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:

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

You can pay gas for relaying callContract using the following method.

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 relaying two-way calls.

A two-way call refers to the scenario of sending a message from a source chain and immediately executing it at a destination chain. Finally, return another message call to the source chain.

Outbound call: a GMP call from chain A to chain B
Returned call: a GMP call returned from chain B to chain A

Once an outbound call is executed on chain B and sends another call to return a message to chain A, the Executor service automatically forwards the remaining gas to relay the returned trip.

Suppose the remaining gas amount is insufficient for the returned trip, the Insufficient Fee tag will show up on Axelarscan UI (see Monitoring state of GMP transactions). In that case, the returned call won’t be relayed until the gas is added. You can increase more gas to relay the call to the destination contract via the Axelar SDK or Axelarscan UI.

Sending messages to multiple destination chains from a single transaction

The Executor Service also supports relaying multiple message calls from a transaction. To do so, the application must pay gas to the Gas Receiver separately for each message. Please see the below message call as an example.

Example: Transaction on the source chain, Message #1 information, Message #2 information

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