Subscription Method

This section explains how to generate random numbers using the subscription method.

Subscriptions

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:

  • Subscription id: 64-bit unsigned integer representing the unique identifier of the subscription.
  • Subscription accounts: An account that holds LINK tokens and makes them available to fund requests to Chainlink VRF v2 coordinators.
  • Subscription owner: The wallet address that creates and manages a subscription account. Any account can add LINK to the subscription balance, but only the owner can add approved consuming contracts or withdraw funds.
  • Consumers: Consuming contracts that are approved to use funding from your subscription account.
  • Subscription balance: The amount of LINK maintained on your subscription account. Requests from consuming contracts will continue to be funded until the balance runs out, so be sure to maintain sufficient funds in your subscription balance to pay for the requests and keep your applications running.

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.

Request and Receive Data

End To End Diagram

VRF v2 subscription method end-to-end diagram

Two types of accounts exist in the Ethereum ecosystem:

  • EOA (Externally Owned Account): An externally owned account that has a private key and can control a smart contract. Transactions can only be initiated by EOAs.
  • Smart contract: A contract that does not have a private key and executes what it has been designed for as a decentralized application.

The Chainlink VRF v2 solution uses both off-chain and on-chain components:

  • VRF v2 Coordinator (on-chain component): A contract designed to interact with the VRF service. It emits an event when a request for randomness is made, and then verifies the random number and proof of how it was generated by the VRF service.
  • VRF service (off-chain component): Listens for requests by subscribing to the VRF Coordinator event logs and calculates a random number based on the block hash and nonce. The VRF service then sends a transaction to the VRFCoordinator including the random number and a proof of how it was generated.

Explanation

Requests to Chainlink VRF v2 follow the Request & Receive Data cycle. The VRF coordinator processes the request and determines the final charge to your subscription using the following steps:

  1. 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 service 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 VRF service 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 maxGasLimit of 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.
  2. The VRF coordinator emits an event.

  3. The event is picked up by the VRF service and will wait for the specified number of block confirmations to respond back to the VRF coordinator with the random values and a proof (requestConfirmations).

  4. The VRF coordinator verifies the proof on-chain then calls back the consuming contract fulfillRandomWords function. 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.

Limits

Chainlink VRF v2 has some subscription limits and coordinator contract limits.

Subscription limits

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.

Coordinator contract limits

You can see the configuration for each network on the Supported networks 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.

  • Each coordinator has a MAX_NUM_WORDS parameter that limits the maximum number of random values you can receive in each request.
  • Each coordinator has a maxGasLimit parameter, which is the maximum allowed callbackGasLimit value for your requests. You must specify a sufficient callbackGasLimit to 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 callbackGasLimit is not sufficient, the callback fails but your subscription is still charged for the work done to generate your requested random values.

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