I'm working on upgrading my Ethereum smart contract, which utilizes upgradable smart contract patterns. Currently, I have a struct named GovernorCountingSimpleStorage with existing fields, and I want to add a new field to this struct in my V2 contract. My concern is maintaining upgradeability and ensuring compatibility with existing deployments.

Here's the relevant code snippet from my V1 contract:

/// @custom:storage-location erc7201:openzeppelin.storage.GovernorCountingSimple
struct GovernorCountingSimpleStorage {
  mapping(uint256 => ProposalVote) _proposalVotes;
  // mapping to store that a user has voted at least one time
  mapping(address => bool) _hasVotedOnce;

// keccak256(abi.encode(uint256(keccak256("openzeppelin.storage.GovernorCountingSimple")) - 1)) & ~bytes32(uint256(0xff))
bytes32 private constant GovernorCountingSimpleStorageLocation =

function _getGovernorCountingSimpleStorage() private pure returns (GovernorCountingSimpleStorage storage $) {
  assembly {
    $.slot := GovernorCountingSimpleStorageLocation

Now, I want to add a new field inside GovernorCountingSimpleStorage in my V2 contract and ensure that the upgrade process is safe and doesn't disrupt existing data. Can I just add a new variable inside GovernorCountingSimpleStorage? Or do I need to create a new storage for the new variables? Should I reserve some slot inside the struct to allow future upgrades?

Is there any good resource where I can study this particular pattern used by OZ in upgradable contracts?

Edit: More about this topic that explains how it functions and the rules can be read here: https://eips.ethereum.org/EIPS/eip-7201

1 Answer 1


In your example, it is safe to add the new field as:

  • GovernorCountingSimpleStorage struct doesn't collide with other storage as has a custom offset
  • the new variables in GovernorCountingSimpleStorage struct added at the end don't collide with existing ones

It will be safe later on to add new variables.

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