Working of Automatic Relayers

Sending the message

The IWormholeRelayer interface has this sendPayloadToEvm function at its core that is used to send a message to a contract on another chain.

function sendPayloadToEvm(
    // Chain ID in Wormhole format
    uint16 targetChain,     
    // Contract Address on target chain we're sending a message to
    address targetAddress,  
    // The payload, encoded as bytes
    bytes memory payload,   
    // How much value to attach to the delivery transaction 
    uint256 receiverValue,  
    // The gas limit to set on the delivery transaction
    uint256 gasLimit        
) external payable returns (
    // Unique, incrementing ID, used to identify a message
    uint64 sequence
);

Here’s the reference implementation of sending a message across different EVM Chains:

// Get a quote for the cost of gas for delivery
(cost, ) = wormholeRelayer.quoteEVMDeliveryPrice(
    targetChain,
    valueToSend,
    GAS_LIMIT
);

// Send the message
wormholeRelayer.sendPayloadToEvm{value: cost}(
    targetChain,
    targetAddress,
    abi.encode(payload),
    valueToSend, 
    GAS_LIMIT
);

Receiving the message

And here’s our receiver function that is implemented in the IWormholeReceiver interface:

function receiveWormholeMessages(
    bytes memory payload,           // Message passed by source contract 
    bytes[] memory additionalVaas,  // Any additional VAAs that are needed (Note: these are unverified) 
    bytes32 sourceAddress,          // The address of the source contract
    uint16 sourceChain,             // The Wormhole chain ID
    bytes32 deliveryHash            // A hash of contents, useful for replay protection
) external payable;