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Gas Free EVM Endpoint

In this tutorial, we will learn how to set up a gas free EVM endpoint for your backend service, all transactions sent through this endpoint will not be charged for gas fees from the transaction sender's account.

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This method only works in situations where you can control the RPC node used to send transactions, such as your backend and some types of embedded wallets. It will not work on the frontend with traditional wallets, such as MetaMask.

Context​

In Using Flow EVM, we know that we can use the RPC endpoint to send EVM transactions to the Flow EVM. The default RPC endpoint is actually the EVM Gateway.

How does the EVM Gateway work?​

EVM transactions sent through the EVM Gateway are wrapped in a Flow Cadence transaction and sent to the Flow network. The Flow EVM Gateway's service account is the payer of that Cadence transaction.

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In Cadence, the proposer, payer, and signer of a transaction are separate, natively.

For EVM transactions inside this Cadence transaction, the transaction fees of the EVM transaction sent from the sender's account and applied to the EVM Gateway's service account. The EVM Gateway pays the Cadence transaction fee, but is reimbursed via the EVM transaction's gas fee, which is directed to its own EVM address as the coinbase.

Here is the key points of the Flow EVM Gateway:

  • The EVM Gateway works as a proxy to send the EVM transactions.
  • The EVM Gateway wraps the EVM transactions into native Flow Cadence transactions.
  • The EVM Gateway has a service account, which is the payer and sender of all native transactions sent through the EVM Gateway.
  • The sender of the EVM transaction is the fee payer of its EVM transaction to the EVM Gateway's service account.

Why we need a gas free EVM endpoint?​

From the Flow transaction model, we know that there is actually a Fee Payer role in native Flow transactions. When the transaction is executed, the fees for the transaction are entirely borne by the Fee Payer role. However, for EVM, the transaction model doesn't separate the signer from the payer. Therefore, when we execute transactions on the EVM, the transaction fees must be covered by the sender of the transaction.

So here is the problem - if we want to send a EVM transaction through the default EVM Gateway, the sender's account has enough balance to cover the transaction fees. However, in some scenarios, developers may prefer to sponsor the transaction fees to create a more user-friendly experience, just can be done in native Cadence transactions, or paymaster solutions on other networks.

Here we provide a solution - you can set up a sponsored transaction EVM endpoint for your backend service and all transactions sent through this endpoint will be sponsored by your account.

Who needs the gas free EVM endpoint?​

Here are several typical business scenarios suitable for its use:

  • Centralized exchanges that wish to improve internal transaction performance for users' deposits and withdrawals.
  • Apps with embedded wallets using social login that don't want to require users to deposit money in a new wallet before they're able to interact with the app.
  • Games needing to complete onchain tasks to onboard new players who wish to try the game.
  • Backends creating and managing large amounts of wallets on behalf of their users.
  • Businesses that wish to provide sponsored transactions as a service.

As long as you can ensure that all your users' transactions are sent through a custom RPC endpoint, you can set up a Gas Free EVM Gateway to enable gas-free transactions for your users.

How to set up a gas free EVM endpoint?​

Mosts of the tasks you need to complete are in the guide to set up your own Custom EVM Gateway. A few highlights:

  1. You need to prepare a Service Account for the EVM Gateway to cover all transaction fees. Please refer to Account Creation for more details.

  2. You need to add enough identical keys to the Service Account to support the concurrent signing of EVM transactions. This is very important, because the EVM Gateway will use the Service Account to pay for the gas fees of the EVM transactions, so the Service Account needs to have enough keys to support the concurrent signing of EVM transactions. Please refer to Account and Key Management for more details.

  3. You need to set the correct environment variables for the EVM Gateway to enable the gas-free feature, adjust the environment variables in the Run the gateway section as follows:

  • COINBASE: The address is used to accept EVM transaction fees. In this case, there won't be fees to accept because all fees will be covered by the service account. But you need to set it with a valid address to ensure the EVM Gateway can start.
  • COA_ADDRESS: This is the service account address, which will be used to pay for the gas fees of the EVM transactions. Please input the address of the Service Account you created in step 1, but without the 0x prefix.
    • Fund this address to cover transaction fees.
  • COA_KEY: You need to set the private key of the Service Account you created in step 1.
  • GAS_PRICE: Critical: set this to 0 to ensure the linked service account will pay for transactions on users behalf.
  1. Please follow the full guide of Custom EVM Gateway to complete the EVM gateway setup, then you will get a custom RPC endpoint which will sponsor 100% of the gas fees for any EVM transaction sent through it.

Conclusion​

In this tutorial, we've explored how to set up a gas-free EVM endpoint for your backend service. This solution provides several significant benefits:

  1. Enhanced User Experience: Users can execute EVM transactions without worrying about gas fees, making the platform more accessible and user-friendly.
  2. Business Flexibility: Service providers can cover transaction costs on behalf of their users.
  3. Cost Management: By centralizing gas fee payments through a service account, businesses can better manage and control their transaction costs.

The implementation requires careful setup of a service account with sufficient keys for concurrent transactions and proper configuration of the EVM Gateway environment variables. While this solution requires more initial setup compared to using the default EVM Gateway, the benefits of providing a gas-free experience to users can significantly enhance your platform's usability and adoption.

Remember that this solution is most effective when you can ensure all user transactions are routed through your custom RPC endpoint. This makes it particularly suitable for centralized services and applications where you have control over the transaction routing.

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