Discussion:
The ability to verify a transaction’s execution with only the post-execution state and the last state (pre-execution state) depends on what we want to verify and the specific details you require for validation. Lets break down what we can prove and what we can not prove with lightclients only having previous state and post state:
What We Can Prove:
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Balances and State Changes: we can prove that the transaction correctly changed the account balances and storage values from the last state to the post-execution state. This includes checking that the sender’s balance decreased by the correct amount, and the recipient’s balance increased as expected.
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Transaction Hash and Signature: We can verify that the transaction hash in the block matches the one provided in the transaction, and we can validate the transaction’s digital signature using the sender’s public key.
What We Cannot Prove:
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Contract Code Execution: We cannot directly prove that the contract method produced the expected output, consumed the expected amount of gas, or adhered to the contract’s internal logic without retaining the contract code. This limitation means you won’t be able to fully verify the correctness of contract execution, especially for complex smart contracts.
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Interaction with Other Contracts: If the transaction interacts with other contracts, we cannot fully validate those interactions, including the inputs and outputs of those interactions, without retaining the contract code for those other contracts.
While we can verify some aspects of a transaction’s execution with only the last state and the post-execution state (e.g., balances and basic transaction integrity), verifying more complex interactions and contract logic would require retaining the contract code. The proposal of partial state caching can be valuable for improving retrieval efficiency and reducing reliance on complex tree processes, but it may not fully address the need for contract code to verify all aspects of execution and interactions with other contracts.
Implementing partial state caching as a proposal should not inherently break the concept of statelessness for light clients in the context of having only the post-execution state and the last state. Statelessness in Ethereum refers to clients, typically light clients, that do not store the entire state but rather access it as needed.
Partial state caching can be seen as a means to enhance the efficiency of stateless clients by allowing them to access specific state fragments more efficiently. However, it does not fundamentally change the stateless nature of these clients. Instead, it provides a mechanism to reduce the complexity and resource requirements associated with verifying transactions and smart contract interactions.
With partial state caching, a light client can still operate in a stateless manner by relying on external sources to access vital state fragments (the last state and the post-execution state) temporarily during transaction verification. This allows the client to verify transactions more efficiently without having to maintain a full state database.
By selectively caching relevant contract data during transaction execution, the light client can validate interactions with other contracts more effectively without retaining the entire contract codebase. This approach allows for a balance between efficient validation and the need for selective contract data access. This approach can reduce the gas fee bounded by state access and contract storage.
Partial caching can help address the challenge of validating interactions with other contracts when we have a portion of the contract code included in a recent block.
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Selective Caching:
- When a transaction interacts with another contract, the light client can selectively cache the contract code, storage, and state related to the contract being interacted with.
- This selective caching should occur dynamically based on the contracts accessed during transaction execution. Only cache what is needed for the specific interactions within the transaction.
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Cache Smart Contract Data:
- Store relevant contract code, storage values, and any state changes introduced by the contract’s methods during the transaction’s execution.
- This data should be cached temporarily and used for validation during and immediately after the transaction’s processing.
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Transaction Validation:
- During validation, the light client can compare the expected inputs and outputs of the contract interactions with the actual data cached during execution.
- It can verify that the inputs to the contract methods match the transaction’s input data and that the outputs produced by the contract match the expected results.
- Ensure that gas consumption is consistent with expectations.
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Dependency Handling:
- If the contract being interacted with depends on other contracts, cache data for those dependent contracts as well.
- Continue to selectively cache data for dependent contracts, allowing for a more complete validation of contract interactions.
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Trust Considerations:
- Maintain trust in the RPC node providing the cached data, as it is essential for the integrity of the validation process.
- Ensure that the cached data is provided by a trusted and reliable source to avoid potential manipulation.
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Data Cleanup:
- After the transaction’s validation and any necessary post-transaction processes, the cached data can be safely removed from the light client’s memory or database.