EIP-7504 introduces a new Ethereum standard for client-friendly, one-to-many proxy contracts. These dynamic smart contracts allow for more flexible upgrades by enabling the proxy contract to delegate calls to multiple implementation contracts. This proposal seeks to standardize how dynamic contracts expose their Application Binary Interface (ABI) for easier client interaction.
Motivation
Dynamic contracts, also known as one-to-many proxy contracts, have been introduced in previous Ethereum Improvement Proposals (EIPs), such as ERC-2535, which focuses on improving upgradeability through multiple implementation contracts.
EIP-7504 aims to promote the adoption of these dynamic contracts by simplifying their usage and making them more accessible for developers and clients.
The primary motivations for this proposal include:
Comprehensibility: Simplifying dynamic contracts' development and interaction process allows developers and users to understand and verify contract code easily.
Client Friendliness: Ensuring that clients, whether graphical interfaces or programmatic systems, can easily interact with dynamic contracts by defining a straightforward method for constructing and interpreting the contract's ABI.
Key Advantages of Dynamic Contracts
Dynamic contracts offer several significant benefits over traditional smart contracts:
Overcoming Code Size Limitations: Ethereum's EIP-170 limits the size of contract code, but dynamic contracts bypass this limitation by using multiple implementation contracts.
Granular Upgrades: Specific functions can be upgraded independently, improving flexibility in maintaining smart contracts.
Reuse of Deployed Contracts: Existing contracts can be reused as implementation contracts, reducing redundancy and increasing efficiency.
Router Interface
This standard's core is the Router interface, which all ERC-7504-compliant contracts must implement. The Router is responsible for delegating incoming function calls to the appropriate implementation contract based on the function selector.
The fallback() function is crucial in dynamic contracts, as it ensures that any call to the contract that does not match a predefined function is routed through the appropriate implementation via delegateCall.
RouterState Interface
In addition to the Router, dynamic contracts must implement the RouterState interface, which defines the structure and metadata for extensions (implementation contracts) that the router interacts with.
Structs and View Functions: The RouterState interface uses several key structures, including Extension, ExtensionMetadata, and ExtensionFunction, which describe the different implementation contracts and the functions they handle.
Key Concepts
Upgradeable Smart Contracts: A system where the proxy contract maintains the state and forwards function calls to the appropriate implementation contract using delegateCall. Upgrading a contract involves updating the mappings between function selectors and implementation contracts.
Router Contract: A proxy contract that determines which implementation contract to call based on the function selector (msg.sig). It enables granular updates by allowing each function to have its own specific implementation contract.
Extensions: Groupings of related functions and implementation contracts allow for modular and organized upgrades, with each of these extensions containing a set of functions, along with metadata that clients can use to interact with the contract.
Rationale
The primary role of the Router contract is to route function calls to the correct implementation contract. The fallback function in the router is triggered when a non-fixed function is called, using the function selector (msg.sig) to determine the appropriate contract. The dynamic contract can be used with or without being behind a minimal proxy contract, depending on the use case.
The standard includes two fixed functions—getImplementationForFunction and getAllExtensions—which provide the necessary information for clients to build and maintain the contract's ABI.
Security Considerations
Accidental Upgrades: To avoid unintentional changes, function selectors should be mapped to the zero address before being remapped to a new implementation. This two-step process helps ensure upgrades are deliberate.
Selfdestruct: Any function in a dynamic contract's extension must not include the selfdestruct operation, as this could lead to the destruction of the router or proxy contracts.
Conclusion
EIP-7504 proposes a structured, client-friendly standard for dynamic contracts that enables granular upgrades, scalability, and client comprehension. By using this standard, developers can build more flexible and robust smart contracts while ensuring easier interaction for end-users and developers.
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