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Fees

Omni charges fees for each xcall. These fees are paid synchronously on the source chain, in the chain's native token. For most rollups, this is ETH.

Fee Calculation​

Fees are determined by an xcall's destination chain, calldata, and gas limit. You can calculate the fee for an xcall via the XApp.feeFor(...) method.

uint256 fee = feeFor(
   destChainId,  // destination chain id
   data,         // abi encoded calldata, ex abi.encodeWithSignature("foo()")
   gasLimit      // gas limit to enforce on execution
)

Fee Payment​

XApp handles calculating and charging fees when making an xcall

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xcall(...) charges fees to your contract by default. To charge users for fees, you can calculate the fee with feeFor(...), and verify msg.value is sufficient. Or better yet, you can require users to send sufficient fees with each contract call:

uint256 fee = xcall(...)
require(msg.value >= fee, "insufficient fee")

You can calculate this fee off chain, and require users send sufficient xcall fees with each contract call.

Example​

In the case of our hello world example, the fee may be different for each greeting, because the length of the greeting message is variable. You can calculate this fee offchain by querying the portal directly (via OmniPortal.feeFor(...)). Or, you can introduce a view function on your contract that calculates the fee.

function greetFee(string calldata greeting) external view {
    feeFor(
        // ...
    );
}

For this simple example, this view function is not that helpful. But for xcalls with calldata that depends on other contract state, a view function like this can be very helpful.