🔮 Oracles

Oracles bring off-chain data (prices, weather, events) onto the blockchain so smart contracts can use it. Contracts cannot fetch URLs or APIs by themselves; oracles provide a trusted or decentralized bridge between the real world and on-chain logic.

🎯 Why Oracles Matter

DeFi — Lending, derivatives, and swaps need price feeds (e.g. ETH/USD)
Gaming & NFTs — Randomness, events, or external triggers
Risk — A single centralized price source can be manipulated; decentralized oracles reduce that risk

📚 Core Concepts

Term	Description
Price feed	On-chain data (e.g. latest ETH/USD) updated by the oracle
Aggregation	Many sources combined (e.g. median) for a single answer
Decentralization	Multiple independent nodes or data sources; no single point of failure
Consumption	Your contract reads the oracle contract (e.g. `latestRoundData()`)

🛠️ Oracle Options

Chainlink (EVM and more)

Chainlink Data Feeds provide decentralized price feeds and other data on many chains (Ethereum, Polygon, Arbitrum, etc.). Contracts read from a feed contract (aggregator).

Other providers

Pyth Network — Low-latency price feeds (Solana, EVM, others)
UMA Oracle — Optimistic oracle for custom data
Band Protocol — Multi-chain oracles
RedStone — Modular oracles

📝 Code: Reading a Chainlink Price Feed (Solidity)


// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
 
import "@chainlink/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol";
 
contract PriceConsumer {
    AggregatorV3Interface internal priceFeed;
 
    // Sepolia ETH/USD feed
    constructor() {
        priceFeed = AggregatorV3Interface(
            0x694AA1769357215DE4FAC081bf1f309aDC325306
        );
    }
 
    function getLatestPrice() public view returns (int256) {
        (
            /* uint80 roundID */,
            int256 price,
            /* uint256 startedAt */,
            /* uint256 timeStamp */,
            /* uint80 answeredInRound */
        ) = priceFeed.latestRoundData();
        return price; // e.g. 2000_00000000 = $2000 (8 decimals)
    }
}

Install: npm install @chainlink/contracts. Use the correct feed address for your network.

📝 Code: Reading a Chainlink Price (JavaScript)


const { ethers } = require("ethers");
 
const abi = [
  "function latestRoundData() external view returns (uint80 roundId, int256 answer, uint256 startedAt, uint256 updatedAt, uint80 answeredInRound)",
];
 
const provider = new ethers.JsonRpcProvider(process.env.RPC_URL);
const feed = new ethers.Contract(
  "0x694AA1769357215DE4FAC081bf1f309aDC325306", // Sepolia ETH/USD
  abi,
  provider
);
 
const data = await feed.latestRoundData();
const price = Number(data.answer) / 1e8;
console.log("ETH/USD:", price);

⚠️ Security Notes

Don’t use a single API in a contract via a custom oracle unless you fully trust it and accept manipulation risk.
Staleness — Check updatedAt (or equivalent) and reject data that is too old.
Decentralized feeds — Prefer aggregated, decentralized oracles (e.g. Chainlink) for critical pricing.
See Smart Contract Security and EVM Smart Contract Security for general practices.