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Pay gas

Pay gas

Overview

Axelar gas services provide methods to pay the relayer gas fee for both callContract and callContractWithToken. The fee can be paid in the native token of the source chain or any token supported by Axelar network. An application that wants Axelar to automatically execute contract calls on the destination chain needs to do three things:

  1. Estimate the gasLimit that the contract call will require on your executable contract on the destination chain.

  2. Call the estimateGasFee method to get the sourceGasFee in the desired gas-payment token on the destination chain. See this code snippet for reference. (As a prerequisite, Axelar SDK must be installed. Refer to AxelarJS SDK.)

  3. Pay the AxelarGasService smart contract on the source chain in the amount calculated in step 2.

Example

For example, assume the following smart contract is deployed on a source chain:

contract SimpleTransferContract {
  ...
  function sendToMany(
      string memory destinationChain,
      string memory destinationContractAddress,
      address[] calldata destinationAddresses,
      string memory symbol,
      uint256 amount
  ) external payable {
      address tokenAddress = gateway.tokenAddresses(symbol);
      IERC20(tokenAddress).transferFrom(msg.sender, address(this), amount);
      IERC20(tokenAddress).approve(address(gateway), amount);
      bytes memory payload = abi.encode(destinationAddresses);

      if(msg.value > 0) {
          // The line below is where we pay the gas fee
          gasReceiver.payNativeGasForContractCallWithToken{value: msg.value}(
              address(this),
              destinationChain,
              destinationContractAddress,
              payload,
              symbol,
              amount,
              msg.sender
          );
      }
      gateway.callContractWithToken(destinationChain, destinationContractAddress, payload, symbol, amount);
  }
}

The msg.value is the gas amount we pay to the AxelarGasService contract.

So, on the front-end side, we need to pass sourceGasFee to msg.value like below:

await contract.sendToMany("moonbeam", "0x...", ["0x.."], "USDC", 1, {
  value: sourceGasFee, // This is the value we get from Step 2.
});

After sending a transaction out, our Executor Service will do the following:

  • Monitor AxelarGasReceiver for receipt of payment, and get the amount paid as amountPaid.
  • Match those to contract calls.
  • Execute the specified contract call, specifying the gasLimit defined above.

Alternative gas payment methods for callContract

There are two available methods to pay gas for relaying callContract. You can choose the one that fits your application design.

payGasForContractcall

This method receives any tokens for the relayer fee. The paid gas for this method must be in tokens Axelar supports. See the list of supported assets for the chains we support: Mainnet | Testnet.

function payGasForContractCall(
    address sender,
    string calldata destinationChain,
    string calldata destinationAddress,
    bytes calldata payload,
    address gasToken,
    uint256 gasFeeAmount,
    address refundAddress
) external;

payNativeGasForContractCall

This method accepts the native tokens of the source chain.

// This is called on the source chain before calling the gateway to execute a remote contract.
function payNativeGasForContractCall(
    address sender,
    string calldata destinationChain,
    string calldata destinationAddress,
    bytes calldata payload,
    address refundAddress
) external payable;

Alternative gas payment methods for callContractWithToken

Similar to the available gas payment methods for callContract, there are two available methods to pay gas for relaying the callContractWithToken.

payGasForContractCallWithToken

This method receives any tokens for the relayer fee. The paid gas for this method must be in tokens Axelar supports. See the list of supported assets: Mainnet | Testnet.

// This is called on the source chain before calling the gateway to execute a remote contract.
function payGasForContractCallWithToken(
    address sender,
    string calldata destinationChain,
    string calldata destinationAddress,
    bytes calldata payload,
    string calldata symbol,
    uint256 amount,
    address gasToken,
    uint256 gasFeeAmount,
    address refundAddress
) external;

payNativeGasForContractCallWithToken

This method accepts the native tokens of the source chain.

// This is called on the source chain before calling the gateway to execute a remote contract.
function payNativeGasForContractCallWithToken(
    address sender,
    string calldata destinationChain,
    string calldata destinationAddress,
    bytes calldata payload,
    string calldata symbol,
    uint256 amount,
    address refundAddress
) external payable;

Methods arguments description

The function names are prety self-explanatory. The following is true for the arguments:

  • For all functions:
    • sender needs to match the address that calls callContract or callContractWithToken on the AxelarGateway. If the AxelarGasReceiver is called by the same contract that will call the Gateway, then simply specify address(this) as sender.
  • For payGasForContractCall and payNativeGasForContractCall, the following need to match the arguments of a contractCall on the AxelarGateway:
    • destinationChain
    • destinationAddress
    • payload
  • For payGasForContractCallWtihToken and payNativeGasForContractCallWithToken, the following need to match the arguments of a contractCallWithToken on the AxelarGateway:
    • destinationChain
    • destinationAddress
    • payload
    • symbol
    • amount
  • For payGasForContractCall and payGasForContractCallWtihToken:
    • gasToken is the address of the token that gas will be paid in. Ensure this token is supported by the network, using the Axelar API.
    • gasFeeAmount is the amount of gasToken to transfer from the sender. The sender needs to have approved the AxelarGasReceiver with the appropriate amount to gasToken first.
  • For payNativeGasForContractCall and payNativeGasForContractCallWithToken, the amount of funds received is specified by msg.value.
  • For all functions, refundAddress is the address that will eventually be able to receive excess amount paid for gas.

Two-way call

The Executor Service supports two-way call, where a message is sent from a source chain, immediately executed at a destination chain, and another message is returned back to the source chain.

The service monitors if there's another contract call immediately executed within the same executed transaction on the destination (to send a message back to the source chain). The service will then automatically use the remaining prepaid gas to relay the second call of the two-way call.

In case the remaining gas amount is insufficient for the second call, the service refunds it to the payer's address. Users can still pay a new gas amount to relay the second call of the transfer through the Axelar SDK or Axelarscan UI.