You are viewing the VRF v2 guide.
If you are using v1, see the VRF v1 guide.
Chainlink VRF (Verifiable Random Function) is a provably fair and verifiable random number generator (RNG) that enables smart contracts to access random values without compromising security or usability.
For each request, Chainlink VRF generates one or more random values and cryptographic proof of how those values were determined. The proof is published and verified on-chain before any consuming applications can use it. This process ensures that results cannot be tampered with or manipulated by any single entity including oracle operators, miners, users, or smart contract developers.
Use Chainlink VRF to build reliable smart contracts for any applications that rely on unpredictable outcomes:
To learn more about the benefits of Chainlink VRF v2, see our blog post Chainlink VRF v2 Is Now Live on Mainnet. For help with your specific use case, contact us to connect with one of our Solutions Architects. You can also ask questions about Chainlink VRF on Stack Overflow.
Chainlink VRF v2 is currently available on the following networks:
See the Supported Networks page for a complete list of coordinator addresses and gas price limits.
VRF v2 requests receive funding from subscription accounts. The Subscription Manager lets you create an account and pre-pay for VRF v2, so you don't provide funding each time your application requests randomness. This reduces the total gas cost to use VRF v2. It also provides a simple way to fund your use of Chainlink products from a single location, so you don't have to manage multiple wallets across several different systems and applications.
Subscriptions have the following core concepts:
For Chainlink VRF v2 to fulfill your requests, you must maintain a sufficient amount of LINK in your subscription balance. Gas cost calculation includes the following variables:
Gas price: The current gas price, which fluctuates depending on network conditions.
Callback gas: The amount of gas used for the callback request that returns your requested random values.
Verification gas: The amount of gas used to verify randomness on-chain.
The gas price depends on current network conditions. The callback gas depends on your callback function, and the number of random values in your request. The cost of each request is final only after the transaction is complete, but you define the limits you are willing to spend for the request with the following variables:
Gas lane: The maximum gas price you are willing to pay for a request in wei. Define this limit by specifying the appropriate
keyHash in your request. The limits of each gas lane are important for handling gas price spikes when Chainlink VRF bumps the gas price to fulfill your request quickly.
Callback gas limit: Specifies the maximum amount of gas you are willing to spend on the callback request. Define this limit by specifying the
callbackGasLimit value in your request.
Two types of accounts exist in the Ethereum ecosystem:
The Chainlink VRF v2 solution uses both off-chain and on-chain components:
VRFCoordinatorincluding the random number and a proof of how it was generated.
Requests to Chainlink VRF v2 follow the Request and Receive Data cycle. The VRF coordinator processes the request and determines the final charge to your subscription using the following steps:
The consuming contract must inherit VRFConsumerBaseV2 and implement the
fulfillRandomWords function, which is the callback VRF function. Submit your VRF request by calling
requestRandomWords of the VRF Coordinator with:
keyHash: Identifier that maps to a job and a private key on the VRF node and that represents a specified gas lane. If your request is urgent, specify a gas lane with a higher gas price limit. The configuration for your network can be found here.
s_subscriptionId: The subscription ID that the consuming contract is registered to. LINK funds are deducted from this subscription.
requestConfirmations: The number of block confirmations the oracle node will wait to respond. The minimum and maximum confirmations for your network can be found here.
callbackGasLimit: The maximum amount of gas a user is willing to pay for completing the callback VRF function. Note that you cannot put a value larger than
maxGasLimitof the VRF Coordinator contract (read coordinator contract limits for more details).
numWords: The number of random numbers to request. The maximum random values that can be requested for your network can be found here.
The VRF coordinator emits an event.
The event is picked up by the VRF node and will wait for the specified number of block confirmations to respond back to the VRF coordinator with the random values and a proof (
The VRF coordinator verifies the proof on-chain then calls back the consuming contract
After the request is complete, the final gas cost is recorded based on how much gas is required for the verification and callback. The total gas cost in wei for your request uses the following formula:
(Gas price * (Verification gas + Callback gas)) = total gas cost
The total gas cost is converted to LINK using the ETH/LINK data feed. In the unlikely event that the data feed is unavailable, the VRF coordinator uses the
fallbackWeiPerUnitLink value for the conversion instead. The
fallbackWeiPerUnitLink value is defined in the coordinator contract for your selected network.
The LINK premium is added to the total gas cost. The premium is defined in the coordinator contract with the
fulfillmentFlatFeeLinkPPMTier1 parameter in millionths of LINK.
(total gas cost + LINK premium) = total request cost
The total request cost is charged to your subscription balance.
Each subscription has the following limits:
Each subscription must maintain a minimum balance to fund requests from consuming contracts. If your balance is below that minimum, your requests remain pending for up to 24 hours before they expire. After you add sufficient LINK to a subscription, pending requests automatically process as long as they have not expired.
The minimum subscription balance must be sufficient for each new consuming contract that you add to a subscription. The required size of the minimum balance depends on the gas lane and the size of the request that the consuming contract makes. For example, a consuming contract that requests one random value will require a smaller minimum balance than a consuming contract that requests 50 random values. In general, you can estimate the required minimum LINK balance using the following formula where max verification gas is always 200,000.
(((Gas lane maximum * (Max verification gas + Callback gas limit)) / (1,000,000,000 Gwei/ETH)) / (ETH/LINK price)) + LINK premium = Minimum LINK
Each subscription supports up to 100 consuming contracts. If you need more than 100 consuming contracts, create multiple subscriptions.
You can see the configuration for each network on the Configuration page. You can also view the full configuration for each coordinator contract directly in Etherscan. As an example, view the Ethereum Mainnet VRF v2 coordinator contract configuration.
MAX_NUM_WORDSparameter that limits the maximum number of random values you can receive in each request.
maxGasLimitparameter, which is the maximum allowed
callbackGasLimitvalue for your requests. You must specify a sufficient
callbackGasLimitto fund the callback request to your consuming contract. This depends on the number of random values you request and how you process them in your
fulfillRandomWords()function. If your
callbackGasLimitis not sufficient, the callback fails but your subscription is still charged for the work done to generate your requested random values.