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Cross-Chain Messaging
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Your First Cross-Chain Message

First Cross-Chain Message

In this tutorial we will create a simple contract that allows sending a message from one chain to another using the Connector API.

Prerequisites​

Set Up Your Environment​

git clone https://github.com/zeta-chain/template

Install dependencies:

cd template
yarn add --dev @openzeppelin/contracts

Create the Contract​

To create a new cross-chain messaging contract you will use the messaging Hardhat task available by default in the template.

npx hardhat messaging CrossChainMessage message:string

The messaging task accepts one or more arguments: the name of the contract and a list of arguments (optionally with types). The arguments define the contents of the message that will be sent across chains.

In the example above the message will have only one field: message of type string. If the type is not specified it is assumed to be string.

The messaging task has created:

  • contracts/CrossChainMessage.sol: a Solidity cross-chain messaging contract
  • tasks/deploy.ts: a Hardhat task to deploy the contract on one or more chains
  • tasks/interact.ts: a Hardhat task to interact with the contract

It also modified hardhat.config.ts to import both deploy and interact tasks.

Cross-Chain Messaging Contract​

Let's review the contents of the CrossChainMessage contract:

contracts/CrossChainMessage.sol
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The contract:

  • inherits from ZetaInteractor, which provides two modifiers that are used to validate the message and revert calls: isValidMessageCall and isValidRevertCall.
  • implements ZetaReceiver, which defines two functions: onZetaMessage and onZetaRevert.

State Variables:

  • CROSS_CHAIN_MESSAGE_MESSAGE_TYPE: a public constant state variable which defines the message type. If your contract supports more than one message type, it's useful to define a constant for each one.
  • _zetaConsumer: a private immutable state variable that stores the address of ZetaTokenConsumer, which is used amond other things for getting ZETA tokens from native tokens to pay for gas when sending a message.
  • _zetaToken: an internal immutable state variable that stores the address of the ZETA token contract.

The contract defines two events: CrossChainMessageEvent emitted when a message is processed on the destination chain and CrossChainMessageRevertedEvent emitted when a message is reverted on the destination chain.

The constructor passes connectorAddress to the ZetaInteractor constructor and initializes both _zetaToken and _zetaConsumer state variables.

The sendMessage function is used to send a message to a recipient contract on the destination chain. It first checks that the destination chain ID is valid. Then it uses ZETA consumer to get the needed amount of ZETA tokens from the provided msg.value (amount of native gas assets sent with the function call), and approves the ZetaConnector to spend the zetaValueAndGas amount of ZETA tokens.

The sendMessage function uses connector.send to send a crosss-chain message with the following arguments wrapped in a struct:

  • destinationChainId: chain ID of the destination chain
  • destinationAddress: address of the contract receiving the message on the destination chain (expressed in bytes since it can be non-EVM)
  • destinationGasLimit: gas limit for the destination chain's transaction
  • message: arbitrary message to be parsed by the receiving contract on the destination chain
  • zetaValueAndGas: amount of ZETA tokens to be sent to the destination chain, ZetaChain gas fees, and destination chain gas fees (expressed in ZETA tokens)
  • zetaParams: optional ZetaChain parameters.

The onZetaMessage function processes incoming cross-chain messages. The function decodes the message to identify its type and content. If the message type matches a predefined constant, the message's reception is logged through the CrossChainMessageEvent. However, if the type is unrecognized, the function reverts to ensure that only specific message types are handled. The function also uses a isValidMessageCall modifier to verify the message's authenticity, ensuring it comes from a trusted source.

The onZetaRevert function handles the reversal of cross-chain messages. Taking in a ZetaInterfaces.ZetaRevert parameter, the function decodes this reverted message to identify its type and content. If the message type aligns with a predefined constant, the function logs the reversal through the CrossChainMessageRevertedEvent. On the other hand, if the type is not recognized, the function reverts the transaction. The function also uses the isValidRevertCall modifier to ensure that the revert message is genuine and originates from the trusted source.

Deploy Task​

The messaging task has created a Hardhat task to deploy the contract.

tasks/deploy.ts
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To establish cross-chain messaging between blockchains via ZetaChain, you need to deploy contracts capable of sending and receiving cross-chain messages to two or more blockchains connected to ZetaChain.

You can specify the desired chains by using a --networks parameter of the deploy task, which accepts a list of network names separated by commas. For instance, --networks goerli_testnet,bsc_testnet.

The main function maintains a mapping of network names to their corresponding deployed contract addresses, iterating over the networks to deploy the contract on each one.

The contract's constructor requires three arguments: the connector contract's address, the ZETA token's address, and the ZETA token consumer contract's address. These addresses are obtained using ZetaChain's getAddress.

The main function subsequently sets interactors for each contract. An interactor is a mapping between a chain ID of the destination and the contract address on that chain.

When deploying to two chains (like Goerli and BSC testnet), you will invoke setInteractorByChainId on a Goerli contract and pass the BSC testnet chain ID (97) and the BSC testnet contract address. You then perform the same operation on a BSC testnet contract, passing the Goerli chain ID (5) and the Goerli contract address. If deploying to more than two chains, you must call setInteractorByChainId for each link between the chains.

Interact Task​

The messaging task has also created a Hardhat task to interact with the contract:

tasks/interact.ts
https://github.com/zeta-chain/example-contracts/blob/main/messaging/message/tasks/interact.ts

The task accepts the following arguments:

  • contract: address of the contract on the source chain
  • amount: amount of native tokens to send with the transaction
  • destination: name of the destination chain
  • message: message to be sent to the destination chain

The main function uses the contract argument to attach to the contract on the source chain. It then uses the destination argument to obtain the destination chain's chain ID. The function subsequently converts the message argument to bytes and sends a transaction to the contract's sendMessage function, passing the destination chain ID and the message.

Finally, the task uses the trackCCTX function from @zetachain/toolkit/helpers to track the token transfer transaction. The function waits for the transaction to appear on ZetaChain and tracks the status of the transaction. Transaction tracking is optional, but helpful to know when the transaction has been processed by ZetaChain.

Create an Account​

To deploy and interact with the contract you will need a wallet with tokens.

Create a new wallet account:

npx hardhat account --save

This command generates a random wallet, prints information about the wallet to the terminal, and saves the private key to a .env file to make it accessible to Hardhat.

Use the Faucet to Request Tokens​

To pay for the transaction fees to deploy and interact with the cross-chain messaging contracts you will need native gas tokens on the connected chains you are deploying contracts to. You can find a list of recommended faucets in the docs.

Check Token Balances​

npx hardhat balances

Deploy the Contract​

Clear the cache and artifacts, then compile the contract:

npx hardhat compile --force

Run the following command to deploy the contract to two networks:

npx hardhat deploy --networks bsc_testnet,goerli_testnet
🚀 Successfully deployed contract on bsc_testnet
📜 Contract address: 0x6Fd784c16219026Ab0349A1a8A6e99B6eE579C93

🚀 Successfully deployed contract on goerli_testnet.
📜 Contract address: 0xf1907bb130feb28D6e1305C53A4bfdb32140d8E6

🔗 Setting interactors for a contract on bsc_testnet
✅ Interactor address for 5 (goerli_testnet) is set to 0xf1907bb130feb28d6e1305c53a4bfdb32140d8e6

🔗 Setting interactors for a contract on goerli_testnet
✅ Interactor address for 97 (bsc_testnet) is set to 0x6fd784c16219026ab0349a1a8a6e99b6ee579c93

Interact with the Contract​

Send a message from BSC testnet to Goerli using the contract address (see the output of the deploy task). Make sure to submit enough native tokens with --amount to pay for the transaction fees.

npx hardhat interact --message hello --network bsc_testnet --destination goerli_testnet --contract 0x6Fd784c16219026Ab0349A1a8A6e99B6eE579C93 --amount 2
🔑 Using account: 0x2cD3D070aE1BD365909dD859d29F387AA96911e1

✅ "sendHelloWorld" transaction has been broadcasted to bsc_testnet
📝 Transaction hash: 0xa3a507d34056f4c00b753e7d0b47b16eb6d35b3c5016efa0323beb274725b1a1

After the cross-chain transaction is processed on ZetaChain, look up the contract on the Goerli explorer by the contract address (0xf1907bb130feb28D6e1305C53A4bfdb32140d8E6) and you should be able to see the emitted HelloWorldEvent event.

Congratulations! 🎉 In this tutorial you have:

  • cloned the Hardhat contract template
  • used npx hardhat messaging to create a new cross-chain messaging contract
  • reviewed the contents of the generated contract and the tasks to deploy and interact with the contract
  • successfully deployed the contract to two connected chains and set interactors on each contract
  • sent a message from one chain to another using the connector.send

Source Code​

You can find the source code for the example in this tutorial here:

https://github.com/zeta-chain/example-contracts/tree/main/messaging/message