2️⃣callWithSyncFeeERC2771

Transactions with on-chain payments and ERC2771 authentication support

If you are using @gelatonetwork/relay-sdk v3 or contracts from the package @gelatonetwork/relay-context v2 please follow this migration guide to migrate to the new versions.

After reading this page:

• You'll know how to use the callWithSyncFeeERC2771 SDK method, using the syncFee payment method.

• You'll see some code which will help you send a relay request within minutes.

• You'll learn how to pay for transactions using the provided values for fee, feeToken and feeCollector.

Please proceed to our Security Considerations page and read it thoroughly before advancing with your implementation. It is crucial to understand all potential security risks and measures to mitigate them.

Overview

callWithSyncFeeERC2771 method uses the syncFee payment method with ERC-2771 support.

Paying for Transactions

When using callWithSyncFeeERC2771 relay method the target contract assumes responsibility for transferring the fee to Gelato's fee collector during transaction execution. For this, the target contract needs to know:

• fee: the transfer amount

• feeToken: the token to be transferred

• feeCollector: the destination address for the fee

Fortunately, Gelato provides some useful tools within the Relay Context Contracts:

1. By inheriting the GelatoRelayContextERC2771 contract in your target contract, you have the ability to transfer the fee through one of two straightforward methods: _transferRelayFee() or _transferRelayFeeCapped(uint256 maxFee). In either case, the inherited contract takes care of decoding the fee, feeToken, and feeCollector behind the scenes. The Gelato Relay backend simplifies the process by automatically calculating the fee for you, using Gelato's Fee Oracle to perform the calculations in the background.

2. Alternatively, you may choose to inherit the GelatoRelayFeeCollectorERC2771 contract. With this approach, Gelato only decodes the feeCollector. You must provide the fee and feeToken on-chain, either by hardcoding them (which is not recommended) or embedding them within the payload to be executed. The suggested way to handle this is to calculate the fee with Gelato's Fee Oracle.

Setting maxFee for Your Transaction

Setting a maximum fee, or maxFee, for your transactions is strongly advised. This practice enables you to ensure that transaction costs remain below a specific limit. The method _transferRelayFeeCapped(uint256 maxFee) in the GelatoRelayContextERC2771 contract provides a convenient way to set the maxFee easily.

If you are utilizing the GelatoRelayFeeCollectorERC2771 contract, the recommended way to pass the maxFee is by calculating the fee with Gelato's Fee Oracle, which is accessible in the relay-sdk. The getEstimatedFee() method is provided to facilitate this calculation.

SDK method: callWithSyncFeeERC2771

const callWithSyncFeeERC2771 = async (
  request: CallWithSyncFeeERC2771Request | CallWithSyncFeeConcurrentERC2771Request,
  options?: RelayRequestOptions
): Promise<RelayResponse>

Arguments:

• request: this is the request body used to send a request.
• options?: relayRequestOptions is an optional object.

Return Object: RelayResponse

type RelayResponse = {
  taskId: string;
};

• taskId: your unique relay task ID which can be used for tracking your request.

Optional Parameters

See Optional Parameters.

Sending a Request

As of today, we support two distinct ways of sending callWithSyncFeeERC2771 requests:

1. Sequentially: This approach ensures that each request is ordered and validated against the nonce stored on-chain. You have two options in this method:

   • Fetch the current nonce value from the smart contract yourself and include it with your request.

   • Allow the relay-sdk to fetch the nonce value for you when handling your relay request.

2. Concurrently: This method enables you to send multiple transactions simultaneously. Replay protection is achieved using a hash-based salt mechanism. Again, you have two options:

   • Provide your own salt value.

   • Allow the relay-sdk to generate a unique salt value for you when processing your relay request.

By default callWithSyncFeeERC2771 requests are using the sequential method.

Concurrent ERC2771 support has been introduced in the relay-sdk version 5.1.0. Please make sure that your package is up-to-date to start using it.

Request Body

type SequentialERC2771Request = {
  chainId: BigNumberish;
  target: string;
  data: BytesLike;
  user: string;
  userDeadline?: BigNumberish;
  feeToken: string;
  isRelayContext?: boolean;
  isConcurrent?: false;
  userNonce?: BigNumberish;
};
type ConcurrentERC2771Request = {
  chainId: BigNumberish;
  target: string;
  data: BytesLike;
  user: string;
  userDeadline?: BigNumberish;
  feeToken: string;
  isRelayContext?: boolean;
  isConcurrent: true;
  userSalt?: string;
};

Common Parameters

• chainId: the chain ID of the chain where the target smart contract is deployed.

• target: the address of the target smart contract.

• data: encoded payload data (usually a function selector plus the required arguments) used to call the required target address.

• user: the address of the user's EOA.

• userDeadline: optional, the amount of time in seconds that a user is willing for the relay call to be active in the relay backend before it is dismissed.

  • This way the user knows that if the transaction is not sent within a certain timeframe, it will expire. Without this, an adversary could pick up the transaction in the mempool and send it later. This could transfer money, or change state at a point in time which would be highly undesirable to the user.

• feeToken: the address of the token that is to be used for payment. Please visit SyncFee Payment Tokens for the full list of supported payment tokens per network.
• isRelayContext: an optional boolean (default: true) denoting what data you would prefer appended to the end of the calldata.

  • If set to true (default), Gelato Relay will append the feeCollector address, the feeToken address, and the uint256 fee to the calldata. In this case your target contract should inherit from the GelatoRelayContextERC2771 contract.
  • If set to false, Gelato Relay will only append the feeCollector address to the calldata. In this case your target contract should inherit from the GelatoRelayFeeCollectorERC2771 contract.

Parameters For Sequential Requests:

• isConcurrent: false (default), optional, represents that the users' requests are validated based on a nonce, which enforces them to be processed sequentially.

• userNonce: optional, this nonce, akin to Ethereum nonces, is stored in a local mapping on the relay contracts. It serves to enforce the nonce ordering of relay calls if the user requires sequential processing. If this parameter is omitted, the relay-sdk will automatically query the current value on-chain.

Parameters For Concurrent Requests:

• isConcurrent: true, indicates that the users' requests are validated based on a unique salt, allowing them to be processed concurrently. Replay protection is still ensured by permitting each salt value to be used only once.

• userSalt: optional, this is a bytes32 hash that is used for replay protection. If the salt is not provided then relay-sdk would generate a unique value based on a random seed and a timestamp.

Example Code GelatoRelayContextERC2771

1. Deploy a GelatoRelayContextERC2771 compatible contract

// SPDX-License-Identifier: MIT
pragma solidity 0.8.17;

import {
    GelatoRelayContextERC2771
} from "@gelatonetwork/relay-context/contracts/GelatoRelayContextERC2771.sol";

import {Address} from "@openzeppelin/contracts/utils/Address.sol";

// Inheriting GelatoRelayContext gives access to:
// 1. _getFeeCollector(): returns the address of Gelato's feeCollector
// 2. _getFeeToken(): returns the address of the fee token
// 3. _getFee(): returns the fee to pay
// 4. _transferRelayFee(): transfers the required fee to Gelato's feeCollector.abi
// 5. _transferRelayFeeCapped(uint256 maxFee): transfers the fee to Gelato
//    only if fee < maxFee
// 6. function _getMsgSender(): decodes and returns the user's address from the
//    calldata, which can be used to refer to user safely instead of msg.sender
//    (which is Gelato Relay in this case).
// 7. _getMsgData(): returns the original msg.data without appended information
// 8. onlyGelatoRelay modifier: allows only Gelato Relay's smart contract
//    to call the function
contract CounterRelayContextERC2771 is GelatoRelayContextERC2771 {
    using Address for address payable;

    mapping(address => uint256) public contextCounter;

    // emitting an event for testing purposes
    event IncrementCounter(address msgSender);

    // `increment` is the target function to call.
    // This function increments a counter variable which is 
    // mapped to every _getMsgSender(), the address of the user.
    // This way each user off-chain has their own counter 
    // variable on-chain.
    function increment() external onlyGelatoRelayERC2771 {
        // Payment to Gelato
        // NOTE: be very careful here!
        // if you do not use the onlyGelatoRelay modifier,
        // anyone could encode themselves as the fee collector
        // in the low-level data and drain tokens from this contract.
        _transferRelayFee();

        // Incrementing the counter mapped to the _getMsgSender()
        contextCounter[_getMsgSender()]++;
        
        emit IncrementCounter(_getMsgSender());
    }
    
    // `incrementFeeCapped` is the target function to call.
    // This function uses `_transferRelayFeeCapped` method to ensure 
    // better control of gas fees. If gas fees are above the maxFee value 
    // the transaction will not be executed.
    // The maxFee will be passed as an argument to the contract call.
    // This function increments a counter variable by 1
    // IMPORTANT: with `callWithSyncFee` you need to implement 
    // your own smart contract security measures, as this 
    // function can be called by any third party and not only by 
    // Gelato Relay. If not done properly, funds kept in this
    // smart contract can be stolen.
    function incrementFeeCapped(uint256 maxFee) external  onlyGelatoRelayERC2771 {

        // Payment to Gelato
        // NOTE: be very careful here!
        // if you do not use the onlyGelatoRelay modifier,
        // anyone could encode themselves as the fee collector
        // in the low-level data and drain tokens from this contract.
  
        _transferRelayFeeCapped(maxFee);

        // Incrementing the counter mapped to the _getMsgSender()
        contextCounter[_getMsgSender()]++;

        emit IncrementCounter(counter);
    }
}

2. Import GelatoRelaySDK into your front-end .js project

import { GelatoRelay, CallWithSyncFeeERC2771Request } from "@gelatonetwork/relay-sdk";
const relay = new GelatoRelay();

3. Send the payload to Gelato

// target contract address
const counter = "<your counter contract address>";

// using a human-readable ABI for generating the payload
const abi = ["function increment()"];

// address of the token used to pay fees
const feeToken = "0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE";

// connect to the blockchain via a front-end provider
const provider = new ethers.BrowserProvider(window.ethereum);
const signer = provider.getSigner();
const user = signer.getAddress();

// instantiate the target contract object
const contract = new ethers.Contract(counter, abi, signer);

// example calling the increment() method
const { data } = await contract.increment.populateTransaction();

// populate the relay SDK request body
const request: CallWithSyncFeeERC2771Request = {
  chainId: (await provider.getNetwork()).chainId,
  target: counter,
  data: data,
  user: user,
  feeToken: feeToken,
  isRelayContext: true,
};
  
// send relayRequest to Gelato Relay API
const relayResponse = await relay.callWithSyncFeeERC2771(request, provider);

// -----------------------------------------------------------------
// the following is an alternative example using Gelato Fee Oracle, 
// setting maxFee, and calling the incrementFeeCapped(maxFee) method

// retrieve the estimate fee from Gelato Fee Oracle
const fee = await relay.getEstimatedFee(
  (await provider.getNetwork()).chainId,
  feeToken,
  gasLimit,
  false,
)

// you can use 2x or 3x to set your maxFee
const maxFee = fee * 2

// example calling the incrementFeeCapped(maxFee) method
const { dataMaxFee } = await contract.incrementFeeCapped.populateTransaction(maxFee);

// populate the relay SDK request body
const requestMaxFee: CallWithSyncFeeERC2771Request = {
  chainId:  (await provider.getNetwork()).chainId,
  target: counter,
  data: dataMaxFee,
  user: user,
  feeToken: feeToken,
  isRelayContext: true,
};
  
// send relayRequest to Gelato Relay API
const relayResponseMAxFee = await relay.callWithSyncFeeERC2771(requestMaxFee, provider);