Building a Relaying Transaction Server

import { Session } from '@0xsequence/auth'
import { ethers } from 'ethers'

// where the <chain_handle> corresponds to https://docs.sequence.xyzhttps://status.sequence.info/
const provider = new ethers.providers.JsonRpcProvider('https://nodes.sequence.app/<chain_handle>');

// Create your server EOA
const walletEOA = new ethers.Wallet(serverPrivateKey, provider)

// Open a Sequence session, this will find or create
// a Sequence wallet controlled by your server EOA
const session = await Session.singleSigner({
  signer: walletEOA,
  projectAccessKey: '<access_key>'
  // OPTIONAL: Multiple wallets could be found for the same EOA
  // to enforce a specific wallet you can use the following callback
  selectWallet: async (wallets: string[]) => {
    const found = wallets.find(w => w === EXPECTED_WALLET_ADDRESS)
    if (!found) throw Error('wallet not found')
    // Returning the wallet address will make the session use it
    // returning undefined will make the session create a new wallet
    return found
  }
})

const signer = session.account.getSigner(137, {
  // OPTIONAL: You can also enforce a specific way to pay for gas fees
  // if not provided the sdk will select one for you
  selectFee: async (
    _txs: any,
    options: FeeOption[]
  ) => {
    // Find the option to pay with native tokens
    const found = options.find(o => !o.token.contractAddress)
    if (!found) throw Error('fee option not found')
    return found
  }
})

// Initialize the contract
const usdc = new ethers.Contract(
  '0x2791bca1f2de4661ed88a30c99a7a9449aa84174', // USDC on Polygon
  ERC_20_ABI,
  signer
)

// Send the transaction
const txnResponse = await usdc.transfer(recipient, 1)

// Check if transaction was successful
if (txnReceipt.status != 1) {
  console.log(`Unexpected status: ${txnReceipt.status}`)
}

// Generate random nonce spaces with ~0% probability of collision
const randomNonceSpace1 = ethers.BigNumber.from(
  ethers.hexlify(ethers.randomBytes(20))
);
const randomNonceSpace2 = ethers.BigNumber.from(
  ethers.hexlify(ethers.randomBytes(20))
);

// Create signers for each nonce space
const signer1 = session.account.getSigner(137, {
  nonceSpace: randomNonceSpace1,
});

const signer2 = session.account.getSigner(137, {
  nonceSpace: randomNonceSpace2,
});

// Generate transactions
const txn1 = {
  to: tokenContract.address,
  data: erc20Interface.encodeFunctionData("transfer", [recipient1, amount1]),
};

const txn2 = {
  to: tokenContract.address,
  data: erc20Interface.encodeFunctionData("transfer", [recipient2, amount2]),
};

// Dispatch transactions, which can now be executed in parallel
await Promise.all([
  signer1.sendTransaction(txn1),
  signer2.sendTransaction(txn2),
]);