Building an EVM-based dApp

deBridge is DeFi's internet of liquidity, enabling any amount of cross-chain interactions (bridging, value transfers, ext calls, etc) in a single transaction. Due to the ability to simultaneously send messages and value deBridge acts as a unified framework for all cross-chain needs and is capable of interconnecting any smart contract on any supported blockchain.

This guide is a great starting point that covers all necessary topics to get you started building cross-chain interactions on top of the deBridge protocol and its' infrastructure.

Build your first deApp

Let's consider the following idea of the conceptual dApp consisting of two smart contacts: a Counter smart contract residing in one chain, where the integer property (counter) is stored and can be incremented by a call initiated by and only by the Incrementor contract from another chain. In other words, when we perform some action against the Incrementor contract, it initiates a cross-chain transaction (a submission, in terms of the deBridge protocol) which is being started on one chain, relayed to, and then executed on another chain; during this cross-chain transaction, a Counter contract is called. The following image presents a high-level overview of a cross-chain transaction we are going to achieve:

This document will guide you through building contracts, scripts and unit tests necessary to make this project happen. A self-contained source code of this dApp can be found on Github: debridge-finance/debridge-cross-chain-dapp-example.git.

Though hypothetical, this example may be used as a foundation for real-life cases, e.g. you may want to track the supply of your token issued across multiple chains on one chain, or send price feeds and trigger events or even actions to buy or sell, etc.

Making the `Counter` contract

Let's start with making the counter contract, responsible for keeping the integer property and accepting calls to increment it. Obviously, there should be a property and a method:

contract CrossChainCounter {
    uint256 public counter;

    function receiveIncrementCommand(uint8 _amount) external {
        counter = counter + _amount;
    }
}

Looks simple! However, we need to put some security restrictions on this method: first, we must ensure it can be called by the deBridgeGate smart contract only, and second, this call should occur only during the cross-chain transaction originating from the Incrementor smart contract at the specific address on another chain. How can this be achieved?

The cross-chain transaction is a message from the origin chain with a payload that includes (among other info) the packed address of the initiator (nativeSender) - i.e. an entity that actually initiated the transaction by calling the deBridgeGate contract on the origin chain. When the cross-chain transaction is being relayed, the deBridgeGate contract on the destination chain temporarily exposes the state of the transaction it currently handles via its properties and then calls a target smart contract (whose address is also a part of a payload) through its helper intermediary. It means that the contract on the target address may access this data.

To make this happen, the Counter must know the deBridgeGate contract's address on the current chain, so it is reasonable to inject it via a constructor:

contract CrossChainCounter {
    IDebridgeGate public deBridgeGate;

    constructor(address deBridgeGate_) {
        deBridgeGate = IDebridgeGate(deBridgeGate_);
    }
}

Then, we can start putting restrictions on the receiveIncrementCommand method using a modifier. The first obvious check we must perform is to ensure this method is called by the deBridgeGate's helper intermediary - a CallProxy contract responsible for performing actual calls (the deBridgeGate contract doesn't make calls directly for security considerations):

contract CrossChainCounter {
    modifier onlyCrossChainIncrementor {
        // caller must be CallProxy
        require(msg.sender == deBridgeGate.callProxy())

        // execute the rest
        _;
    }
}

It is implied by the protocol that such calls may occur only while deBridgeGate handles some cross-chain transactions relayed from another chain.

The not-so-obvious second check is related to our business logic: we want to ensure that a transaction is originating from the chain we know and from the contract we trust. To make such validation happen, we must preliminarily store the trusted address in the Counter contract, for example like this:

contract CrossChainCounter {
    uint256 trustedChain;
    bytes trustedCrossChainCaller;

    function addChainSupport(
        uint256 _trustedChain,
        address _trustedIncrementor
    ) external onlyAdmin {
        trustedChain = _trustedChain;
        trustedCrossChainCaller = abi.encodePacked(_trustedIncrementor);
    }
}

Note that we store the packed version of the caller's address (mind that trustedCrossChainCaller is defined as bytes rather than the address): this happens because the deBridgeGate smart contract stores the byte representation of the native sender address to ensure future compatibility with non-EVM chains (e.g. Solana).

Later, after you deploy the Incrementor contract and get its address, we may let the Counter contract know about it by calling the addChainSupport method.

As soon as the Counter contract starts storing the caller's address, the cross-chain transaction is allowed to originate from, we may reuse this data and add the additional validation logic to the modifier:

contract CrossChainCounter {
    modifier onlyCrossChainIncrementor {
        // take the callProxy instance
        ICallProxy callProxy = ICallProxy(deBridgeGate.callProxy());

        // caller must be CallProxy
        require(address(callProxy) == msg.sender);

        // origin chain must be known
        require(callProxy.submissionChainIdFrom() == trustedChain);

        // native sender (initiator of the txn on the origin chain) must be trusted
        // Bytes can't be compared directly, so take the hashes of them
        require(
            keccak256(callProxy.submissionNativeSender())
            == keccak256(trustedCrossChainCaller)
        );

        // execute the rest
        _;
    }
}

That's it! Now if we apply the given modifier to the target receiveIncrementCommand method, it becomes properly protected from unauthorized calls and ready to receive commands from the trusted contract on another chain:

contract CrossChainCounter {
    function receiveIncrementCommand(uint8 _amount)
        external
        onlyCrossChainIncrementor // <-- mind the modifier applied
    {
        counter = counter + _amount;
    }
}

Making the `Incrementor` contract

Now after we have the Counter contract and know its interface, we may design the Incrementor contract, which is responsible for initiating the cross-chain call to Counter's receiveIncrementCommand(). First things first, we must let the Incrementor know where the Counter contract actually resides, so we inject it with the chain ID and address of the Counter contract, and we also specify the address of the deBridgeGate contract as well:

contract CrossChainIncrementor {
    IDebridgeGate deBridgeGate;
    uint256 counterResidenceChainID;
    address counterResidenceAddress;

    constructor(
        address deBridgeGate_,
        uint256 counterResidenceChainID_,
        address counterResidenceAddress_
    ) {
        deBridgeGate = IDebridgeGate(deBridgeGate_);
        counterResidenceChainID = counterResidenceChainID_
        counterResidenceAddress = counterResidenceAddress_;
    }
}

For the sake of simplicity, let's assume that anyone may invoke the Incrementor, so let its interface be as simple as follows:

contract Incrementor {
    function increment(uint8 _amount) external payable {
        deBridgeGate.send{value: msg.value}(/* ... */)
    }
}

Crafting the deBridge submission (a cross-chain transaction) spins around the deBridgeGate's send() method — the only entry point to initiate a transaction. It accepts plenty of non-trivial variables and structs. Let's overview all of them to make our submission happen.

A protocol fee

Worth mentioning that the send() method is marked as payable (meaning that it accepts ether during a call) and it is necessary to bypass enough ether to cover the protocol fee (or global fixed native fee, according to the internal definition) taken in the native currency of the chain. How much? The fee varies from chain to chain: for example, at the time of writing the fee on Ethereum is 0.001 ETH and the fee on Polygon is 0.5 MATIC. Since fees can be changed by deBridge governance and are expected to be reduced as protocol scales, you are advised to retrieve the actual fee amount by reading deBridgeGate's globalFixedNativeFee property either on-chain:

uint protocolFee = deBridgeGate.globalFixedNativeFee;

or by making a call to the RPC node:

// ethers.js
const protocolFee = await deBridgeGate.globalFixedNativeFee();

// or web3.js
const protocolFee = await deBridgeGate.methods.globalFixedNativeFee().call();

Then pass the retrieved amount of ether to the call:

deBridgeGate.send{value: protocolFee}(/* ... */);

Submission params

The gate accepts a variety of parameters through the SubmissionAutoParamsTo struct (see its definition), so it is important to understand each.

executionFee (or included gas) is the amount of the bridged asset that will be transferred to anyone who will deliver the message in the destination chain. In other words, this is a prepayment for potential gas expenses on the target chain, that will be transferred by the protocol to the address that claims the message. Anyone can run the keeper service to deliver messages and earn the executionFee. This is an advanced topic that runs out of the scope of this document, so for the sake of simplicity just set this value to zero.

flags is a bitmask of toggles affecting the behavior of the gate. The following flags are important to be set in our case:

REVERT_IF_EXTERNAL_FAIL tells the CallProxy to revert the whole claim transaction in case the call to the receiver address (the callee contract on the destination chain; it is the Counter contract in our case) fails. Select the proper behavior wisely, ensuring it is aligned with the design of the callee contract: for example, the contract may fail deliberately and irretrievably so it may be reasonable to handle this call gracefully and mark the whole cross-chain transaction as succeeded. Keep in mind that once the claim transaction succeeds it submissionId is marked as used, so you cannot replay the transaction on the destination chain.
PROXY_WITH_SENDER tells the CallProxy to expose the address that initiated the cross-chain transaction (submission) on the origin chain. Again, choose wisely: as for our case, the Counter contract expects this data, so we need to ensure it's presented on the destination chain.

A complete list of flags with their description can be found on EVM smart contract interfaces page

fallbackAddress is the address on the destination chain where the bridged funds will be transferred to in case the call to the receiver address fails AND REVERT_IF_EXTERNAL_FAIL is not set. Since we don't bridge any funds (only the calldata), this field is not very important though it is mandatory to set it. Mind that this address must be packed into bytes.

data is the field for the calldata to execute on the destination chain. Not a big deal if the contracts reside on different chains: we can encode the call using the interface of the contract to call. In our case, we can import the interface of the Counter (ICrossChainCounter) and use it with the encodeWithSelector to produce valid instructions aligned with the interface of Counter's receiveIncrementCommand method:

bytes memory counterCalldata = abi.encodeWithSelector(
    ICrossChainCounter.receiveIncrementCommand.selector,
    _amountToIncrementBy
);

Summing it up, here is the complete snippet that produces an autoParams struct with settings that suit our needs:

IDeBridgeGate.SubmissionAutoParamsTo memory autoParams;

autoParams.executionFee = _executionFee;

// Exposing nativeSender must be requested explicitly
// We request it bc of CrossChainCounter's onlyCrossChainIncrementor modifier
autoParams.flags = Flags.setFlag(
    autoParams.flags,
    Flags.PROXY_WITH_SENDER,
    true
);

// If something happens, we need to revert the transaction to avoid this call being lost
autoParams.flags = Flags.setFlag(
    autoParams.flags,
    Flags.REVERT_IF_EXTERNAL_FAIL,
    true
);

autoParams.data = abi.encodeWithSelector(
    ICrossChainCounter.receiveIncrementCommand.selector,
    _amountToIncrementBy
);

autoParams.fallbackAddress = abi.encodePacked(msg.sender);

Of course, you can craft the autoParams struct either on-chain (as in the example above) or off-chain using ethers.js or web3.js.

The `send()` args

The last major step towards successful submission is the understanding of args of the send() method.

_tokenAddress is the address of the ERC-20 token contract whose tokens you are willing to bridge additionally along with the calldata. If you are willing to bridge the native currency (e.g. ETH from Ethereum), use the zero address (address(0)).
_amount is the amount of tokens (of the contract specified in the first arg) you are willing to bridge. Dealing with bridged assets is an advanced topic which out of the scope of this document, so in this example, it is enough to set this arg to zero. The following things are worth mentioning: first, the gate cuts a small 0.1% fee off the bridged asset; second, if you bridge the native currency of the origin blockchain, you must not forget to supply an additional amount to cover the protocol fee, but not include it in this arg value; third, the aforementioned executionFee (included gas) is counted in the currency of this bridged asset, so its decimals must be in sync with this asset; fourth, ERC-20 tokens should not be transferred in/out explicitly, use allowance and safeTransferFrom instead.
_chainIdTo sets the destination chain ID. Consider looking at chainlist.org for known chain IDs, see the list of supported chains in our docs, or query deBridgeGate.getChainToConfig on-chain property for programmatic access to the list of chains supported by deBridge.
_receiver defines the address on the destination chain to receive bridged assets (if any) and be called by the CallProxy contract in case the call data is given. In the given example, we must set this arg to the address of the Counter smart contract.
_permit allows the caller to specify EIP-2612-compliant signed approval for the deBridgeGate contract to transfer the tokens specified in the first arg. Not applicable here.
_useAssetFee allows paying the protocol fee in the currency of the asset being bridged rather than the native currency of the blockchain. Not applicable here.
_referralCode is used to mark the submission with your own code, which will be used later.
_autoParams is the encoded autoParams struct we've crafted in the previous chapter.

If you integrate with or build applications on top of the deBridge infrastructure, make sure you specify your referral code that can be generated by pressing the WAGMI button at https://app.debridge.finance/. Governance may thank you later for being an early builder.

The list of args is enormously long due to the internal complexity and the wide range of features deBridge protocol provides, but, however, there are only three args you must care about right now: _chainIdTo, _receiver and _autoParams. The snippet that actually makes a call to the deBridgeGate contract may look like this:

deBridgeGate.send{value: _protocolFee}(
    address(0), // _tokenAddress, N/A
    0, // _amount, N/A
    counterResidenceChainID, // _chainIdTo
    abi.encodePacked(counterResidenceAddress), // _receiver
    "", // _permit, N/A
    true, // _useAssetFee, N/A
    0, // _referralCode, N/A
    abi.encode(autoParams) // _autoParams
);

Of course, this call may be crafted on-chain in your own contract or off-chain. After this call to deBridgeGate is made within a blockchain, the cross-chain transaction is being initiated.

Accompanying and finishing a submission

Consider reading the Lifecycle of a cross-chain call to get yourself familiar with how the cross-chain calls are handled.

After our smart contract (Incrementor in our case) submits a new cross-chain call, the deBridgeGate contract emits a Sent event containing all necessary details about the cross-chain call, including the submissionId — the global cross-chain identifier of such a call. The submissionId is the important thing to identify our submission, so we must capture it either by parsing the event manually or using deBridge SDK (deSDK) which does this action for us:

// find all submissions submitted in your transaction by its hash
// Obviously, a single transaction may contain multiple submissions:
// a contract may call deBridgeGate.send() multiple times, e.g. to submit data
// to different chains simultaneously - that's why Submission.findAll()
// returns an array of Submission objects
const submissions = await evm.Submission.findAll(transactionHash, context);

// take the first submission.
// DO YOUR OWN SANITY CHECKS TO ENSURE IT CONTAINS THE EXPECTED NUMBER OF SUBMISSIONS
const [submission] = submissions;

Checking the status of the submission

The submission gets accepted by the validators after a transaction (containing the cross-chain call has been submitted) receives 12 block confirmations (256 for the Polygon chain). This is a required transaction finality validators are waiting for to avoid the consequences of the network divergence. You can monitor the finality of the transaction in a few ways: either using the preferred library (web3.js, ethers.js, or whatever) or with a little help of deSDK:

// check if submission if confirmed: validator nodes wait a specific block
// confirmations before sign the message. Currently, 12 blocks is expected
// for most supported EVM chains (256 for Polygon).
const isConfirmed = await submission.hasRequiredBlockConfirmations();

The number of block confirmations required for sent messages can be found in Fees and Supported Chains section

Pulling signatures

After the origin transaction receives enough block confirmations, we may start pulling the signatures. Currently, signatures are available through the deBridge API: you can query them manually by calling the API directly, or use deSDK which additionally checks if enough signatures have been published already:

if (isConfirmed) {
    const claim = await submission.toEVMClaim(evmDestinationContext);

    // check if claim has been signed by enough validators
    await isSigned = await claim.isSigned();
}

Crafting transactions to claiming a submission

After the submission has been confirmed and signed by enough validators, it's time to craft a claiming transaction that will land down the submission and execute the message on the destination chain.

To claim a submission, a call to the deBridgeGate.claim() method on the destination chain must be crafted using the data from taken from various sources, which isn't an easy task, so there is deSDK which takes the burden of data preparation:

// the resulting tuple of args to be then passed to the deBridgeGate.claim() method
const claimArgs = await claim.getEncodedArgs();

// e.g. using ethers.js:
// await deBridgeGate.claim(...claimArgs, { gasLimit: 8_000_000 });

Then you can pass the args to the deBridgeGate.claim() method, and finally sign and broadcast your transaction and wait for the Claimed event. This will indicate a successful submission completion.

Keep in mind that estimating gas for such transaction may have undesirable pitfalls that we have covered in our small research - this may be the case if you turn off the REVERT_IF_EXTERNAL_FAIL flag. We recommend using professional transaction simulation services (offered by Tenderly or Blocknative) rather than calling your RPC's eth_estimateGas endpoint.

Build your first deApp

Making the Counter contract

Making the Incrementor contract

A protocol fee

Submission params

The send() args

Accompanying and finishing a submission

Checking the status of the submission

Pulling signatures

Crafting transactions to claiming a submission

Further reading

Making the `Counter` contract

Making the `Incrementor` contract

The `send()` args