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MATIC to Ethereum

The mechanism for transferring data from Polygon PoS to Ethereum differs from the process of transferring data from Ethereum to Polygon PoS. Validators create checkpoint transactions on the Ethereum chain to facilitate this transfer. These checkpoints serve as periodic summaries of the PoS chain’s state, ensuring data integrity and consistency when moving data back to Ethereum. The flow of this process is briefly described below.

  1. A transaction is created on Polygon PoS. It is crucial to emit an event and ensure that the event logs include the data intended for transfer to Ethereum. This process is essential for tracking and verifying the data transfer, as the event logs serve as a reliable record that can be referenced on the Ethereum network.
  2. Within a time frame of approximately 10 to 30 minutes, this transaction is check-pointed on the Ethereum chain by the validators.
  3. Once checkpointing is complete, the hash of the transaction created on PoS can be submitted as a proof on the RootChainManager contract on the Ethereum chain. This contract validates the transaction, verifies whether this transaction is included in the checkpoint, and finally decodes the event logs from this transaction.
  4. The decoded event log data can then be used to perform changes on the root contract deployed on the Ethereum chain.
  5. We use a predicate contract which is a special type of contract that can be only triggered by the RootChainManager contract to ensure the state update on Ethereum is secure. This architecture ensures that the state changes on Ethereum happens only when the transaction on Polygon is check pointed and verified on the Ethereum chain by the RootChainManager contract.

Overview

  • A transaction is executed on the child contract deployed on the Polygon chain.
  • An event is also emitted in this transaction. The parameters of this event includes the data which has to be transferred from Polygon PoS to Ethereum.
  • The validators on the PoS network pick up this transaction in a specific interval of time( probably 10-30 mins), validate them and add them to the checkpoint on Ethereum.
  • A checkpoint transaction is created on the RootChain contract and the checkpoint inclusion can be checked using this script
  • Once the checkpoint addition is completed, the matic.js library can be used to call the exit function of the RootChainManager contract. Here’s an example.
  • Running the script, verifies the inclusion of the Polygon transaction hash on Ethereum chain, and then in turn calls the exitToken function of the predicate contract.

Securing state changes

The important thing to note is that the verification of the transaction hash from Polygon PoS and triggering the predicate contract happens in a single transaction, thus ensuring security of any state change in the root contract.

Implementation

This is a simple demonstration of how data can be transferred from Polygon PoS to Ethereum. This tutorial shows an example of transferring a uint256 value across the chain. But you can transfer any type of data. However, it is necessary to encode the data in bytes and then emit it from the child contract, which can finally be decoded by the root contract.

First, create the root chain and child chain contract. Ensure that the function that does the state change also emits an event. This event must include the data to be transferred as one of its parameters. A sample format of how the child and root contract must look like is given below. This is a very simple contract that has a data variable whose value is set by using a setData function. Calling the setData function emits the Data event. Rest of the things in the contract will be explained in the upcoming sections of this tutorial.

Child contract
contract Child {

    event Data(address indexed from, bytes bytes_data);

    uint256 public data;

    function setData(bytes memory bytes_data) public {
     data = abi.decode(bytes_data,(uint256));
     emit Data(msg.sender,bytes_data);
    }

}

Pass this 0x1470E07a6dD1D11eAE439Acaa6971C941C9EF48f as the value for _predicate in the root contract constructor.

Root contract
contract Root {

    address public predicate;
    constructor(address _predicate) public{
        predicate=_predicate;
    }

   modifier onlyPredicate() {
        require(msg.sender == predicate);
        _;
    }

    uint256 public data;

    function setData(bytes memory bytes_data) public onlyPredicate{
        data = abi.decode(bytes_data,(uint256));
    }

}

Once the child and root contract is deployed on the Polygon and Ethereum chain respectively, these contracts have to be mapped using the PoS bridge. This mapping ensures that a connection is maintained between these two contracts across the chains. For doing this mapping,the Polygon team can be reached on Discord.

One important thing to note is that in the root contract, there is a onlyPredicate modifier. It is recommended to use this modifier always because it ensures that only the predicate contract makes the state change on the root contract. The predicate contract is a special contract that triggers the root contract only when the transaction that happened on the Polygon PoS chain is verified by the RootChainManager on Ethereum chain. This ensures secure change of state on the root contract.

For testing the above implementation, we can create a transaction on the Polygon chain by calling the setData function of the child contract. We need to wait at this point for the checkpoint to be completed. The checkpoint inclusion can be checked using this script. Once checkpoint is completed, call the exit function of the RootChainManager using matic.js SDK.

const txHash =
  "0xc094de3b7abd29f23a23549d9484e9c6bddb2542e2cc0aa605221cb55548951c";

const logEventSignature =
  "0x93f3e547dcb3ce9c356bb293f12e44f70fc24105d675b782bd639333aab70df7";

const execute = async () => {
  try {
    const tx = await maticPOSClient.posRootChainManager.exit(
      txHash,
      logEventSignature
    );
    console.log(tx.transactionHash); // eslint-disable-line
  } catch (e) {
    console.error(e); // eslint-disable-line
  }
};

As shown in the above screenshot, the txHash is the transaction hash of the transaction that happened on the child contract deployed on Polygon chain.

The logEventSignature is the keccack-256 hash of the Data event. This is the same hash that we have included in the predicate contract. All the contract code used for this tutorial and the exit script can be found here

Once the exit script is completed, the root contract on Ethereum chain can be queried to verify if the value of the variable data that was set in child contract has also been reflected in the data variable of the root contract.

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