Building Relaying Server with Sequence

With Sequence, you can create a smart contract wallet your server can use to dispatch transactions for your users without you having to be worried about transaction speed, throughput and re-orgs.

Sequentual Transactions

By default, Sequence transactions will be executed sequentially.

Nodejs Server

Your server will need an EOA wallet that will be able to sign messages. It will be the owner of your server-side Sequence wallet which will be used to dispatch transactions.

This Sequence wallet should have the correct ownership at your contract level, not the EOA. Also, the Sequence wallet should be sufficiently funded in order to pay fees needed by the relayer to dispatch your transactions.

import { RpcRelayer } from '@0xsequence/relayer'
import { Wallet } from '@0xsequence/wallet'
import { ethers } from 'ethers'

// Get a provider
const provider = new ethers.providers.JsonRpcProvider('https://nodes.sequence.app/polygon')

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

// Create your rpc relayer instance with relayer node you want to use
const relayer = new RpcRelayer({url: 'https://polygon-relayer.sequence.app', provider: provider})

// Create your Sequence server wallet, controlled by your server EOA, and connect it to the relayer
const wallet = (await Wallet.singleOwner(walletEOA)).connect(provider, relayer)

// Craft your transaction
const erc721Interface = new ethers.utils.Interface([
  'function safeTransferFrom(address _from, address _to, uint256 _tokenId)'
])

const data = erc721Interface.encodeFunctionData(
  'safeTransferFrom', [senderAddress, recipientAddress, id]
)

const txn = {
  to: erc721TokenAddress,
  data
}

// Request the possible fee options the relayer will accept for this transaction
const [config, context] = await Promise.all([wallet.getWalletConfig(), wallet.getWalletContext()])
const { options, quote } = await relayer.getFeeOptions(config[0], context, txn /* , txn2, txn3, etc... */)



// Choose a fee from the list of options returned by the relayer
// MATIC is native to Polygon and needs to be handled differently than other ERC-20 tokens like USDC

// === vvv To pay the fee in native MATIC: vvv ===
const option = options.find(option => option.token.symbol === 'MATIC')
if (!option) {
  throw Error(`relayer doesn't support MATIC fees`)
}

// Craft the MATIC fee payment transaction
// revertOnError: true is required for fee payments
const feeTxn = {
  to: option.to,
  value: option.value,
  gasLimit: option.gasLimit,
  revertOnError: true
}
// === ^^^ MATIC fee ^^^ ===



// === vvv To pay the fee in USDC: vvv ===
const option = options.find(option => option.token.symbol === 'USDC')
if (!option) {
  throw Error(`relayer doesn't support USDC fees`)
}

const erc20Interface = new ethers.utils.Interface([
  'function transfer(address _to, uint256 _value)'
])

// Craft the USDC fee payment transaction
// revertOnError: true is required for fee payments
const feeTxn = {
  to: option.token.contractAddress,
  gasLimit: option.gasLimit,
  data: erc20Interface.encodeFunctionData('transfer', [option.to, option.value]),
  revertOnError: true
}
// === ^^^ USDC fee ^^^ ===



// Send your transaction with the fee and quote to the relayer for dispatch
const txnResponse = await wallet.sendTransaction([txn, feeTxn], undefined, undefined, quote)

// Wait for transaction to be mined
const txnReceipt = await txnResponse.wait()

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

Parallel Transactions

If you want to send multiple independent transactions without needing to batch them, you can also send them in distinct nonce spaces. Using distinct nonce spaces for your transactions signals to the relayer that there's no dependency between them and that they can be executed on-chain in any order.

This allows the transactions to be dispatched immediately in an unbuffered way without having to wait for a full batch. Here is an example of how to do that:

// Import sequence nonce encoding function
import { encodeNonce } from '@0xsequence/transactions';

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

// Generate transactions
const txn1 = {
  to: tokenContract.address,
  data: erc20Interface.encodeFunctionData(
    'transfer', [recipient1, amount1]
  ),
  nonce: encodeNonce(randomNonceSpace1, 0)
}

const txn2 = {
  to: tokenContract.address,
  data: erc20Interface.encodeFunctionData(
    'transfer', [recipient2, amount2]
  ),
  nonce: encodeNonce(randomNonceSpace2, 0)
}

// Dispatch transactions, which can now be executed in parallel
const txnResponse = await wallet.sendTransactionBatch([txn1, txn2])

If batching transactions is not a problem for your use-case, you can call await wallet.sendTransactionBatch(txns). You can read more about batch transactions in Sending Batched Transactions.