# VRF integration

In this section, we will discuss how to integrate your smart contracts with the Band VRF. This section is separated into two sub-sections: Requesting and Resolving.

Typically, when building on-chain applications that rely on an unpredictable outcome, such as lottery apps or games, the system requires a reliable source of randomness, and that is when the Band VRF comes into play.

# VRF Flow


As shown in the VRF workflow, a request and callback model is used to obtain new random data. We will summarize the steps shown in the figure above into 3 main steps to make it easier to understand; steps 1 and 3 are on the EVM side, and step 2 is on the Bandchain side.

  1. The process begins with a transaction that contains a consumer's request for random data from Band's VRFProvider contract.
  2. After the transaction in step 1 is confirmed, an off-chain entity will grab parameters from its log to make a request transaction on Bandchain, producing the VRF result.
  3. The last step is grabbing the result in step 2 with the proof of availability on Bandchain for making a relay transaction on the EVM side to resolve the request in the first step.

# Contracts integration

# Requesting

Assume that you are building an on-chain application that uses the Band VRF as a reliable source of randomness. Your contract(s) should contain a reference to Band's VRFProvider contract to be able to request random values.

First, let's define an interface for the VRFProvider contract.

Copy interface IVRFProvider { /// @dev The function for consumers who want random data. /// Consumers can simply make requests to get random data back later. /// @param seed Any string that used to initialize the randomizer. function requestRandomData(string calldata seed) external payable; }

Then, the consumer only needs to call requestRandomData with a string parameter called seed.

  • Please note that the seed is a generated string on the consumer side.
    Any two different consumers may be using the same seed. However, every consumer must use a unique seed for all their requests, as the VRFProvider has a mapping to check this condition.
    Copy contract VRFProvider { ... // Mapping that enforces the client to provide a unique seed for each request mapping(address => mapping(string => bool)) public hasClientSeed; ... }

After including the IVRFProvider, the consumer can now make a request-call to the VRFProvider contract, as shown in the example implementation below.

Copy contract MockVRFConsumer { IVRFProvider public provider; constructor(IVRFProvider _provider) { provider = _provider; } function requestRandomDataFromProvider(string calldata seed) external payable { provider.requestRandomData{value: msg.value}(seed); } }

When calling requestRandomData(seed), the consumer can specify msg.value to incentivize others to resolve the random data request. However, consumers can choose not to provide any incentive and resolve the request themselves.

After the consumer knows how to request from the VRFProvider, the consumer must also implement a callback function for the VRFProvider contract to call after the corresponding VRF result has been relayed.

The implementation below takes the previous step's code and adds the consume function.

Copy contract MockVRFConsumer { IVRFProvider public provider; constructor(IVRFProvider _provider) { provider = _provider; } function requestRandomDataFromProvider(string calldata seed) external payable { provider.requestRandomData{value: msg.value}(seed); } // ======================================================================================= string public latestSeed; uint64 public latestTime; bytes32 public latestResult; function consume(string calldata seed, uint64 time, bytes32 result) external override { require(msg.sender == address(provider), "Caller is not the provider"); latestSeed = seed; latestTime = time; latestResult = result; } }

As shown above, the consume function implements a logic that verifies whether the caller is the VRFProvider contract or not. This is to ensure that no one can call this function except the VRFProvider contract. With regards to the remaining logic in the example, the callback function only saves the callback data from the VRFProvider contract to its state.

Finally, we got our complete MockVRFConsumer contract that can make a request called the VRFProvider and then consume the VRF result once that request is resolved from the VRFProvider side.

# Resolving

Anyone can get any existing requests in the VRFProvider contract by tracking the mapping called tasks. A task(VRF request) was designed to be resolved in a decentralized way. Therefore, there is no permission to resolve any task. The only thing that needs to resolve any unresolved tasks is the Merkle proof of the VRF result available on Bandchain.

The code below shows what a task looks like and the data structure(mapping) that helps track the tasks.

Copy contract VRFProvider { ... struct Task { bool isResolved; uint64 time; address caller; uint256 taskFee; bytes32 seed; bytes32 result; string clientSeed; } // A global variable representing the number of all tasks in this contract uint64 public taskNonce; // Mapping from nonce => task mapping(uint64 => Task) public tasks; ... function relayProof(bytes calldata _proof, uint64 _taskNonce) external nonReentrant { ... } ... }

When a resolver(a self-implemented worker, a bot, a bounty hunter, etc.) finds an unresolved request, the resolver can resolve it by requesting the VRF randomness on the BandChain. After the VRF result is finalized on the BandChain, the resolver can retrieve the Merkle proof of availability of the result and then relay the proof via a relayProof function on the VRFProvider contract. The resolver also needs to specify the nonce of the task it wants to resolve.

The next step will demonstrate how to manually request and resolve the VRF randomness using the user interfaces of Cosmoscan(Band) and Etherscan(EVM).

# Manually request and resolve

This section will demonstrate how to request random data from the VRFProvider and then resolve the request manually using Goerli (opens new window) and Laozi-Testnet6 (opens new window) UI.

Firstly, go to the VRFProvider contract on Goerli (opens new window) to view some of its global variables.


To request the VRF randomness on Bandchain, we need to know the oracleScriptID. The VRF oracle script is the ID 335 (opens new window) on the laozi-Testnet6.

Now, let's move to the MockVRFConsumer (opens new window) contract to begin the VRF flow started by calling a function requestRandomDataFromProvider.


The video below shows the flow of the MockConsumer that requests the VRF random value from the VRFProvider.

Recommended Oracle Script Request Settings

Parameter Value
Prepare Gas 100000
Execute Gas 400000
Gas Limit (Leave Blank)
Ask Count 4
Min Count 3

# Implement your own resolver

An alternative to manually resolve the request is to use a resolver bot. Anyone can implement their own version of resolver bot to automate the resolving process. We provide an open-source version of Band's VRF worker bot, which is available at VRFWorkerV1 repository (opens new window).