Understanding Eth_createAccessList: Access Lists Explained

When working with Ethereum, especially with nodes like Hyperledger Besu, understanding the nuances of RPC calls is crucial for efficient development and debugging. One such call, eth_createAccessList, plays a vital role in optimizing transaction gas costs. However, a common point of confusion arises regarding its response, particularly when a transaction might revert. This article aims to clarify the eth_createAccessList response, ensuring you know what to expect and how to interpret the data, even in scenarios where the underlying transaction fails. We'll dive deep into what an access list is, why it's important, and how Besu handles the response, drawing insights from recent developments like the PR#9358 in the Hyperledger Besu repository.

What is an Access List and Why Does it Matter?

At its core, an access list is a way to pre-declare the state that a transaction will likely interact with. Before the introduction of EIP-2930 (which defined access lists), Ethereum transactions would implicitly read and write to any part of the state they touched. This meant that every piece of state accessed by a transaction incurred a gas cost. For complex transactions involving multiple contract calls or storage slots, this could lead to significant gas expenses. Moreover, if a transaction touched many state variables that were not actually used, gas was still spent. This is where the access list comes in, offering a more gas-efficient approach. By including an access list with your transaction, you are essentially telling the Ethereum node which storage slots and contract addresses your transaction intends to access. This allows the node to optimize its state management and, more importantly, enables the Ethereum Virtual Machine (EVM) to charge gas more precisely. The gas saved comes from a reduced cost for accessing pre-declared state variables. Instead of paying a higher gas fee for every state access, you pay a lower fee for pre-declared accesses and an additional, but usually smaller, fee for accessing items not on the list. This mechanism is particularly beneficial for smart contracts that have a predictable set of state interactions, as it can significantly reduce the overall transaction fee. The adoption of access lists is a key step towards making Ethereum transactions more affordable and predictable, especially as the network continues to scale and handle more complex decentralized applications (dApps).

How eth_createAccessList Works

The eth_createAccessList RPC method is designed to help you construct this access list. When you send a transaction's details (from, to, gas, gas price, value, data, nonce) to eth_createAccessList, the node simulates the execution of that transaction. During this simulation, it meticulously tracks all the storage slots and contract addresses that would be read from or written to. The primary goal of eth_createAccessList is to identify these state elements and compile them into the structured format required for an EIP-2930 compliant transaction. The process involves taking the proposed transaction parameters and executing them in a sandboxed environment. The EVM, during this simulation, logs every storage read, storage write, and contract creation/call. These logged interactions are then aggregated. Contract addresses that are interacted with are added to the accessList under the address key. For each contract address, a list of storageKeys is generated, which are the specific storage slot addresses (hashes) that were accessed. The eth_createAccessList method then returns this compiled list. This generated list can then be used when sending the actual transaction using eth_sendRawTransaction by including it in the transaction object. The simulation doesn't alter the blockchain state; it's purely a read-only operation for the purpose of analyzing state interactions. This makes it a safe and essential tool for developers looking to optimize their transaction costs before broadcasting them to the network. The accuracy of the generated access list is paramount, as an incomplete list could negate the gas savings, while an overly broad list might not provide the maximum benefit. Therefore, robust simulation and state tracking are key features of a reliable eth_createAccessList implementation. The method acts as a proactive gas optimization tool, allowing developers to anticipate and manage costs effectively.

Interpreting the eth_createAccessList Response: The Revert Scenario

Now, let's address the critical aspect: what happens if the transaction simulation within eth_createAccessList results in a revert? A common misconception is that if the transaction reverts, eth_createAccessList should also fail or return an error. However, this is not the case. The Hyperledger Besu team, following best practices and ensuring predictable behavior, has clarified that eth_createAccessList returns a success response (the access list) even if the transaction is reverted. This is a deliberate design choice. The purpose of eth_createAccessList is to determine the state accesses of a transaction, not its ultimate success or failure. Even a reverted transaction interacts with the state – it reads certain values, potentially attempts to write to others, and might even trigger contract calls that lead to the revert. These interactions still need to be accounted for in the access list for accurate gas calculation. Therefore, the RPC method diligently records all these state accesses during its simulation, regardless of the final outcome. The returned access list will accurately reflect all the storage slots and addresses that would have been accessed had the transaction not reverted. This ensures that even for transactions that are destined to fail, you can still generate an accurate access list, which is crucial for gas estimation and the overall gas mechanism defined by EIP-2930. The underlying principle is that the potential state access is what determines the access list, not the successful execution of the transaction. This behavior aligns with the broader goals of gas optimization, where anticipating all possible state interactions is key to predicting and managing costs. It also simplifies the developer experience, as they don't need to handle separate error conditions for eth_createAccessList based on transaction revert status. The response is always an access list, which is then used by the transaction sender to potentially reduce gas costs, irrespective of the transaction's execution outcome.

Besu's Implementation and Best Practices

Hyperledger Besu's implementation of eth_createAccessList adheres to this principle of returning a successful response with the access list, even for reverted transactions. This consistency is vital for developers building applications on Ethereum networks powered by Besu. The team's commitment to following the Ethereum Improvement Proposals (EIPs) ensures that Besu behaves predictably and in line with the wider Ethereum ecosystem. The decision to return the access list even on revert, as highlighted in Hyperledger Besu PR#9358, reinforces the method's sole purpose: identifying state interactions for gas optimization. It's not a transaction execution tool; it's a state analysis tool. For developers, this means you can confidently use eth_createAccessList to generate your access lists. If you receive a response, you have an access list. You should then use this list when constructing your actual transaction. The success or failure of the transaction itself will be determined when you send it using eth_sendRawTransaction and it's processed by the network. Besu's approach simplifies the development workflow by providing a single, consistent output for eth_createAccessList. This predictability helps in writing more robust and efficient smart contracts and dApps. Developers should always refer to the official Hyperledger Besu documentation for the most up-to-date information on RPC methods and their behavior. Understanding these details allows for better integration and optimization, contributing to a more efficient and cost-effective use of the Ethereum network. The commitment to EIP compliance means that Besu aims to provide a reliable foundation for decentralized applications.

Conclusion: A Reliable Tool for Gas Optimization

In summary, the eth_createAccessList RPC method is a powerful tool for optimizing Ethereum transaction gas costs by providing a mechanism to declare state interactions. It's crucial for developers to understand that this method's primary function is to simulate state access and compile an access list, not to guarantee transaction success. As clarified by Hyperledger Besu and its implementation, eth_createAccessList will always return a success response containing the generated access list, even if the simulated transaction ultimately reverts. This predictable behavior simplifies development and ensures that gas cost optimization is still possible for transactions that might fail execution. By leveraging eth_createAccessList, developers can proactively manage gas expenses, leading to more efficient and user-friendly decentralized applications. Always remember that the generated access list is a preparatory step; the actual transaction execution and its success or failure occur when the transaction is broadcast and processed by the network.

For more in-depth information on Ethereum development and best practices, I highly recommend exploring resources like the Ethereum Yellow Paper, which provides the formal specification of the Ethereum protocol, and the official Ethereum Foundation website for broader insights into the ecosystem.