Proxy Pattern

The most common design pattern for upgradable contracts is the Proxy Pattern. This involves separating the contract’s data (storage) and logic (code).

Normal contract (Storage)

Proxy contract

Basic example

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

contract StorageProxy {
    uint public num;
    address implementation;

    constructor(address _implementation) {
        num = 0;
        implementation = _implementation;
    }

    function setNum(uint _num) public {
        (bool success, ) = implementation.delegatecall(
            abi.encodeWithSignature("setNum(uint256)", _num)
        );
        require(success, "Error while delegating call");
    }

    function setImplementation(address _implementation) public {
        implementation = _implementation;
    }

}

contract Implementationv1 {
    uint public num;

    function setNum(uint _num) public {
        num = _num;
    }
}

contract Implementationv2 {
    uint public num;

    function setNum(uint _num) public {
        num = _num * 2;
    }
}

contract Implementationv3 {
    uint public num;

    function setNum(uint _num) public {
        num = _num * 3;
    }
}