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 learn how to use our helper functions to get useful information from directly within your target contract.
  • These allow you access to the feeCollector, feeToken address , fee, and _msgSenderamount from within your target contract.

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Overview

When using callWithSyncFeeERC2771, you need to pay Gelato’s fee collecting contract when your target function is called, otherwise your relay request will not execute. To carry out this payment, your target contract needs to know the address of the fee collector so that you can transfer funds during the call. Furthermore, you need to know in what token to pay the fee in, and how much to pay the fee collector. Gelato Relay appends this useful information to the end of the calldata, when using the callWithSyncFeeERC2771 SDK method. Gelato Relay’s Context contracts give you helper functions which you can use via inheritance in your target contract allowing you to decode information from the relay calldata, giving you access to:
  • uint256 _getFee(): a value denoting how much fee to pay.
  • address _getFeeToken(): the address of the token the fee is paid in.
  • address _getFeeCollector(): the address to which to send your payment.
  • address _getMsgSender(): the address of the off-chain signer.
If you need target function needs to know all four variables from the relay calldata, see GelatoRelayContextERC2771.If you only need the feeCollector address (i.e. you already encode the fee and feeToken inside your own function parameters), see GelatoRelayFeeCollectorERC2771.

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Getting Started

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Installing relay-context

relay-context is an extremely simple way to create a Gelato Relay compatible smart contract, with just one import. 
npm install --save-dev @gelatonetwork/relay-context
or 
yarn add -D @gelatonetwork/relay-context
:::note Please make sure to use version v3.0.0 and above. :::

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Smart Contract

import {
    GelatoRelayContextERC2771 
} from "@gelatonetwork/relay-context/contracts/GelatoRelayContextERC2771.sol";
for GelatoRelayContextERC2771. OR: 
import {
    GelatoRelayFeeCollectorERC2771
} from "@gelatonetwork/relay-context/contracts/GelatoRelayFeeCollectorERC2771.sol";
for GelatoRelayFeeCollectorERC2771.

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GelatoRelayContextERC2771

GelatoRelayContextERC2771 allows your smart contract to retrieve the following information from the relay calldata:
  • Gelato’s fee collector address, a contract specifically deployed for collecting relay fees securely. This allows a smart contract to transfer fees directly if you are using the syncFee payment method.
  • The fee token address specifying which address the fee will be paid in, which Gelato resolved from the original relay-SDK request body.
  • The fee itself, which includes the gas cost + Gelato’s 10% fee on top.
  • The _getMsgSender() address, the off-chain signer address.
Below is an example: 
// SPDX-License-Identifier: MIT
pragma solidity 0.8.17;

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

contract Counter is GelatoRelayContextERC2771 {
    mapping(address => uint256) public counter;
    
    event IncrementCounter();
    
    // `increment` is the target function to call
    // this function increments a counter variable after payment to Gelato
    function increment() external onlyGelatoRelay {
        // 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();

        counter[_getMsgSender()] += 1;

        emit IncrementCounter();
    }
}
Line 21 inherits the GelatoRelayContextERC2771 contract, giving access to these features:

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Verifying the caller:

  • onlyGelatoRelay: a modifier which will only allow Gelato Relay to call this function.
  • _isGelatoRelay(address _forwarder): a function which returns true if the address matches Gelato Relay’s address.

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Decoding the calldata:

  • _getFeeCollector() : a function to retrieve the fee collector address.
  • _getFee(): a function to retrieve the fee that Gelato will charge.
  • _getFeeToken(): a function to retrieve the address of the token used for fee payment.
  • _getMsgSender(): a function to retrieve the off-chain signer address.
  • _getMsgData(): a function to access the original msg.data without appended information.

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Transferring Fees to Gelato

As you are using the callWithSyncFeeERC2771 SDK method, you can use the below helper functions to pay directly to Gelato:
  • _transferRelayFee(): a function which transfers the fee amount to Gelato, with no cap.
  • _transferRelayFeeCapped(uint256 _maxFee): a function which transfers the fee amount to Gelato which a set cap from the argument maxFee in wei. This helps limit fees on function calls in case of gas volatility or just for general budgeting.

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GelatoRelayFeeCollectorERC2771

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Why are there two different contracts that I can inherit?

You can choose to inherit either GelatoRelayContextERC2771 or GelatoRelayFeeCollectorERC2771. GelatoRelayContextERC2771 gives you access to all four pieces of information: feeCollector, feeToken, fee, and msg.sender whereas GelatoRelayFeeCollectorERC2771 assumes only the feeCollector and the msg.sender addresses are appended to the calldata.

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Which contract should I inherit?

In the majority of scenarios, inheriting from GelatoRelayContextERC2771 is recommended. This approach provides the most convenient way to handle fees, as it only requires you to call either the _transferRelayFee() or _transferRelayFeeCapped(uint256 _maxFee) method. All other processes are managed seamlessly behind the scenes. If the fee is known in advance - for instance, if you have already queried our fee oracle for the fee amount and a user has given their approval on this amount and the token to be used for payment via a front-end interface - you would only need to inform your smart contract where to direct this fee. In this case, you would require only the feeCollector address. For this purpose, please inherit from GelatoRelayFeeCollectorERC2771. Refer to the following details.

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Recommendation: maximum fee signing

The fee oracle allows you to query and display a fee to your users before sending their transaction via Gelato Relay. Therefore, you could also give them the option to sign off on a certain fee. In this case, you might want to pass the fee you receive from the oracle directly to your target function as an argument. This makes sense, but be wary that due to gas price volatility, a smoother UX might be to query the fee oracle and calculate a maximum fee by adding some overhead, and displaying this maximum fee to the user. This way, if gas prices do fluctuate more than normal, you can be certain that your user’s transaction is executed. You can choose to set a very large overhead, or a smaller one, depending on your own trade-off between execution likelihood and cost to the user. This way, you can also integrate a fee check from inside target function to avoid any overpayments.

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GelatoRelayFeeCollectorERC2771 Integration

GelatoRelayFeeCollector allows your smart contract to retrieve the following information from the relay calldata:
  • Gelato’s fee collector address, a contract specifically deployed for collecting relay fees securely. This allows a smart contract to transfer fees directly if you are using the syncFee payment method.
  • The _getMsgSender() address, the off-chain signer address.
Below is an example: 
// SPDX-License-Identifier: MIT
pragma solidity 0.8.17;

import {IERC20} from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import {Address} from "@openzeppelin/contracts/utils/Address.sol";
import {
    SafeERC20
} from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
import {
    GelatoRelayFeeCollectorERC2771
} from "@gelatonetwork/relay-context/contracts/GelatoRelayFeeCollectorERC2771.sol";

contract Counter is GelatoRelayFeeCollectorERC2771 {
    using SafeERC20 for IERC20;
    mapping(address => uint256) public counter;
    
    event IncrementCounter();
    
    // `increment` is the target function to call
    // this function increments a counter variable after payment to Gelato
    function increment() external onlyGelatoRelay {
        // Retreiving the feeCollector address, using the nativeToken
        address feeCollector = _getFeeCollector();
        address nativeToken = "0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE"; 
        // Hardcoding the fee to 100 wei - NOTE: this is just for example
        // If you do not pay enough to feeCollector, 
        // your relay request will not go through
        // In reality, you should pass in user signatures TODO
        uint256 fee = 100;
        
        // 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.
        transfer(nativeToken, feeCollector, fee);

        counter[_getMsgSender()] += 1;
        emit IncrementCounter();
    }
    
    
    function transfer(
        address _token,
        address _to,
        uint256 _amount
    ) internal {
        if (_amount == 0) return;
        _token == NATIVE_TOKEN
            ? Address.sendValue(payable(_to), _amount)
            : IERC20(_token).safeTransfer(_to, _amount);
    }
}
Line 13 inherits the GelatoRelayFeeCollectorERC2771 contract, giving access to these features:

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Verifying the caller:

  • onlyGelatoRelay: a modifier which will only allow Gelato Relay to call this function.
  • _isGelatoRelay(address _forwarder): a function which returns true if the address matches Gelato Relay’s address.

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Decoding the calldata:

  • _getFeeCollector() : a function to retrieve the fee collector address.
  • _getMsgSender(): a function to retrieve the off-chain signer address.
  • _getMsgData(): a function to access the original msg.data without appended information.