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Broadcasting

X402 Protocol broadcasting is responsible for securely transmitting validated payment payloads to the blockchain for settlement. The Blockia relayer service handles all on-chain execution, gas sponsorship, and transaction monitoring.

Relayer Responsibilities​

  • Receives validated ERC-3009 payloads from the backend
  • Checks relayer account balance and chain status
  • Broadcasts transferWithAuthorization to the USDC contract
  • Waits for transaction confirmation and returns the tx hash
  • Handles errors (insufficient balance, network issues, contract reverts)

On-Chain Execution​

The relayer submits the transaction using the correct chain and contract address, ensuring:

  • Correct EIP-712 domain separator and chainId
  • Gas fees are paid by the relayer (merchant-sponsored)
  • Only valid, non-replayed, and non-expired payloads are broadcast

Error Handling​

If broadcasting fails, the relayer returns a detailed error to the backend, which is surfaced to the API client. Common errors include:

  • RELAY_BALANCE_LOW: Relayer has insufficient funds
  • CONTRACT_REVERTED: Smart contract rejected the transfer
  • NETWORK_ERROR: RPC/network issues

Monitoring & Observability​

  • All broadcast attempts are logged with status and tx hash
  • Relayer balance and health are monitored
  • Failed transactions trigger alerts for investigation

Future Enhancements​

  • Multi-chain relayer support
  • Batch transaction broadcasting
  • Automated relayer failover and redundancy