EIP-1167: Minimal Proxy Contract

https://solidity-by-example.org/app/minimal-proxy/

EIP-1167, which is also referred to as the minimal proxy contract, is a commonly used solidity pattern for cheaply creating proxy clones.

Why a minimal proxy?

• For instances when you need to deploy the same contract multiple times.

• The initialization data could be different for each individual contract, but the code would be the same.

• Because deploying large contracts can be quite expensive, instead of deploying a huge contract multiple times, we just deploy a super-cheap minimal proxy contract that points to the huge contract already on chain.

The cost to this pattern is that every execution has a delegate call overhead.

What does minimal proxy do?

• All the proxy/clone contract will do is delegate all calls to the implementation

• State is updated on the proxy, while the master contract holds the necessary logic

• Several clones can point to the same implementation contract. Clones cannot be upgraded.

• The address of the implementation contract is stored in the bytecode. This saves gas compared to storage and prevents the clone from pointing to another implementation.

EIP 1167 has nothing to do with upgradeability nor tries to replace it.

How does it look like?

EVM bytecode

• The dummy address 0xbebebebebebebebebebebebebebebebebebebebe is replaced with the implementation contract address.

• bytecode is 55 bytes in size, or 0x37

Essentially, the minimal proxy contract looks like this in pseudocode:

Implementation in Solidity

• we have to create the proxy/clone contract using the 55 byte EVM bytecode above, after updating it with the implementation contract address.

• this is done with assembly in solidity

• With assembly, we will update 0xbebebe... with the implementation address, load the entire bytes string into memory and execute it with the create opcode

• We see this process in: https://solidity-by-example.org/app/minimal-proxy/

Implementation with initialization

• We may want to have different initialization for clones.

• For example, if we were cloning an ERC20 token, every clone would have the same totalSupply, which might not be desirable.

• The initializer pattern allows us to deploy the clone as if it has a constructor that takes arguments

Process:

1. Create an implementation contract

2. Clone the contract with the EIP-1167 standard

3. Deploy the clone and call the initialization function, which can only be called once

contractImplementationContract{
    boolprivate isInitialized;      

    //initializer function that will be called once, during deployment.
    function initializer() external {              
        require(!isInitialized);
        // do stuff with state vars
        isInitialized =true;     
    }          
    
    // rest of the implementation functions go here 
}

• The initializer function has to be included in both the master as well as clone contracts.

• With this function, we can initialize state variables differently, as per use case.

• Deployed example: https://goerli.etherscan.io/address/0x8c7a30C5e64a088e1b03E2012E9ff42398AD0DA8#code

This restriction to only call once is necessary, or someone might alter the critical parameter we set after deployment, such as changing the total supply.

MinimalProxyFactory

contract MinimalProxyFactory {
    
    address[] public proxies;

    function deployClone(address _implementationContract) external returns (address) {
        // convert the address to 20 bytes
        bytes20 implementationContractInBytes = bytes20(_implementationContract);
        //address to assign a cloned proxy
        address proxy;
        
    
        // as stated earlier, the minimal proxy has this bytecode
        // <3d602d80600a3d3981f3363d3d373d3d3d363d73><address of implementation contract><5af43d82803e903d91602b57fd5bf3>

        // <3d602d80600a3d3981f3> == creation code which copies runtime code into memory and deploys it

        // <363d3d373d3d3d363d73> <address of implementation contract> <5af43d82803e903d91602b57fd5bf3> == runtime code that makes a delegatecall to the implentation contract
 

        assembly {
            /*
            reads the 32 bytes of memory starting at the pointer stored in 0x40
            In solidity, the 0x40 slot in memory is special: it contains the "free memory pointer"
            which points to the end of the currently allocated memory.
            */
            let clone := mload(0x40)
            // store 32 bytes to memory starting at "clone"
            mstore(
                clone,
                0x3d602d80600a3d3981f3363d3d373d3d3d363d73000000000000000000000000
            )

            /*
              |              20 bytes                |
            0x3d602d80600a3d3981f3363d3d373d3d3d363d73000000000000000000000000
                                                      ^
                                                      pointer
            */
            // store 32 bytes to memory starting at "clone" + 20 bytes
            // 0x14 = 20
            mstore(add(clone, 0x14), implementationContractInBytes)

            /*
              |               20 bytes               |                 20 bytes              |
            0x3d602d80600a3d3981f3363d3d373d3d3d363d73bebebebebebebebebebebebebebebebebebebebe
                                                                                              ^
                                                                                              pointer
            */
            // store 32 bytes to memory starting at "clone" + 40 bytes
            // 0x28 = 40
            mstore(
                add(clone, 0x28),
                0x5af43d82803e903d91602b57fd5bf30000000000000000000000000000000000
            )

            /*
            |                 20 bytes                  |          20 bytes          |           15 bytes          |
            0x3d602d80600a3d3981f3363d3d373d3d3d363d73b<implementationContractInBytes>5af43d82803e903d91602b57fd5bf3 == 45 bytes in total
            */
            
            
            // create a new contract
            // send 0 Ether
            // code starts at the pointer stored in "clone"
            // code size == 0x37 (55 bytes)
            proxy := create(0, clone, 0x37)
        }
        
        // Call initialization
        ImplementationContract(proxy).initializer();
        proxies.push(proxy);
        return proxy;
    }
}

• array of addresses, proxies, is optional. Serves to keep a list of clones deployed.

Links

• https://www.youtube.com/watch?v=9xqoK2nKkM4
• https://www.rareskills.io/post/eip-1167-minimal-proxy-standard-with-initialization-clone-pattern
• https://solidity-by-example.org/app/minimal-proxy/
• https://blog.openzeppelin.com/deep-dive-into-the-minimal-proxy-contract/