Getting Started

This section walks through how to build an example cross-chain application on top of the Teleporter protocol, recreating the ExampleCrossChainMessenger contract that sends arbitrary string data from one chain to another. Note that this tutorial is meant for education purposes only. The resulting code is not intended for use in production environments.

Step 1: Create Initial Contract​

Create a new file called MyExampleCrossChainMessenger.sol in a new directory:

mkdir teleporter/contracts/src/CrossChainApplications/MyExampleCrossChainMessenger/
touch teleporter/contracts/src/CrossChainApplications/MyExampleCrossChainMessenger/MyExampleCrossChainMessenger.sol

At the top of the file define the Solidity version to work with, and import the necessary types and interfaces.

pragma solidity 0.8.18;

import {ITeleporterMessenger, TeleporterMessageInput, TeleporterFeeInfo} from "@teleporter/ITeleporterMessenger.sol";
import {ReentrancyGuard} from "@openzeppelin/[email protected]/security/ReentrancyGuard.sol";

Next, define the initial empty contract. The contract inherits from ReentrancyGuard to prevent reentrancy attacks.

contract MyExampleCrossChainMessenger is
    ReentrancyGuard
{

}

Finally, add the following struct and event declarations into the body of the contract, which will be integrated in later:

    /**
     * @dev Messages sent to this contract.
     */
    struct Message {
        address sender;
        string message;
    }

    /**
     * @dev Emitted when a message is submited to be sent.
     */
    event SendMessage(
        bytes32 indexed destinationBlockchainID,
        address indexed destinationAddress,
        address feeTokenAddress,
        uint256 feeAmount,
        uint256 requiredGasLimit,
        string message
    );

    /**
     * @dev Emitted when a new message is received from a given chain ID.
     */
    event ReceiveMessage(
        bytes32 indexed sourceBlockchainID,
        address indexed originSenderAddress,
        string message
    );

Step 2: Integrating Teleporter Messenger​

Now that the initial empty MyExampleCrossChainMessenger is defined, it's time to integrate with ITeleporterMessenger, which will provide the functionality to deliver cross chain messages.

Create a state variable of ITeleporterMessenger type called teleporterMessenger. Then create a constructor that takes in an address where the Teleporter Messenger would be deployed on this chain, and set the corresponding state variable.

    ITeleporterMessenger public immutable teleporterMessenger;

    constructor(address teleporterMessengerAddress) {
        teleporterMessenger = ITeleporterMessenger(teleporterMessengerAddress);
    }

Step 3: Send and Receive​

Now that MyExampleCrossChainMessenger has an instantiation of ITeleporterMessenger, the next step is to add in the functionality of sending and receiving arbitrary string data between chains.

To start, create the function declaration for sendMessage, which will send string data cross-chain to the specified destination address' receiver. This function allows callers to specify the destination chain ID, destination address to send to, relayer fees, required gas limit for message execution at the destination address.

    /**
     * @dev Send a new message to another chain.
     */
    function sendMessage(
        bytes32 destinationBlockchainID,
        address destinationAddress,
        address feeTokenAddress,
        uint256 feeAmount,
        uint256 requiredGasLimit,
        string calldata message
    ) external returns (bytes32 messageID) {

    }

MyExampleCrossChainMessenger also needs to implement ITeleporterReceiver. First, add the import of this interface:

import {ITeleporterReceiver} from "@teleporter/ITeleporterReceiver.sol";

Then declare that the contract will implement it:

  contract MyExampleCrossChainMessenger is
-     ReentrancyGuard
+     ReentrancyGuard,
+     ITeleporterReceiver
  {

And then finally add the method receiveTeleporterMessage that receives the cross-chain messages from Teleporter.

    /**
     * @dev Receive a new message from another chain.
     */
    function receiveTeleporterMessage(
        bytes32 sourceBlockchainID,
        address originSenderAddress,
        bytes calldata message
    ) external {

    }

Now it's time to implement the methods, starting with sendMessage. First, add the necessary imports.

import {SafeERC20TransferFrom, SafeERC20} from "@teleporter/SafeERC20TransferFrom.sol";
import {IERC20} from "@openzeppelin/[email protected]/token/ERC20/IERC20.sol";

Next, add a using directive to the top of the contract body specifying SafeERC20 as the IERC20 implementation to use:

    using SafeERC20 for IERC20;

Then add a check to the sendMessage function for whether feeAmount is greater than zero. If it is, transfer and approve the amount of IERC20 asset at feeTokenAddress to the Teleporter Messenger saved as a state variable.

        // For non-zero fee amounts, first transfer the fee to this contract, and then
        // allow the Teleporter contract to spend it.
        uint256 adjustedFeeAmount;
        if (feeAmount > 0) {
            adjustedFeeAmount = SafeERC20TransferFrom.safeTransferFrom(
                IERC20(feeTokenAddress),
                feeAmount
            );
            IERC20(feeTokenAddress).safeIncreaseAllowance(
                address(teleporterMessenger),
                adjustedFeeAmount
            );
        }

Note: Relayer fees are an optional way to incentivize relayers to deliver a Teleporter message to its destination. They are not strictly necessary, and may be omitted if a relayer is willing to relay messages with no fee, such as with a self-hosted relayer.

Next, to the end of the sendMessage function, add the event to emit, as well as the call to the TeleporterMessenger contract with the message data to be executed when delivered to the destination address. Form a TeleporterMessageInput and call sendCrossChainMessage on the TeleporterMessenger instance to start the cross chain messaging process. The message must be ABI encoded so that it can be properly decoded on the receiving end.

Note: allowedRelayerAddresses is empty in this example, meaning any relayer can try to deliver this cross chain message. Specific relayer addresses can be specified to ensure only those relayers can deliver the message.

        emit SendMessage({
            destinationBlockchainID: destinationBlockchainID,
            destinationAddress: destinationAddress,
            feeTokenAddress: feeTokenAddress,
            feeAmount: adjustedFeeAmount,
            requiredGasLimit: requiredGasLimit,
            message: message
        });
        return
            teleporterMessenger.sendCrossChainMessage(
                TeleporterMessageInput({
                    destinationBlockchainID: destinationBlockchainID,
                    destinationAddress: destinationAddress,
                    feeInfo: TeleporterFeeInfo({
                        feeTokenAddress: feeTokenAddress,
                        amount: adjustedFeeAmount
                    }),
                    requiredGasLimit: requiredGasLimit,
                    allowedRelayerAddresses: new address[](0),
                    message: abi.encode(message)
                })
            );

With the sending side complete, the next step is to implement ITeleporterReceiver.receiveTeleporterMessage. The receiver in this example will just receive the arbitrary string data, and check that the message is sent through Teleporter. To the receiveTeleporterMessage function, add:

        // Only the Teleporter receiver can deliver a message.
        require(msg.sender == address(teleporterMessenger), "Unauthorized.");

        // do something with message.

The base of sending and receiving messages cross chain is complete. MyExampleCrossChainMessenger can now be expanded with functionality that saves the received messages, and allows users to query for the latest message received from a specified chain.

Step 4: Storing the Message​

Start by adding a map to the body of the contract, in which the key is the sourceBlockchainID and the value is the latest message sent from that chain. The message is of type Message, which is already declared in the contract.

    mapping(bytes32 sourceBlockchainID => Message message) private _messages;

Next, update receiveTeleporterMessage to save the message into the mapping after it is received and verified that it's sent from Teleporter. At the end of that function, ABI decode the message bytes into a string, and emit the ReceiveMessage event.

        // Store the message.
        string memory messageString = abi.decode(message, (string));
        _messages[sourceBlockchainID] = Message(
            originSenderAddress,
            messageString
        );
        emit ReceiveMessage(
            sourceBlockchainID,
            originSenderAddress,
            messageString
        );

Next, add a function to the contract called getCurrentMessage that allows users or contracts to easily query the contract for the latest message sent by a specified chain.

    /**
     * @dev Check the current message from another chain.
     */
    function getCurrentMessage(
        bytes32 sourceBlockchainID
    ) external view returns (address, string memory) {
        Message memory messageInfo = _messages[sourceBlockchainID];
        return (messageInfo.sender, messageInfo.message);
    }

Step 5: Upgrade Support​

At this point, the contract is now fully usable, and can be used to send arbitrary string data between chains. However, there are a few more modifications that need to be made to support upgrades to TeleporterMessenger. For a more in-depth explanation of how to support upgrades, see the Upgrades README here.

The first change to make is to inherit from TeleporterOwnerUpgradeable instead of ITeleporterReceiver. TeleporterOwnerUpgradeable integrates with the TeleporterRegistry via TeleporterUpgradeable to easily utilize the latest TeleporterMessenger implementation. TeleporterOwnerUpgradeable also ensures that only an admin address for managing Teleporter versions, specified by the constructor argument teleporterManager, is able to upgrade the TeleporterMessenger implementation used by the contract.

To start, replace the import for ITeleporterReceiver with TeleporterOwnerUpgradeable:

- import {ITeleporterReceiver} from "@teleporter/ITeleporterReceiver.sol";
+ import {TeleporterOwnerUpgradeable} from "@teleporter/upgrades/TeleporterOwnerUpgradeable.sol";

Also, replace the contract declaration to inherit from TeleporterOwnerUpgradeable instead of ITeleporterReceiver:

contract MyExampleCrossChainMessenger is
     ReentrancyGuard,
-    ITeleporterReceiver
+    TeleporterOwnerUpgradeable
{

Next, update the constructor to invoke the TeleporterOwnerUpgradeable constructor.

-     constructor(address teleporterMessengerAddress) {
-         teleporterMessenger = ITeleporterMessenger(teleporterMessengerAddress);
-     }
+     constructor(
+         address teleporterRegistryAddress,
+         address teleporterManager
+     ) TeleporterOwnerUpgradeable(teleporterRegistryAddress, teleporterManager) {}

Then, remove the teleporterMessenger state variable:

-     ITeleporterMessenger public immutable teleporterMessenger;

And at the beginning of sendMessage() add a call to get the latest ITeleporterMessenger implementation from TeleporterRegistry:

        ITeleporterMessenger teleporterMessenger = teleporterRegistry.getLatestTeleporter();

And finally, change receiveTeleporterMessage to _receiveTeleporterMessage, mark it as internal override, and change the data location of its message parameter to memory. It's also safe to remove the check against teleporterMessenger in _receiveTeleporterMessage, since that same check is handled in TeleporterOwnerUpgradeable's receiveTeleporterMessage function.

-     function receiveTeleporterMessage(
+     function _receiveTeleporterMessage(
          bytes32 sourceBlockchainID,
          address originSenderAddress,
-         bytes calldata message
+         bytes memory message
-     ) external {
+     ) internal override {
-          // Only the Teleporter receiver can deliver a message.
-          require(msg.sender == address(teleporterMessenger), "Unauthorized.");

MyExampleCrossChainMessenger is now a working cross-chain dApp built on top of Teleporter! Full example here.

Step 6: Testing​

For testing, scripts/local/e2e_test.sh sets up a local test environment consisting of three avalanche-l1s deployed with Teleporter, and a lightweight inline relayer implementation to facilitate cross chain message delivery. An end-to-end test for ExampleCrossChainMessenger is included in tests/flows/example_messenger.go, which performs the following:

1. Deploys the ExampleERC20 token to avalanche-l1 A.
2. Deploys ExampleCrossChainMessenger to both avalanche-l1s A and B.
3. Approves the cross-chain messenger on avalanche-l1 A to spend ERC20 tokens from the default address.
4. Sends "Hello, world!" from avalanche-l1 A to avalanche-l1 B's cross-chain messenger to receive.
5. Calls getCurrentMessage on avalanche-l1 B to make sure the right message and sender are received.

To run this test against the newly created MyExampleCrossChainMessenger, first generate the ABI Go bindings by running ./scripts/abi_bindings.sh --contract MyExampleCrossChainMessenger from the root of this repository. Then, add to the generated Go package the SendMessageRequiredGas constant, which is required by the tests, in a new file abi-bindings/go/CrossChainApplications/MyExampleCrossChainMessenger/MyExampleCrossChainMessenger/constants.go:

package myexamplecrosschainmessenger
 
import "math/big"
 
var SendMessageRequiredGas = big.NewInt(300000)

Next, modify tests/utils/utils.go, which is used by tests/flows/example_messenger.go, to use the ABI bindings for MyExampleCrossChainMessenger instead of ExampleCrossChainMessenger. First replace the import:

-       examplecrosschainmessenger "github.com/ava-labs/teleporter/abi-bindings/go/CrossChainApplications/examples/ExampleMessenger/ExampleCrossChainMessenger"
+       myexamplecrosschainmessenger "github.com/ava-labs/teleporter/abi-bindings/go/CrossChainApplications/MyExampleCrossChainMessenger/MyExampleCrossChainMessenger"

Then, in that same utils.go, replace all instances of to examplecrosschainmessenger with myexamplecrosschainmessenger and all instances of ExampleCrossChainMessenger with MyExampleCrossChainMessenger.

Finally, from the root of the repository, invoke the tests with an extra bit of configuration that tells the Ginkgo test framework to focus only on the tests of this example contract (excluding all of the broader tests of Teleporter):

GINKGO_FOCUS="Example cross chain messenger" scripts/local/e2e_test.sh