Solidity Deep Dive

Transparent Proxy (OpenZeppelin):

• Proxy contract delegates all calls to implementation
• Admin can upgrade implementation address
• ifAdmin modifier prevents admin from accidentally calling implementation functions
• Storage lives in proxy, logic lives in implementation

• Upgrade logic lives in the implementation, not the proxy
• Lighter proxy contract (cheaper deployment)
• Risk: if implementation doesn't include upgrade function, contract becomes non-upgradeable forever
• Preferred by OpenZeppelin for new projects

• Multiple proxies point to a single beacon
• Beacon stores implementation address
• Upgrade beacon → all proxies upgrade simultaneously
• Perfect for factory patterns (many instances of same logic)

// BAD: Implementation V1
contract V1 {
    uint256 public value; // slot 0
}
// BAD: Implementation V2
contract V2 {
    address public owner; // slot 0 — COLLISION!
    uint256 public value; // slot 1
}

// GOOD: Always append new variables
contract V2 {
    uint256 public value; // slot 0 (preserved)
    address public owner; // slot 1 (new)
}

Use EIP-1967 storage slots for proxy admin/implementation to avoid collision with implementation storage.