VRF Best Practices

These are example best practices for using Chainlink VRF. To explore more applications of VRF, refer to our blog.

Getting a random number within a range

If you need to generate a random number within a given range, use modulo to define the limits of your range. Below you can see how to get a random number in a range from 1 to 50.

function fulfillRandomWords(
  uint256, /* requestId */
  uint256[] memory randomWords
) internal override {
  // Assuming only one random word was requested.
  s_randomRange = (randomWords[0] % 50) + 1;
}

Getting multiple random values

If you want to get multiple random values from a single VRF request, you can request this directly with the numWords argument:

  • If you are using the VRF v2 subscription method, see the Get a Random Number guide for an example where one request returns multiple random values.
  • If you are using the VRF v2 direct funding method, see the Get a Random Number guide for an example where one request returns multiple random values.

Processing simultaneous VRF requests

If you want to have multiple VRF requests processing simultaneously, create a mapping between requestId and the response. You might also create a mapping between the requestId and the address of the requester to track which address made each request.

mapping(uint256 => uint256[]) public s_requestIdToRandomWords;
mapping(uint256 => address) public s_requestIdToAddress;
uint256 public s_requestId;

function requestRandomWords() external onlyOwner returns (uint256) {
  uint256 requestId = COORDINATOR.requestRandomWords(
    keyHash,
    s_subscriptionId,
    requestConfirmations,
    callbackGasLimit,
    numWords
  );
  s_requestIdToAddress[requestId] = msg.sender;

  // Store the latest requestId for this example.
  s_requestId = requestId;

  // Return the requestId to the requester.
  return requestId;
}

function fulfillRandomWords(
    uint256 requestId,
    uint256[] memory randomWords
  ) internal override {
  // You can return the value to the requester,
  // but this example simply stores it.
  s_requestIdToRandomWords[requestId] = randomWords;
}

You could also map the requestId to an index to keep track of the order in which a request was made.

mapping(uint256 => uint256) s_requestIdToRequestIndex;
mapping(uint256 => uint256[]) public s_requestIndexToRandomWords;
uint256 public requestCounter;

function requestRandomWords() external onlyOwner {
  uint256 requestId = COORDINATOR.requestRandomWords(
    keyHash,
    s_subscriptionId,
    requestConfirmations,
    callbackGasLimit,
    numWords
  );
  s_requestIdToRequestIndex[requestId] = requestCounter;
  requestCounter += 1;
}

function fulfillRandomWords(
    uint256 requestId,
    uint256[] memory randomWords
  ) internal override {
  uint256 requestNumber = s_requestIdToRequestIndex[requestId];
  s_requestIndexToRandomWords[requestNumber] = randomWords;
}

Processing VRF responses through different execution paths

If you want to process VRF responses depending on predetermined conditions, you can create an enum. When requesting for randomness, map each requestId to an enum. This way, you can handle different execution paths in fulfillRandomWords. See the following example:

// SPDX-License-Identifier: MIT
// An example of a consumer contract that relies on a subscription for funding.
// It shows how to setup multiple execution paths for handling a response.
pragma solidity ^0.8.7;

import "@chainlink/contracts/src/v0.8/interfaces/VRFCoordinatorV2Interface.sol";
import "@chainlink/contracts/src/v0.8/VRFConsumerBaseV2.sol";

/**
 * THIS IS AN EXAMPLE CONTRACT THAT USES HARDCODED VALUES FOR CLARITY.
 * THIS IS AN EXAMPLE CONTRACT THAT USES UN-AUDITED CODE.
 * DO NOT USE THIS CODE IN PRODUCTION.
 */

contract VRFv2MultiplePaths is VRFConsumerBaseV2 {
    VRFCoordinatorV2Interface COORDINATOR;

    // Your subscription ID.
    uint64 s_subscriptionId;

    // Goerli coordinator. For other networks,
    // see https://docs.chain.link/docs/vrf/v2/supported-networks/#configurations
    address vrfCoordinator = 0x2Ca8E0C643bDe4C2E08ab1fA0da3401AdAD7734D;

    // The gas lane to use, which specifies the maximum gas price to bump to.
    // For a list of available gas lanes on each network,
    // see https://docs.chain.link/docs/vrf/v2/supported-networks/#configurations
    bytes32 keyHash =
        0x79d3d8832d904592c0bf9818b621522c988bb8b0c05cdc3b15aea1b6e8db0c15;

    uint32 callbackGasLimit = 100_000;

    // The default is 3, but you can set this higher.
    uint16 requestConfirmations = 3;

    // For this example, retrieve 1 random value in one request.
    // Cannot exceed VRFCoordinatorV2.MAX_NUM_WORDS.
    uint32 numWords = 1;

    enum Variable {
        A,
        B,
        C
    }

    uint256 public variableA;
    uint256 public variableB;
    uint256 public variableC;

    mapping(uint256 => Variable) public requests;

    // events
    event FulfilledA(uint256 requestId, uint256 value);
    event FulfilledB(uint256 requestId, uint256 value);
    event FulfilledC(uint256 requestId, uint256 value);

    constructor(uint64 subscriptionId) VRFConsumerBaseV2(vrfCoordinator) {
        COORDINATOR = VRFCoordinatorV2Interface(vrfCoordinator);
        s_subscriptionId = subscriptionId;
    }

    function updateVariable(uint256 input) public {
        uint256 requestId = COORDINATOR.requestRandomWords(
            keyHash,
            s_subscriptionId,
            requestConfirmations,
            callbackGasLimit,
            numWords
        );

        if (input % 2 == 0) {
            requests[requestId] = Variable.A;
        } else if (input % 3 == 0) {
            requests[requestId] = Variable.B;
        } else {
            requests[requestId] = Variable.C;
        }
    }

    function fulfillRandomWords(
        uint256 requestId,
        uint256[] memory randomWords
    ) internal override {
        Variable variable = requests[requestId];
        if (variable == Variable.A) {
            fulfillA(requestId, randomWords[0]);
        } else if (variable == Variable.B) {
            fulfillB(requestId, randomWords[0]);
        } else if (variable == Variable.C) {
            fulfillC(requestId, randomWords[0]);
        }
    }

    function fulfillA(uint256 requestId, uint256 randomWord) private {
        // execution path A
        variableA = randomWord;
        emit FulfilledA(requestId, randomWord);
    }

    function fulfillB(uint256 requestId, uint256 randomWord) private {
        // execution path B
        variableB = randomWord;
        emit FulfilledB(requestId, randomWord);
    }

    function fulfillC(uint256 requestId, uint256 randomWord) private {
        // execution path C
        variableC = randomWord;
        emit FulfilledC(requestId, randomWord);
    }
}

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