports, Remix might select another contract to deploy by default. In the Contract section, select the PriceConsumerV3 contract to make sure that Remix deploys the correct contract.
   

6. Click Deploy to deploy the contract to the Goerli testnet. MetaMask opens and asks you to confirm payment for deploying the contract. Make sure MetaMask is set to the Goerli network before you accept the transaction. Because these transactions are on the blockchain, they are not reversible.
   

7. In the MetaMask prompt, click Confirm to approve the transaction and spend your testnet ETH required to deploy the contract.
   

8. After a few seconds, the transaction completes and your contract appears under the Deployed Contracts list in Remix. Click the contract dropdown to view its variables and functions.
   

9. Click getLatestPrice to show the latest price from the aggregator contract. The latest price appears just below the button. The returned price is an integer, so it is missing its decimal point.
   

You can run your own oracle networks that provide data to smart contracts similar to the AggregatorV3Interface, but first, you should learn how to configure your contracts to pay oracles using LINK tokens. Follow the Generate Random Numbers to learn how.