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.

The only difference from using a typical Sequence wallet when sending transactions to the blockchain, is at the smart contract level the msg.sender is one of the Sequence Relayers wallet addresses. For Sequence Builder standard contracts, this is not a problem when combined with a relayed Transactions API request.

By default, Sequence transactions will be executed sequentially.

The following steps will guide you through how to create you server and mint collectibles to a wallet address:

  1. Environment Setup with Express Server: Create a NodeJs based server using the library Express to accept HTTP requests
  2. Project & Access Key Management: Claim a public access key to interact with the Sequence stack
  3. Deploy Collectible Contract: Deploy a collectible contract in order to be able to submit transactions to the blockchain to mint tokens to a wallet address
  4. Construct Sponsored Relayer with Transactions API: Craft a function to be used in an Express route to call the Sequence Transactions API from a sponsored contract

Additonal Features:

  • (Optional) Relay with Wallet Owned Currency: Craft a function to be used in an Express route to call the Sequence Transactions API paid for using a wallet owned currency
  • (Optional) Relay Parallel Transactions: Perform transaction batching to send a currency
1

Environment Setup with Express Server

Ensure that pnpm (or some other node package manager) is installed with the following command:

curl -fsSL https://get.pnpm.io/install.sh | sh -

Then, clone down the following express template code

Express is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications, which will be used in this guide.

After the code is locally on your machine, run your server and client with the following command:

pnpm run start

Contained in the code is a route called /mint that can be called from the cli for testing.

Give a try with this example curl request:

curl -X POST http://localhost:3000/mint -d '{"tokenID": 0, "address": "0x"}'

You should see the following output:

{"txHash":"0x"}
2

Project & Access Key Management

First start by following this walkthrough for how to sign up to the Sequence Builder and to learn how to create a project.

Then, in order to use the Transactions API, you’ll need to upgrade your Billing to Developer which can be with this walkthrough.

Finally, a Public Access Key is required for the Transactions API, which can be acquired following this walkthough.

Finally update the update the .env.example to .env with the following:

CHAIN_HANDLE='<CHAIN_HANDLE>' # e.g. `mainnet`, `xr-sepolia`, etc.

PROJECT_ACCESS_KEY='<PUBlIC_ACCESS_KEY>'
3

Deploy Collectible Contract

Follow this walkthrough to deploy a collectible contract.

If you’re using a non-testnet and need to sponsor your contract, you can perform this action by following this walkthrough

Finally, update the .env with your deployed collectible contract:

...

COLLECTIBLE_CONTRACT_ADDRESS="<ADDRESS>"

​
Construct Sponsored Relayer with the Transactions API

The full code for this section can be found here

First, using the template code provided in step #1, we’ll need to add a few packages

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

import { findSupportedNetwork, NetworkConfig } from '@0xsequence/network'

Then, 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.

Openning a session may trigger a migration of your Sequence wallet to a new version, this could be v1 to v2 or v2 to future versions.

Migration is a one-way process, once your wallet is migrated it cannot be reverted to a previous version.

To catch any unwanted migration, you can use the onMigration callback.

To implement the callContract function, include the following code that uses a single signer to relay transactions:

const callContract = async (address: string, tokenID: number): Promise<ethers.providers.TransactionResponse> => {

	

	const chainConfig: NetworkConfig = findSupportedNetwork(process.env.CHAIN_HANDLE!)!

	const provider = new ethers.providers.StaticJsonRpcProvider({

		url: chainConfig.rpcUrl

	})



	const walletEOA = new ethers.Wallet(process.env.PKEY!, provider);

	const relayerUrl = `https://${chainConfig.name}-relayer.sequence.app`



	// Create a single signer sequence wallet session

	const session = await Session.singleSigner({

		signer: walletEOA,

		projectAccessKey: process.env.PROJECT_ACCESS_KEY!

	})



	const signer = session.account.getSigner(chainConfig.chainId)

	

	// Standard interface for ERC1155 contract deployed via Sequence Builder

	const collectibleInterface = new ethers.Interface([

		'function mint(address to, uint256 tokenId, uint256 amount, bytes data)'

	])

		

	const data = collectibleInterface.encodeFunctionData(

		'mint', [`${address}`, `${tokenID}`, "1", "0x00"]

	)



	const txn = {

		to: process.env.COLLECTIBLE_CONTRACT_ADDRESS, 

		data: data

	}



	try {

		return await signer.sendTransaction(txn)

	} catch (err) {

		console.error(`ERROR: ${err}`)

		throw err

	}

}

Finally, update the .env with a private key for a wallet that can be generated from the following app which is used for demo purposes. For production, we recommend to generate private keys securely locally on your computer via this example script.

Then, update the PKEY variable with the key:

...

PKEY='<WALLET_PRIVATE_KEY>'

​
Grant Minter Role to Relayer Wallet Address

One must update the role access of the contract in the Builder to only receive requests from the minter wallet address.

You can do this in Sequence Builder by providing minter permission to your Sequence Wallet Transactions API Address.

To do so, open your project, navigate to the Contracts page, select your Linked contracts and under Write Contract tab expand the grantRole method.

Complete with the following details:

bytes32 role: 0x9f2df0fed2c77648de5860a4cc508cd0818c85b8b8a1ab4ceeef8d981c8956a6

address account: <Generated Sequence Transactions API Wallet Address>

Where the role string inputted is the result of keccak256("MINTER_ROLE") in solidity or ethers.solidityPackedKeccak256(ethers.toUtf8Bytes("MINTER_ROLE")) in javascript

This makes it so that only your specific address can mint from the contract, it will error otherwise.

Complete the role update by clicking write and sign the sponsored transaction.

You application is now ready for you to send a test transaction from the client frontend by signing into your wallet and clicking mint.

Give it a try!

Don’t forget to update the access key in the client in the initWallet function

​
(Optional) Relay with Wallet Owned Currency

You can also enforce a specific way to pay for gas fees:

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}`)

}

​
(Optional) Relay 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 transactions API 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:

// 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),

]);

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