Get a Random Number#

This guide explains how to get random values using a simple contract to request and receive random values from OKTC VRF without managing a subscription.

Requirements#

This guide assumes that you know how to create and deploy smart contracts on OKTC Maine using the following tools:

• The Remix IDE
• MetaMask

Create and deploy a VRF compatible contract#

For this example, use the VRFDirectFundingConsumer.sol sample contract. This contract imports the following dependencies:

• VRFWrapperConsumerBase.sol(link)

The contract also includes pre-configured values for the necessary request parameters such as callbackGasLimit, requestConfirmations, the number of random words numWords, the VRF Wrapper address wrapperAddress, and the OKT token address OKTAddress. You can change these parameters if you want to experiment on different .

Build and deploy the contract on OKTC Mainnet.

1. Open the VRFDirectFundingConsumer.sol contract in Remix.

2. On the Compile tab in Remix, compile the VRFDirectFundingConsumer contract.

3. Configure your deployment. On the Deploy tab in Remix, select the Injected Web3 Environment and select the VRFDirectFundingConsumer contract from the contract list.

4. Fill the constructor arguments like below and Click the Deploy button to deploy your contract on-chain. MetaMask opens and asks you to confirm the transaction.

   Item	Value
   address _vrfV2Wrapper	0xB1A0323E202B27300f8530740A37162b2d7e62cB

5. After you deploy your contract, copy the address from the Deployed Contracts list in Remix. Before you can request randomness from VRF , you must fund your consuming contract with enough OKT tokens in order to request for randomness. Next, fund your contract.

Fund Your Contract#

Requests for randomness will fail unless your consuming contract has enough OKT. For this example, transfering 0.01 OKT to your consumer contracts should be sufficient.

Request random values#

The deployed contract requests random values from OKTC VRF, receives those values, builds a struct RequestStatus containing them, and stores the struct in a mapping s_requests. Run the requestRandomWords() function on your contract to start the request.

1. Return to Remix and view your deployed contract functions in the Deployed Contracts list.

2. Click the requestRandomWords() function to send the request for random values to OKTC VRF. MetaMask opens and asks you to confirm the transaction.

   After you approve the transaction, OKTC VRF processes your request. OKTC VRF fulfills the request and returns the random values to your contract in a callback to the fulfillRandomWords() function. At this point, a new key requestId is added to the mapping s_requests. Depending on current conditions, it might take a few minutes for the callback to return the requested random values to your contract.

3. To fetch the request ID of your request, call lastRequestId().

4. After the oracle returns the random values to your contract, the mapping s_requests is updated. The received random values are stored in s_requests[_requestId].randomWords.

5. Call getRequestStatus() and specify the requestId to display the random words.

Analyzing the contract#

In this example, the consuming contract uses static configuration parameters.

// SPDX-License-Identifier: MIT
// An example of a consumer contract that directly pays for each request.
pragma solidity ^0.8.7;


contract VRFV2WrapperConsumerExample is VRFV2WrapperConsumerBase {
    event RequestFulfilled(
        uint256 requestId,
        uint256[] randomWords,
        uint256 payment
    );

    constructor(
        address _vrfV2Wrapper
    ) VRFV2WrapperConsumerBase(_vrfV2Wrapper) {}

    function fulfillRandomWords(
        uint256 _requestId,
        uint256[] memory _randomWords
    ) internal override {
        require(s_requests[_requestId].paid > 0, "request not found");

        s_requests[_requestId].fulfilled = true;
        s_requests[_requestId].randomWords = _randomWords;
        emit RequestFulfilled(
            _requestId,
            _randomWords,
            s_requests[_requestId].paid
        );
    }
}

The parameters define how your requests will be processed. You can find the values for your network in the Supported networks page.

• uint32 callbackGasLimit: The limit for how much gas to use for the callback request to your contract’s fulfillRandomWords() function. It must be less than the maxGasLimit limit on the coordinator contract minus the wrapperGasOverhead. See the VRF Direct funding limits for more details. Adjust this value for larger requests depending on how your fulfillRandomWords() function processes and stores the received random values. If your callbackGasLimit is not sufficient, the callback will fail and your consuming contract is still charged for the work done to generate your requested random values.
• uint16 requestConfirmations: How many confirmations the OKTC node should wait before responding. The longer the node waits, the more secure the random value is. It must be greater than the minimumRequestBlockConfirmations limit on the coordinator contract.
• uint32 numWords: How many random values to request. If you can use several random values in a single callback, you can reduce the amount of gas that you spend per random value. The total cost of the callback request depends on how your fulfillRandomWords() function processes and stores the received random values, so adjust your callbackGasLimit accordingly.

The contract includes the following functions:

• requestRandomWords(): Takes your specified parameters and submits the request to the VRF Wrapper contract.
• fulfillRandomWords(): Receives random values and stores them with your contract.
• getRequestStatus(): Retrive request details for a given _requestId.
• withdrawOKT(): At any time, the owner of the contract can withdraw outstanding OKT balance from it.

Clean up#

After you are done with this contract, you can retrieve the remaining OKT to use with other examples.

1. Call withdrawOKT() function. MetaMask opens and asks you to confirm the transaction. After you approve the transaction, the remaining OKT will be transfered from your consuming contract to your wallet address.

What's next#

• › Security Considerations
• › Best Practices
• [› Supported Networks](/dev/oktc-solutions/oktc-vrf/Direct-Funding-Method/Suppo rted-Networks/Supported-Networks.html)