Abstract

It has been a significant challenge for developers attempting to create cloneable and upgradeable contracts on the Ethereum Virtual Machine (EVM). While ERC-2535 Diamonds and other existing proxy standards offer partial solutions, a comprehensive answer has remained elusive. Our proposal addresses this gap through the introduction of two main features.

Function-Level Upgradeability

In alignment with ERC-2535, this functionality permits the selective redirection of implementation contracts for individual function calls. This granular control over upgrades allows for modifications on a per-function basis. Moreover, segmenting implementation contracts by function helps mitigate the limitations posed by the contract size cap (24.576kB as of EVM version Shanghai or earlier).

Factory/Clone-Friendly & Simultaneous Upgradeability

Drawing on the Beacon model from ERC-1967, our method aims to streamline the process of cloning and updating Proxy contracts simultaneously. This approach is designed to maintain consistent functionality across different instances, each with its own state. Typically, proxies are limited to basic upgradeability features or follow the ERC-1167 standard. However, our solution combines both functionalities into a compact proxy.

Motivation

Smart contract development often encounters hurdles due to the inherent limitations of the Ethereum Virtual Machine (EVM), such as the contract size limit and stack depth. Additionally, addressing vulnerabilities in both the smart contract logic and its compiler are persistent issues. While there is a desire to minimize reliance on trusted third parties for upgradeability, introducing complex governance structures for upgrade management can significantly increase the workload for crypto DevOps, adding to the apprehension developers may feel towards advancing their projects. This apprehension can restrict the complexity and innovation within smart contract development. Our approach seeks to simplify smart contract programming, making it more accessible and enjoyable. It does so by clearly delineating DevOps concerns from business logic, thereby enhancing codebase clarity, facilitating audits, and allowing for more focused analysis through Language Model (LM) techniques, tailored to specific infrastructure and domain needs.

Use Cases

Over time, various smart contract design patterns have been proposed and utilized. This Upgradeable Clone Standard (UCS) is intended for scenarios where these existing patterns may not suffice. To clarify it, we define some key terms:

  • Contract-Level Upgradeability: One Proxy contract corresponds to one Implementation contract, responsible for all logic of the Proxy.
  • Function-Level Upgradeability: One Proxy contract corresponds to multiple Implementation contracts, basically each responsible for a specific function.
  • Factory: A contract that clones Proxies with a common Implementation(s). In the context of upgradeability, it allows for the simultaneous upgrade of these cloned Proxies.

Here are the use cases:

  1. For basic needs without Upgradeability or a Factory, Regular smart contract deployment suffices.
  2. When a Factory is needed without Upgradeability, ERC-1167 is suitable.
  3. For Contract-Level Upgradeability without a Factory, ERC-1822 can be used.
  4. For Contract-Level Upgradeability with a Factory, the Beacon from ERC-1967 is applicable.
  5. For Function-Level Upgradeability without a Factory, ERC-2535 is available.
  6. For Function-Level Upgradeability with a Factory, this Upgradeable Clone Standard is the ideal choice.

Fig. Use Cases

Specification

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119 and RFC 8174.

In the EVM, contract accounts are characterized by four primary fields: nonce, balance, code, and storage. This ERC’s architecture modularizes these functionalities into three distinct types of contracts, each serving a specific purpose when combined to represent a single account:

  1. Proxy Contract: Maintains the state of the contract account, such as nonce, balance, and storage. This contract delegatecalls to the Function Contract as registered in the Dictionary Contract, ensuring the state and logic are separated but effectively integrated.
  2. Dictionary Contract: Acts as a dispatcher that routes function calls based on their selectors to the appropriate Function Contract. It manages the dynamic aspects of contract behavior, facilitating function upgrades and dynamic addressing. By externalizing this contract from the Proxy Contract, it becomes factory/clone-friendly and supports simultaneous upgradeability.
  3. Function (Implementation) Contract: Implements the executable logic for function calls. When delegatecalled by the Proxy Contract, it performs the actual computations or logic as defined in the contract’s code.

This architecture not only aligns with the core attributes of an EVM contract account but also significantly enhances the modularity, upgradeability, and scalability of smart contracts by clarifying account state, function dispatching, and logic implementation.

Proxy Contract

This contract requests the Dictionary Contract to retrieve the associated Function Contract address based on its function selector, and then delegatecall to it.

Storage & Events

This contract SHOULD store the Dictionary Contract address in the storage slot 0x267691be3525af8a813d30db0c9e2bad08f63baecf6dceb85e2cf3676cff56f4, obtained as bytes32(uint256(keccak256('erc7546.proxy.dictionary')) - 1), in accordance with the method defined in ERC-1967. This ensures that the address is stored in a secure and predictable slot.

Changes to the Dictionary address SHOULD emit events. When such an event is emitted, it MUST use the signature:

event DictionaryUpgraded(address dictionary);

Functions

For every invocation made via CALL or STATICCALL, this contract MUST perform a delegatecall to the corresponding Function Contract address retrieved from the Dictionary Contract using the getImplementation(bytes4 functionSelector) function. This contract MUST also process the return value from this delegatecall to ensure the intended functionality is executed correctly. Furthermore, to avoid potential collisions with function selectors registered in the Dictionary Contract, the Proxy SHOULD NOT define any external functions.

Dictionary Contract

This contract manages a mapping of function selectors to corresponding Function Contract addresses. It uses this mapping to handle requests from the Proxy Contract.

Storage & Events

The Dictionary MUST maintain a mapping of function selectors to Function Contract addresses.

Changes to this mapping SHOULD be communicated through an event (or log).

event ImplementationUpgraded(bytes4 functionSelector, address implementation);

Functions

getImplementation

This contract MUST implement this function to return Function Implementation Contract address.

function getImplementation(bytes4 functionSelector) external view returns(address implementation);
setImplementation

This contract SHOULD implement this function to update or add new function selectors and their corresponding Function Implementation Contract addresses to the mapping.

function setImplementation(bytes4 functionSelector, address implementation) external;
supportsInterface

This contract is RECOMMENDED to implement the supportsInterface(bytes4 interfaceID) function defined in ERC-165 to indicate which interfaces are supported by the contracts referenced in the mapping.

supportsInterfaces

This contract is RECOMMENDED to implement the supportsInterfaces() to return a list of registered interfaceIDs.

function supportsInterfaces() public view returns (bytes4[] memory);

Function (Implementation) Contract

This contract acts as the logic implementation contract that the Proxy Contract delegatecalls and it’s address is registered with the function selector in the Dictionary Contract.

Storage & Events

This contract SHOULD NOT use its storage but SHOULD store to the Proxy Contract through delegatecall.

The Proxy Contract shares storage layout with several Function Contracts. For example, using sequential slot allocation starting from slot 0, as is the default compiler option, can lead to storage conflicts.

In order to prevent storage conflict, this contract MUST manage the storage layout properly. The matter of storage management techniques has been a subject of debate for years, both at the ERC level and the language level. However, there is still no definitive standard. Therefore, this ERC does not go into the specifics of storage management techniques.

It is RECOMMENDED to choose the storage management method that is considered most appropriate at the time.

For instance, the storage could be arranged according to useful storage layout patterns, such as ERC-7201.

Functions

This contract MUST have the same function selector registered in the Dictionary Contract. If not, the Proxy’s delegatecall will fail. So it is RECOMMENDED for each Function Contract to implement ERC-165’s supportsInterface(bytes4 interfaceID) to ensure that it correctly implements the function selector being registered when added to the Dictionary.

Rationale

Comparison with ERC-2535

While both this ERC and ERC-2535 offer Function-Level Upgradeability, there is a key distinction in their approaches. ERC-2535 maintains a mapping of implementation contracts (referred to as Facets in ERC-2535) within the Proxy itself. In contrast, this ERC stores the mapping in an external Dictionary Contract. This externalization of the mapping facilitates another significant feature of this standard: Factory/Clone-Friendly & Simultaneous Upgradeability. By separating the mapping from the Proxy, this design allows for easier cloning of contracts and their simultaneous upgrade, which is not as straightforward in the ERC-2535 framework.

Fig. Comparison with Diamond

Separating the Dictionary and Proxy contracts:

The separation of the Dictionary from the Proxy was driven by aligning with Factory/Clone-Friendly & Simultaneous Upgradeability.

To achieve this, the management functionality of Function Implementation Contract addresses were externalized as the Dictionary Contract instead of including them within the Proxy Contract, a concept akin to the Beacon Proxy approach.

If the functionality is within the Proxy Contract, each proxy requires its implementation to be upgraded. By externalizing this, a common implementation can be cloned and upgraded simultaneously.

Fig. Comparison with Beacon

Utilizing the mapping of function selectors and implementation addresses:

The utilization of the mapping of function selectors to corresponding Function Implementation Contract addresses of the Dictionary Contract by the Proxy Contract, followed by delegatecalling to the returned implementation address, aligns with Function-Level Upgradeability.

By adopting this approach, the Proxy emulates the behavior of possessing a set of Function Implementation Contracts registered within the Dictionary Contract. This specification closely resembles the pattern outlined in the Diamond Standard.

Reference Implementation

There are reference implementations and tests as a foundry project.

It includes the following contents:

Security Considerations

Delegation of Implementation Management

This pattern of delegating all implementations for every call to the Dictionary Contract relies on the assumption that the Dictionary Contract’s admin acts in good faith and does not introduce vulnerabilities through negligence.

You should not connect your proxy with the Dictionary Contract provided by an untrusted admin. Moreover, providing an option to switch to another Dictionary Contract managed by a different (or potentially more trustworthy) admin is recommended.

While it is possible to store the Dictionary Contract address in the code area (e.g., using Solidity’s immutable or constant), it SHOULD be designed with caution, considering the possibility that if the Dictionary Contract’s admin is not the same as the Proxy Contract’s admin, the ability to manipulate the implementation could be permanently lost.

Storage Conflict

As mentioned in the above Storage section. This design pattern involves multiple Function Implementation Contracts sharing a single Proxy Contract storage. Therefore, it’s important to take care for preventing storage conflicts by using the storage management method that is considered most appropriate at the time.

Mismatch Function Selector

The Dictionary Contract returns the Function Implementation Contract address based on the Proxy Contract’s invoked function selector.

If there is a mismatch between function selectors registered in the Dictionary Contract and those implemented in the Function Implementation Contract, the execution will fail. To prevent unexpected behavior, it’s recommended to check that the Function Implementation Contract includes the function selector (interface) being registered during the process for setting implementation address to the Dictionary Contract.

Handling of CALL and STATICCALL

The Proxy Contract is designed primarily to respond to CALL and STATICCALL opcodes. Should a DELEGATECALL be made to this Proxy Contract, it will attempt to request the Dictionary Contract for a corresponding implementation via the getImplementation(bytes4 functionSelector) function, using the stored Dictionary Contract address within its own storage. Although this action may not lead to the intended outcome if the calling contract’s storage layout does not align with expectations, it does not constitute a direct threat to the Proxy Contract itself. Developers are cautioned that invoking this Proxy Contract via DELEGATECALL could result in unexpected and potentially non-functional outcomes, making it an unsuitable method for interaction.

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