Lesson 9: Purpose of fulfillRandomWords() not understood.

[Mark-Four]

Why is the fulfillRandomWords function called that? As per my understanding, the code we have written within the function is similar to a withdraw function that has been passed a random number as a parameter that helps it pick a random winner, and all the funds are sent to that winner address.
What is the function fulfilling actually?

[alymurtazamemon]

@Mark-Four Which fulfullRandomWords function the one that we have override or the one inside VRFCoordinator?

[Mark-Four]

Hi @alymurtazamemon The one that's in our raffle.sol.
That also brings up a question as to why the function exists in the VRFCoordinator as well since it has no data in it and is expecting to be overridden.

[alymurtazamemon]

@Mark-Four I have meeting I will answer you after it, hope someone will answer it before that.

[alymurtazamemon]

@Mark-Four fulfillRandomWords of VRFCoordinator will be called by Chainlink nodes when we will request for randomWords, they will do all the off-chain calculations and will give us random numbers. These random numbers will be passed to our overridden fulfillRandomWords parameters along with the request-id.

Now on our side of fulfillRandomWords, we are taking those random numbers and getting an index of a winner from it and doing other things like clearing variables and emitting events, etc.

[Mark-Four]

Got it! Thanks yet again @alymurtazamemon ! you're a saviour!!!

[krakxn]

Can you quote the relevant code?

[Mark-Four]

Hi @krakxn,

 function fulfillRandomWords(
        uint256, /*requestId*/
        uint256[] memory randomWords
    ) internal override {
        uint256 indexOfWinner = randomWords[0] % s_players.length;
        address payable recentWinner = s_players[indexOfWinner];
        s_recentWinner = recentWinner;
        (bool success, ) = recentWinner.call{value: address(this).balance}("");
        if (!success) {
            revert Raffle__TransferFailed();
        }
        emit WinnerPicked(recentWinner);
    }

I'd like to know the logic behind 'Fulfilling' a random number.
As I understand from the chainlink documentation, the fulfillRandomWords function checks to see if the random number was indeed provided by a verified chainlink node, before using the random number in the contract, in order to prevent attacks.
But as given in the code above, we haven't used any functionality that points to anything outside the contract.

[krakxn]

Uhm.. sorry if I remember incorrectly but pretty sure we use VRFCoordinator somewhere, which does the functionality you are describing.

Whereas here, the functionality is for generating the winner.

[Mark-Four]

Thanks @krakxn. You're right and you remember correctly. I was getting ahead of myself. Understood it's use later when we're deploying. Thank you again!!!

[krakxn]

That is awesome, all the best!