This guide uses the Polygon Testnet (Mumbai) which in itself is mapped to the Goerli Network to demonstrate asset transfer between the two blockchains. It's important to note that for the purposes of this tutorial, you should use a proxy address whenever is possible. This is because the implementation contract address is liable to change when a new update is added to the contract code. The proxy never changes and the proxy redirects all the incoming calls to the latest implementation. If you use the proxy address, you won't need to worry about any changes happening on the implementation contract before you're ready.
For example, please use the RootChainManagerProxy address for interactions instead of the RootChainManager address. Deployment details like the PoS contract addresses, ABI, Test Token Addresses can be found here
Mapping your assets is a necessary step for integrating the PoS bridge on your application so if you haven't done it, please submit a mapping request here. For the purposes of this tutorial, the team has deployed test tokens and mapped them to the PoS bridge. Request the asset you want to use on the faucet and if the test tokens are unavailable, reach out to the team on Discord. We'll make sure to reply you immediately.
High Level Flow
- Call the depositEtherFor function on the RootChainManager and send the required Ether.
Burn your token on the Polygon chain. This involves sending your token to Polygon's zero address so the token is no longer usable.
Call exit function on RootChainManager to submit the transaction proof of burn hash. This call can be made after the checkpoint is submitted for the block containing burn transaction.
Details and an Explanation of Terms
ETH can be deposited to the Polygon chain by calling depositEtherFor on the RootChainManager contract. The Polygon PoS client exposes the depositEtherForUser method to make this call. Since ETH is deposited as ERC20 on the Polygon chain, withdrawing it follows the same process as ERC20 tokens.
The depositEtherForUser method looks like this:
Sidenote: Deposits from Ethereum to Polygon happen using the state sync mechanism and this takes about 5-7 minutes. After waiting for this time interval, it is recommended to check the balance using web3.js/matic.js library or using Metamask. The explorer will show the balance only if at least one asset transfer has happened on the child chain. This link explains how to track the deposit events.
To burn the tokens and engage the withdrawal process, please call the withdraw function of the MaticWETH contract. Since Ether is an ERC20 token on the Polygon chain, using the burnERC20 method that the Polygon PoS client exposes kickstarts the process.
The burnERC20 method looks like this
Make sure you store the transaction hash returned from this call because you will use this while generating burn proof.
Once the checkpoint has been submitted for the block containing burn transaction, please call the exit function of the RootChainManager contract and submit the proof of burn. Upon submitting valid proof, tokens are transferred to the user. The Polygon PoS client exposes the exitERC20 method to make this call. This function can be called only after the checkpoint is included in the main chain. The checkpoint inclusion can be tracked by following this guide.
The exitERC20 method looks like this