Web3 & DApp Development Fundamentals · Lesson

Fetching Off-Chain Data

Implement smart contracts that request and receive data from Chainlink oracles, such as price feeds or external API calls.

Fetching Off-Chain Data is a free Web3 & DApp Development Fundamentals lesson on CoddyKit — lesson 3 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Web3 & DApp Development Fundamentals learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Data Beyond the Blockchain

Smart contracts are powerful, but they operate in isolation on the blockchain. They can't directly access information from the 'real world' like current stock prices, weather data, or external API responses.

This limitation is known as the oracle problem. To overcome it, we use oracles.

Chainlink's Data Solution

Chainlink is a decentralized oracle network designed to securely connect smart contracts with off-chain data and services.

It acts as a bridge between the blockchain and external systems.
Chainlink nodes collect data from various sources.
They then deliver this data to smart contracts in a reliable and verifiable way.

Real-Time Price Feeds

One of the most common and critical uses of Chainlink is providing price feeds. These feeds deliver up-to-date market prices for cryptocurrencies, commodities, and other assets directly to your smart contracts.

This is crucial for DeFi applications, lending protocols, and derivatives.

Interacting with Price Feeds

To get data from a Chainlink price feed, your smart contract interacts with a specific data feed contract deployed on the blockchain.

These data feed contracts all implement a standard interface called AggregatorV3Interface. This makes it easy for your contract to read the data.

How to Get Price Data

The primary function we use to fetch the latest price from an AggregatorV3Interface is latestRoundData().

It returns several pieces of information, including the actual price, when it was updated, and the round ID.

Build a Price Reader

Let's write a simple Solidity contract that reads the latest ETH/USD price from a Chainlink price feed. You can try running this in Remix, using the Sepolia testnet.

The address 0x694AA1769357215Ee4f0fFf93d2d05d454e5f5c4 is for the ETH/USD price feed on Sepolia.

pragma solidity ^0.8.0;

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

contract PriceReader {
    AggregatorV3Interface internal priceFeed;

    constructor() {
        // ETH/USD price feed address on Sepolia testnet
        priceFeed = AggregatorV3Interface(0x694AA1769357215Ee4f0fFf93d2d05d454e5f5c4);
    }

    function getLatestPrice() public view returns (
        uint80 roundId,
        int answer,
        uint256 startedAt,
        uint256 updatedAt,
        uint80 answeredInRound
    ) {
        // Get the latest price data from the feed
        (roundId, answer, startedAt, updatedAt, answeredInRound) = priceFeed.latestRoundData();
    }

    function getDecimals() public view returns (uint8) {
        // Get the number of decimals used by the price feed
        return priceFeed.decimals();
    }
}

Decoding Price Data

The answer returned by latestRoundData() is the actual price, but it's an int and scaled by a certain number of decimals.

You must call getDecimals() to know how many decimal places to account for. For example, if answer is 180000000000 and decimals is 8, the price is 1800.00000000.

Beyond Prices: Custom Data

While price feeds are common, Chainlink can also fetch almost any other type of off-chain data, or even generate verifiable randomness.

This requires a more generalized approach where your contract makes a request to a Chainlink node for specific data, and the node responds by calling back your contract.

The Request & Fulfill Pattern

For custom data, Chainlink uses a two-step process:

Request: Your smart contract sends a request to a Chainlink oracle contract, specifying the data source (e.g., an API endpoint) and the job ID.
Fulfill: A Chainlink node processes the request, fetches the off-chain data, and then sends a transaction back to your smart contract, calling a designated 'fulfill' function with the requested data.

This ensures data is delivered securely on-chain.

Oracle Integration Check

Consider a smart contract that needs to get the current weather conditions for a specific city. Which of the following best describes how a Chainlink oracle would typically facilitate this?

Lesson Summary

We've explored how smart contracts fetch off-chain data using Chainlink. You learned about:

The oracle problem and Chainlink's solution.
Interacting with Chainlink Price Feeds using AggregatorV3Interface.
The latestRoundData() function and interpreting its output.
The general 'request and fulfill' pattern for custom data requests.

Fetching external data securely is vital for building robust DApps!

Frequently asked questions

Is the “Fetching Off-Chain Data” lesson free?

Yes — the full text of “Fetching Off-Chain Data” is free to read here on the web, and the Web3 & DApp Development Fundamentals course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Web3 & DApp Development Fundamentals course, upgrade to CoddyKit PRO.

What will I learn in “Fetching Off-Chain Data”?

Implement smart contracts that request and receive data from Chainlink oracles, such as price feeds or external API calls. You practise Web3 & DApp Development Fundamentals with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Web3 & DApp Development Fundamentals?

No prior experience is required. Web3 & DApp Development Fundamentals on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 3, so you can start here or from the beginning and move at your own pace.

How long does the “Fetching Off-Chain Data” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Web3 & DApp Development Fundamentals lesson?

Yes. Every Web3 & DApp Development Fundamentals lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

← Back to Web3 & DApp Development Fundamentals