Working with Blockchain Oracles - Introduction

Getting funds

You can get testnet LINK token using the links mentioned in chainlink docs https://docs.chain.link/docs/link-token-contracts/

Add LINK token to metamask to see your token balance in the metamask assets panel

• Check Chainlink data feeds
• Check Chainlink market
• Make an API Request
• Check core adapters
• Lottery Example, if that does not work, check this
• For v1 links, click here
• For creating a local chainlink node and oracle - https://docs.chain.link/chainlink-nodes/

Example usage of Oracle

This example is borrowed from https://docs.chain.link/docs/historical-price-data/

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

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

contract HistoricalPriceConsumerV3 {

    AggregatorV3Interface internal priceFeed;
    
    /**
     * Network: Kovan
     * Aggregator: ETH/USD
     * Address: 0x9326BFA02ADD2366b30bacB125260Af641031331
     */
    constructor() {
        priceFeed = AggregatorV3Interface(0x9326BFA02ADD2366b30bacB125260Af641031331);
    }

    /**
     * Returns historical price for a round id.
     * roundId is NOT incremental. Not all roundIds are valid.
     * You must know a valid roundId before consuming historical data.
     *
     * ROUNDID VALUES:
     *    InValid:      18446744073709562300
     *    Valid:        18446744073709562301
     *    
     * @dev A timestamp with zero value means the round is not complete and should not be used.
     */
    function getHistoricalPrice(uint80 roundId) public view returns (int256) {
        (
            uint80 id, 
            int price,
            uint startedAt,
            uint timeStamp,
            uint80 answeredInRound
        ) = priceFeed.getRoundData(roundId);
        require(timeStamp > 0, "Round not complete");
        return price;
    }
}

Random Number Generator

A copy of https://github.com/smartcontractkit/truffle-starter-kit/blob/master/contracts/RandomNumberConsumer.sol

// This example code is designed to quickly deploy an example contract using Remix.

pragma solidity 0.6.6;

import "https://raw.githubusercontent.com/smartcontractkit/chainlink/master/evm-contracts/src/v0.6/VRFConsumerBase.sol";

contract RandomNumberConsumer is VRFConsumerBase {
    
    bytes32 internal keyHash;
    uint256 internal fee;
    
    uint256 public randomResult;
    
     /**
      * Moving to Polygon testnet
      * Reference: https://blog.chain.link/how-to-get-a-random-number-on-polygon/
      * Contract addresses for Chainlink
      * Reference: https://docs.chain.link/docs/vrf-contracts/ 
      * 
      * REMINDER: TODO: Change the contract address based on the mainnet deployment
     */
    constructor() 
        VRFConsumerBase(
            0x8C7382F9D8f56b33781fE506E897a4F1e2d17255, // VRF Coordinator
            0x326C977E6efc84E512bB9C30f76E30c160eD06FB  // LINK Token
        ) public
    {
        keyHash = 0x6e75b569a01ef56d18cab6a8e71e6600d6ce853834d4a5748b720d06f878b3a4;
        fee = 100000000000000; // 0.1 LINK
    }
    
    /** 
     * Requests randomness from a user-provided seed
     ************************************************************************************
     *                                    STOP!                                         * 
     *         THIS FUNCTION WILL FAIL IF THIS CONTRACT DOES NOT OWN LINK               *
     *         ----------------------------------------------------------               *
     *         Learn how to obtain testnet LINK and fund this contract:                 *
     *         ------- https://docs.chain.link/docs/acquire-link --------               *
     *         ---- https://docs.chain.link/docs/fund-your-contract -----               *
     *                                                                                  *
     ************************************************************************************/
    function getRandomNumber() public returns (bytes32 requestId) {
        require(LINK.balanceOf(address(this)) >= fee, "Not enough LINK - fill contract with faucet");
        return requestRandomness(keyHash, fee);
    }

    /**
     * Callback function used by VRF Coordinator
     */
    function fulfillRandomness(bytes32 requestId, uint256 randomness) internal override {
        randomResult = randomness;
    }
    
    /**
     * Withdraw LINK from this contract
     * 
     * DO NOT USE THIS IN PRODUCTION AS IT CAN BE CALLED BY ANY ADDRESS.
     * THIS IS PURELY FOR EXAMPLE PURPOSES.
     */
    function withdrawLink() external {
        require(LINK.transfer(msg.sender, LINK.balanceOf(address(this))), "Unable to transfer");
    }
}

Last Updated on Apr 24, 2022