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Protocol Upgradability

To allow for future updates to the zkEVM Protocol implementation (either in the case of adding new features, fixing bugs, or optimizations upgrades), the following contracts are deployed using a Transparent Upgradeable Proxy (TUP) pattern:

  • PolygonZkEVM.sol (Consensus Contract)
  • PolygonZkEVMGlobalExitRoot.sol
  • PolygonZkEVMBridge.sol

To inherit security and avoid prolonging and making the audit process more complex, the Polygon zkEVM team has chosen to use the OpenZeppelin’s openzeppelin-upgrades library in order to implement this functionality.

Why Use OpenZeppelin Libraries?

OpenZeppelin is a reputated and well-known brand in the industry because of its audits and open-source libraries of implementations of Ethereum standards, and its openzeppelin-upgrades library has been already audited and battle tested.

Furthermore, openzeppelin-upgrades is more than just a set of contracts; it also includes Hardhat and Truffle plugins to help with proxy deployment, upgrades, and administrator rights management.

As shown in the diagram below, Open Zeppelin's TUP pattern separates the protocol implementation of storage variables using delegated calls and the fallback function, allowing the implementation code to be updated without changing the storage state or the contract's public address.

tup pattern schema

Following OpenZeppelin's recommendations, an instance of the contract ProxyAdmin.sol, which is also included in the openzeppelin-upgrades library, is deployed and its address is set as the proxy contract's admin. The Hardhat and Truffle plugins make these operations safe and simple.

Each ProxyAdmin.sol instance serves as the actual administrative interface for each proxy, and the administrative account is the owner of each ProxyAdmin.sol instance. During the zkEVM Protocol's deployment, ownership of ProxyAdmin.sol will be transferred to the Admin role.