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Introduction

In this article, we’ll detail how to deploy, link, and interact with multi-chain smart contracts across 2 different shards on Quai Network. This method can be extended to deploy trustless cross-chain contracts across all 9 shards. We’ll be using the basic implementation of a QRC721 token, an adapted version of the ERC721 standard, to showcase cross-chain contracts for this tutorial.

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Prerequisites

To deploy multi chain smart contracts on Quai, we’ll need a few tool-kits and dependencies. Here’s an overview of all of the dependencies we’ll be using:
NodeJSJavascript runtime environment. Use the LTS version.
hardhat-exampleA Hardhat project with sample contracts and deploy scripts for Quai Network.
Quais.jsA JavaScript library for interacting with Quai Network.
quai-hardhat-pluginA plugin built for Hardhat that provides support for the SolidityX compiler.

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Environment Setup

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Dependencies

Start by cloning the hardhat-example repository, navigating to the SolidityX/ directory we’ll be using for this tutorial, and installing the dependencies via npm. 
git clone https://github.com/dominant-strategies/hardhat-example.git
cd hardhat-example/SolidityX
npm install
If you’ve already cloned the hardhat-example repository for the Single-Chain Deployment Tutorial, you can skip the cloning step. Just navigate to the SolidityX/ directory and run npm install.

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Smart Contracts

The SolidityX directory comes with 2 sample contracts: QRC20.sol and QRC721.sol inside of the contracts/ directory. Both of the included contracts are the initial SolidityX/cross-chain implementations of existing token standards. Source code for the contracts can be found in the SolidityX-Contracts Repository As mentioned above, we’ll be deploying the QRC721 smart contract. Before configuring and deploying the QRC721, we recommend getting familiar with the contract specs.

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Environment Variables

We’ve included a sample .env.dist environment file at the root of the hardhat-example repo to hold token details, private keys, and RPC URLs in a secure fashion.
The .env.dist file is a template file and should not be used as is. You should copy the .env.dist file to a new .env file in the repository root using the following command:This file lives at the root of the hardhat-example repository and serves as the config file for both the Solidity and SolidityX directories.
Copy the .env.dist file in the root to a new .env file in the repository root using the following command: 
cp ../.env.dist ../.env
Open the .env file and add your private keys, RPC URLs, and token args for the contract you’d like to deploy. The .env file should look like this:
.env
# Unique Privkey for each deployment address
CYPRUS1_PK="0x3700000000000000000000000000000000000000000000000000000000000000" # pubkey starting with 0x00
CYPRUS2_PK="0x9400000000000000000000000000000000000000000000000000000000000000" # pubkey starting with 0x01
...more priv keys

# Chain ID (local: 1337, testnet and devnet: 9000)
CHAIN_ID="9000"

# RPC endpoints
RPC_URL="https://rpc.quai.network"
...more rpc urls

# Token Arguments
...more token args
The PK values must all be for unique addresses and correspond to the zone name, i.e. your CYPRUS1PK should be the private key of your Cyprus1 address.
Further information on RPC endpoints can be found in the local network specifications section for local nodes and the devnet specifications section for remote nodes. The hardhat-example repository uses the Quais SDK to configure network connections using only a single RPC URL. To learn more about how the SDK configures network providers, visit the SDK provider examples section. After filling in your private keys, RPC URL, we’re now ready to securely consume them inside of hardhat.config.js.

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Hardhat Configuration

Hardhat uses hardhat.config.js to configure smart contract deployments. The config file allows you to define deployment networks, tasks, compilers, etc. hardhat-example/SolidityX contains a prebuilt hardhat.config.js file with configurations for compiling, deploying, verifying SolidityX smart contracts on Quai. The below configuration file has two main differences from the hardhat.config.js file use for basic Solidity contract deployment:
  • It imports the quai-hardhat-plugin to handle SolidityX compiler download
  • Inclusion of the optional solidityx object to specify a locally built SolidityX compiler (if you don’t want to use the plugin to download the compiler)
When deploying or verifying a contract, hardhat.config.js will pull your private keys and RPC URLs from the .env file and use them to deploy and verify your contracts.

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SolidityX Compiler

To be able to properly compile and deploy SolidityX contracts, we’ll need the SolidityX. There are two methods of installing the SolidityX compiler for use with Hardhat:
  • Install the SolidityX compiler via quai-hardhat-plugin (Recommended)
  • Install and build the SolidityX compiler from source
As noted above, the hardhat.config.js file already includes the quai-hardhat-plugin to handle the SolidityX compiler download. If you’ve followed the above steps, you’re already set up to use the plugin to download the SolidityX compiler.

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Deploy

1

Compile with SolidityX

SolidityX contract compilation with Hardhat is simple and can be done using npx in the CLI.Compile all of the contracts inside the contracts/ directory with:
npx hardhat compile
Which should output something like:
Info Using SolidityX at: /Users/user/hardhat-example/SolidityX/solc
Warning: This is a pre-release compiler version, please do not use it in production.

Compiled 2 Solidity files successfully (evm target: istanbul).
2

Configure deployment scripts

Inside the scripts/ directory, you’ll find a deploy script for both QRC20 and QRC721 contracts: deployQRC20.js and deployQRC721.js. For this tutorial, we’ll be using the QRC721 contract.The deployQRC721.js script works by pulling your specified network/accounts config from hardhat.config.js and the QRC721 arguments specified in the .env file at the root of the repository and uses them to deploy your contract.Token arguments are consumed via the tokenArgs array:
const tokenArgs = [process.env.QRC721_NAME, process.env.QRC721_SYMBOL, process.env.QRC721_BASE_URI]
Your specified network configuration is consumed inside of the provider and wallet variables in tandem with the compiled contract ABI and bytecode to create a new contract instance:
const provider = new quais.JsonRpcProvider(hre.network.config.url, undefined, { usePathing: true })
const wallet = new quais.Wallet(hre.network.config.accounts[0], provider)
const QRC721 = new quais.ContractFactory(QRC721Json.abi, QRC721Json.bytecode, wallet)
We’ll use these ideas to properly modify the token args and network specification to deploy our contracts in the next step.
3

Deploy contracts

For this tutorial, we’ll be deploying two instances of our QRC721 contract on two different chains. You can extend the methodology used here to deploy and link contracts to any combination of shards within Quai Network.We’ll be deploying the first QRC721 contract on Cyprus-1 chain. To do this, we’ll pass cyprus1 as the network flag in the deployment command like below:
npx hardhat run scripts/deployQRC721.js --network cyprus1
Running this should output:
Transaction broadcasted: 0x018100ff42c92c99ddfe8f577ce63743769ae2daf46ad2032eec3bc3f803961d
Contract deployed to: 0x00735E9B2c731Fd3eCC8129a3653ACb99dF969cC
Now, we can deploy an identical QRC721 contract to another shard within Quai, like Cyprus-2. Like before, you’ll pass cyprus2 as the network flag in the deployment command.
When deploying QRC721s, we recommend changing the baseURI variable for each chain to prevent duplicate mints or additionally modifying the QRC721 contract to handle minting on different shards. This variable can be changed in the .env file at the root of the repository.
npx hardhat run scripts/deployQRC721.js --network cyprus2
Which again should output something like this:
Transaction broadcasted: 0x01b40095e3d8e9d498a1ee8eb0500b832a3c98abf9b787fcc63a91c37585fbe7
Contract deployed to: 0x0172F38EC31f58B1419E2CcE2B05B095625218ea
We’ve now deployed our test QRC721 contract to both the Cyprus-1 and Cyprus-2 chains!
Make sure to save these two contract addresses, we’ll need them in the next section.
To complete our cross-chain NFT deployment, we’ll need to link the two deployed contracts. “Linking” the two QRC721 contracts can be done by adding the deployed contract addresses of our QRC721s to the approved contracts array within each contract. This can be done using the AddApprovedAddresses method. It accepts 2 arrays as arguments: chain indexes and approved addresses. The AddApprovedAddresses method seen below can be used to add as few as 1 or as many as 8 sister contracts to the approvedAddresses array of a QRC721 or QRC20 contract.
QRC721.sol
  function AddApprovedAddresses(uint8[] calldata chain, address[] calldata addr) external {
    require(msg.sender == _deployer, "Sender is not deployer");
    require(chain.length == addr.length, "chain and address arrays must be the same length");
    for(uint8 i = 0; i < chain.length; i++) {
        require(chain[i] < 9, "Max 9 zones");
        require(ApprovedAddresses[chain[i]] == address(0), "The approved address for this zone already exists");
        ApprovedAddresses[chain[i]] = addr[i];
    }
  }
Once the sister contract addresses have been added to the respective ApprovedAddresses of each of the QRC721 contracts, the cross-chain functionality of the transferFrom method becomes available, which allows anyone who owns a token from the collection to trustlessly send their it between shards that the contracts are deployed to.

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Contract Linking Script

To link the sister contracts, we’ll utilize quais.js and some of the Hardhat Runtime Environment that we used in the deploy script. Start by creating another file in the scripts directory named addApprovedAddresses.js. 
touch scripts/addApprovedAddresses.js
Then, paste the following code into addApprovedAddresses.js:
addApprovedAddresses.js
const quais = require('quais')
const QRC721 = require('../artifacts/contracts/QRC721.sol/QRC721.json')

async function AddApprovedQRC721Addresses() {
	const provider = new quais.JsonRpcProvider(hre.network.config.url, undefined, { usePathing: true })
	const privateKey = hre.network.config.accounts[0]
	const wallet = new quais.Wallet(privateKey, provider)
	const contractAddress = 'contract address you want to change the address array for' // contract address to add approved addresses to
	const qrc721 = new quais.Contract(contractAddress, QRC721.abi, wallet)
	try {
		const tx = await qrc721.AddApprovedAddresses(
			[0, 1], // chain indexes (cyprus1 is 0, cyprus2 is 1, etc.)
			['0x00...', '0x01....'] // contract addresses (must be in same order as chain indexes)
		)
		const txRecipt = await tx.wait()
		console.log('Transaction mined with hash', txReceipt.hash)
	} catch (error) {
		console.error('Error sending transaction:', error)
	}
}

AddApprovedQRC721Addresses()
	.then(() => process.exit(0))
	.catch((error) => {
		console.error(error)
		process.exit(1)
	})
The addApprovedAddresses.js script uses the QRC721.sol ABI to compose and send a transaction that inserts new addresses to the approvedAddresses array in any deployed QRC721 contract.
After you’ve pasted the code for the linking script into the addApprovedAddresses.js file, we’re ready to start linking our sister contracts.
1

Build linking transactions

Now that we’ve set up our script, we’re ready to link our two deployed contracts.Start by grabbing the addresses of the two contracts we deployed in the deploy section.
Cyprus 1 contract address: 0x00735E9B2c731Fd3eCC8129a3653ACb99dF969cC
Cyprus 2 contract address: 0x0172F38EC31f58B1419E2CcE2B05B095625218ea
We’ll take these contract addresses and use them to build the transaction data passed to the addApprovedAddresses method.
You can pass the same transaction data to every contract you want to link, as the addApprovedAddresses method can take in and handle its own contract address as an argument. This removes the need to alter the transaction data for each contract you want to link.
The transaction data we’ll need to pass to the addApprovedAddresses method is (notice the order of the arrays):
  • chainIndex array: [0, 1]
  • address array: ['0x00735E9B2c731Fd3eCC8129a3653ACb99dF969cC', '0x0172F38EC31f58B1419E2CcE2B05B095625218ea']
The built transaction should look similar to this:
const tx = await qrc721.AddApprovedAddress(
  [0, 1], // chain indexes [cyprus1, cyprus2]
  ['0x00735E9B2c731Fd3eCC8129a3653ACb99dF969cC', '0x0172F38EC31f58B1419E2CcE2B05B095625218ea'] // contract addresses [cyprus1, cyprus2]
)
You can extend this transaction data structure to link as many contracts as you’d like by adding additional chain indexes and contract addresses to the arrays. Always make sure to add the same number of chain indexes and contract addresses to the arrays in matching order.
2

Send linking transactions

First, we’re going to send the linking transaction to our Cyprus 1 contract. To do this, start by changing the contractAddress variable to our Cyprus 1 contract address in the addApprovedAddresses.js script:
const contractAddress = '0x00735E9B2c731Fd3eCC8129a3653ACb99dF969cC'
Now, we’re ready to run the script and complete the Cyprus 1 contract linkage. Make sure to pass the --network cyprus1 flag when sending transactions to the Cyprus 1 contract.
npx hardhat run scripts/addApprovedAddresses.js --network cyprus1
The script should output something like this:
Transaction sent: 0x00499178c3f0046b4d44a57a966f9e224759c1b3158af984fcb5a1432b16ee8e
Transaction mined with hash: 0x00499178c3f0046b4d44a57a966f9e224759c1b3158af984fcb5a1432b16ee8e
We’ve now linked our Cyprus 1 contract to our Cyprus 2 contract, but we’re not done yet.To finish linking these two sister contracts, we’ll need to send the exact same transaction data to the Cyprus 2 contract. In the addApprovedAddresses.js script, change the contractAddress variable to our Cyprus 2 contract address:
const contractAddress = '0x0172F38EC31f58B1419E2CcE2B05B095625218ea'
Lastly, send the linkage transaction to our Cyprus 2 token by running the script with the --network cyprus2 flag:
Transaction sent: 0x018e8dea20b73089b51e6b3d2b3abd8a9e8ca63e06be20375cf721e13aabd590
Transaction mined with hash: 0x018e8dea20b73089b51e6b3d2b3abd8a9e8ca63e06be20375cf721e13aabd590
Once the second transaction is confirmed, our two QRC721 contracts have been successfully linked across chains. After minting a token, you can now send your NFTs from Cyprus 1 to Cyprus 2 without a bridge or external service! This deployment and linking process can be repeated for any number of chains within Quai Network purely by deploying the contract to the desired chains and linking them with the addApprovedAddresses method. You now have the tools to deploy and link contracts across all zone chains within Quai Network.
The same deploy and link method can be used for any other SolidityX based contract with cross-chain logic, including the QRC-20 Token.