Proxies

This document is better viewed at https://docs.openzeppelin.com/contracts/api/proxy

This is a low-level set of contracts implementing different proxy patterns with and without upgradeability. For an in-depth overview of this pattern check out the Proxy Upgrade Pattern page.

Most of the proxies below are built on an abstract base contract.

  • Proxy: Abstract contract implementing the core delegation functionality.

In order to avoid clashes with the storage variables of the implementation contract behind a proxy, we use EIP1967 storage slots.

  • ERC1967Upgrade: Internal functions to get and set the storage slots defined in EIP1967.

  • ERC1967Proxy: A proxy using EIP1967 storage slots. Not upgradeable by default.

There are two alternative ways to add upgradeability to an ERC1967 proxy. Their differences are explained below in Transparent vs UUPS Proxies.

Using upgradeable proxies correctly and securely is a difficult task that requires deep knowledge of the proxy pattern, Solidity, and the EVM. Unless you want a lot of low level control, we recommend using the OpenZeppelin Upgrades Plugins for Truffle and Hardhat.

A different family of proxies are beacon proxies. This pattern, popularized by Dharma, allows multiple proxies to be upgraded to a different implementation in a single transaction.

In this pattern, the proxy contract doesn’t hold the implementation address in storage like an ERC1967 proxy, instead the address is stored in a separate beacon contract. The upgrade operations that are sent to the beacon instead of to the proxy contract, and all proxies that follow that beacon are automatically upgraded.

Outside the realm of upgradeability, proxies can also be useful to make cheap contract clones, such as those created by an on-chain factory contract that creates many instances of the same contract. These instances are designed to be both cheap to deploy, and cheap to call.

  • Clones: A library that can deploy cheap minimal non-upgradeable proxies.

Transparent vs UUPS Proxies

The original proxies included in OpenZeppelin followed the Transparent Proxy Pattern. While this pattern is still provided, our recommendation is now shifting towards UUPS proxies, which are both lightweight and versatile. The name UUPS comes from EIP1822, which first documented the pattern.

While both of these share the same interface for upgrades, in UUPS proxies the upgrade is handled by the implementation, and can eventually be removed. Transparent proxies, on the other hand, include the upgrade and admin logic in the proxy itself. This means TransparentUpgradeableProxy is more expensive to deploy than what is possible with UUPS proxies.

UUPS proxies are implemented using an ERC1967Proxy. Note that this proxy is not by itself upgradeable. It is the role of the implementation to include, alongside the contract’s logic, all the code necessary to update the implementation’s address that is stored at a specific slot in the proxy’s storage space. This is where the UUPSUpgradeable contract comes in. Inheriting from it (and overriding the _authorizeUpgrade function with the relevant access control mechanism) will turn your contract into a UUPS compliant implementation.

Note that since both proxies use the same storage slot for the implementation address, using a UUPS compliant implementation with a TransparentUpgradeableProxy might allow non-admins to perform upgrade operations.

By default, the upgrade functionality included in UUPSUpgradeable contains a security mechanism that will prevent any upgrades to a non UUPS compliant implementation. This prevents upgrades to an implementation contract that wouldn’t contain the necessary upgrade mechanism, as it would lock the upgradeability of the proxy forever. This security mechanism can be bypassed by either of:

  • Adding a flag mechanism in the implementation that will disable the upgrade function when triggered.

  • Upgrading to an implementation that features an upgrade mechanism without the additional security check, and then upgrading again to another implementation without the upgrade mechanism.

Core

Proxy

import "@openzeppelin/contracts/proxy/Proxy.sol";

This abstract contract provides a fallback function that delegates all calls to another contract using the EVM instruction delegatecall. We refer to the second contract as the implementation behind the proxy, and it has to be specified by overriding the virtual _implementation function.

Additionally, delegation to the implementation can be triggered manually through the _fallback function, or to a different contract through the _delegate function.

The success and return data of the delegated call will be returned back to the caller of the proxy.

_delegate(address implementation) internal

Delegates the current call to implementation.

This function does not return to its internall call site, it will return directly to the external caller.

_implementation() → address internal

This is a virtual function that should be overriden so it returns the address to which the fallback function and _fallback should delegate.

_fallback() internal

Delegates the current call to the address returned by _implementation().

This function does not return to its internall call site, it will return directly to the external caller.

fallback() external

Fallback function that delegates calls to the address returned by _implementation(). Will run if no other function in the contract matches the call data.

receive() external

Fallback function that delegates calls to the address returned by _implementation(). Will run if call data is empty.

_beforeFallback() internal

Hook that is called before falling back to the implementation. Can happen as part of a manual _fallback call, or as part of the Solidity fallback or receive functions.

If overriden should call super._beforeFallback().

ERC1967

ERC1967Proxy

import "@openzeppelin/contracts/proxy/ERC1967/ERC1967Proxy.sol";

This contract implements an upgradeable proxy. It is upgradeable because calls are delegated to an implementation address that can be changed. This address is stored in storage in the location specified by EIP1967, so that it doesn’t conflict with the storage layout of the implementation behind the proxy.

constructor(address _logic, bytes _data) public

Initializes the upgradeable proxy with an initial implementation specified by _logic.

If _data is nonempty, it’s used as data in a delegate call to _logic. This will typically be an encoded function call, and allows initializating the storage of the proxy like a Solidity constructor.

_implementation() → address impl internal

Returns the current implementation address.

ERC1967Upgrade

import "@openzeppelin/contracts/proxy/ERC1967/ERC1967Upgrade.sol";

This abstract contract provides getters and event emitting update functions for EIP1967 slots.

Available since v4.1.

_getImplementation() → address internal

Returns the current implementation address.

_upgradeTo(address newImplementation) internal

Perform implementation upgrade

Emits an Upgraded event.

_upgradeToAndCall(address newImplementation, bytes data, bool forceCall) internal

Perform implementation upgrade with additional setup call.

Emits an Upgraded event.

_upgradeToAndCallSecure(address newImplementation, bytes data, bool forceCall) internal

Perform implementation upgrade with security checks for UUPS proxies, and additional setup call.

Emits an Upgraded event.

_getAdmin() → address internal

Returns the current admin.

_changeAdmin(address newAdmin) internal

Changes the admin of the proxy.

Emits an AdminChanged event.

_getBeacon() → address internal

Returns the current beacon.

_upgradeBeaconToAndCall(address newBeacon, bytes data, bool forceCall) internal

Perform beacon upgrade with additional setup call. Note: This upgrades the address of the beacon, it does not upgrade the implementation contained in the beacon (see {UpgradeableBeacon-_setImplementation} for that).

Emits a BeaconUpgraded event.

Upgraded(address implementation) event

Emitted when the implementation is upgraded.

AdminChanged(address previousAdmin, address newAdmin) event

Emitted when the admin account has changed.

BeaconUpgraded(address beacon) event

Emitted when the beacon is upgraded.

Transparent Proxy

TransparentUpgradeableProxy

import "@openzeppelin/contracts/proxy/transparent/TransparentUpgradeableProxy.sol";

This contract implements a proxy that is upgradeable by an admin.

To avoid proxy selector clashing, which can potentially be used in an attack, this contract uses the transparent proxy pattern. This pattern implies two things that go hand in hand:

  1. If any account other than the admin calls the proxy, the call will be forwarded to the implementation, even if that call matches one of the admin functions exposed by the proxy itself.

  2. If the admin calls the proxy, it can access the admin functions, but its calls will never be forwarded to the implementation. If the admin tries to call a function on the implementation it will fail with an error that says "admin cannot fallback to proxy target".

These properties mean that the admin account can only be used for admin actions like upgrading the proxy or changing the admin, so it’s best if it’s a dedicated account that is not used for anything else. This will avoid headaches due to sudden errors when trying to call a function from the proxy implementation.

Our recommendation is for the dedicated account to be an instance of the ProxyAdmin contract. If set up this way, you should think of the ProxyAdmin instance as the real administrative interface of your proxy.

Modifiers

ifAdmin() modifier

Modifier used internally that will delegate the call to the implementation unless the sender is the admin.

constructor(address _logic, address admin_, bytes _data) public

Initializes an upgradeable proxy managed by _admin, backed by the implementation at _logic, and optionally initialized with _data as explained in ERC1967Proxy.constructor.

admin() → address admin_ external

Returns the current admin.

Only the admin can call this function. See ProxyAdmin.getProxyAdmin.
To get this value clients can read directly from the storage slot shown below (specified by EIP1967) using the eth_getStorageAt RPC call. 0xb53127684a568b3173ae13b9f8a6016e243e63b6e8ee1178d6a717850b5d6103

implementation() → address implementation_ external

Returns the current implementation.

Only the admin can call this function. See ProxyAdmin.getProxyImplementation.
To get this value clients can read directly from the storage slot shown below (specified by EIP1967) using the eth_getStorageAt RPC call. 0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc

changeAdmin(address newAdmin) external

Changes the admin of the proxy.

Emits an AdminChanged event.

Only the admin can call this function. See ProxyAdmin.changeProxyAdmin.

upgradeTo(address newImplementation) external

Upgrade the implementation of the proxy.

Only the admin can call this function. See ProxyAdmin.upgrade.

upgradeToAndCall(address newImplementation, bytes data) external

Upgrade the implementation of the proxy, and then call a function from the new implementation as specified by data, which should be an encoded function call. This is useful to initialize new storage variables in the proxied contract.

Only the admin can call this function. See ProxyAdmin.upgradeAndCall.

_admin() → address internal

Returns the current admin.

_beforeFallback() internal

Makes sure the admin cannot access the fallback function. See Proxy._beforeFallback.

ProxyAdmin

import "@openzeppelin/contracts/proxy/transparent/ProxyAdmin.sol";

This is an auxiliary contract meant to be assigned as the admin of a TransparentUpgradeableProxy. For an explanation of why you would want to use this see the documentation for TransparentUpgradeableProxy.

getProxyImplementation(contract TransparentUpgradeableProxy proxy) → address public

Returns the current implementation of proxy.

Requirements:

  • This contract must be the admin of proxy.

getProxyAdmin(contract TransparentUpgradeableProxy proxy) → address public

Returns the current admin of proxy.

Requirements:

  • This contract must be the admin of proxy.

changeProxyAdmin(contract TransparentUpgradeableProxy proxy, address newAdmin) public

Changes the admin of proxy to newAdmin.

Requirements:

  • This contract must be the current admin of proxy.

upgrade(contract TransparentUpgradeableProxy proxy, address implementation) public

Upgrades proxy to implementation. See TransparentUpgradeableProxy.upgradeTo.

Requirements:

  • This contract must be the admin of proxy.

upgradeAndCall(contract TransparentUpgradeableProxy proxy, address implementation, bytes data) public

Upgrades proxy to implementation and calls a function on the new implementation. See TransparentUpgradeableProxy.upgradeToAndCall.

Requirements:

  • This contract must be the admin of proxy.

Beacon

BeaconProxy

import "@openzeppelin/contracts/proxy/beacon/BeaconProxy.sol";

This contract implements a proxy that gets the implementation address for each call from a UpgradeableBeacon.

The beacon address is stored in storage slot uint256(keccak256('eip1967.proxy.beacon')) - 1, so that it doesn’t conflict with the storage layout of the implementation behind the proxy.

Available since v3.4.

constructor(address beacon, bytes data) public

Initializes the proxy with beacon.

If data is nonempty, it’s used as data in a delegate call to the implementation returned by the beacon. This will typically be an encoded function call, and allows initializating the storage of the proxy like a Solidity constructor.

Requirements:

  • beacon must be a contract with the interface IBeacon.

_beacon() → address internal

Returns the current beacon address.

_implementation() → address internal

Returns the current implementation address of the associated beacon.

_setBeacon(address beacon, bytes data) internal

Changes the proxy to use a new beacon. Deprecated: see _upgradeBeaconToAndCall.

If data is nonempty, it’s used as data in a delegate call to the implementation returned by the beacon.

Requirements:

  • beacon must be a contract.

  • The implementation returned by beacon must be a contract.

IBeacon

import "@openzeppelin/contracts/proxy/beacon/IBeacon.sol";

This is the interface that BeaconProxy expects of its beacon.

Functions

implementation() → address external

Must return an address that can be used as a delegate call target.

BeaconProxy will check that this address is a contract.

UpgradeableBeacon

import "@openzeppelin/contracts/proxy/beacon/UpgradeableBeacon.sol";

This contract is used in conjunction with one or more instances of BeaconProxy to determine their implementation contract, which is where they will delegate all function calls.

An owner is able to change the implementation the beacon points to, thus upgrading the proxies that use this beacon.

constructor(address implementation_) public

Sets the address of the initial implementation, and the deployer account as the owner who can upgrade the beacon.

implementation() → address public

Returns the current implementation address.

upgradeTo(address newImplementation) public

Upgrades the beacon to a new implementation.

Emits an Upgraded event.

Requirements:

  • msg.sender must be the owner of the contract.

  • newImplementation must be a contract.

Upgraded(address implementation) event

Emitted when the implementation returned by the beacon is changed.

Minimal Clones

Clones

import "@openzeppelin/contracts/proxy/Clones.sol";

EIP 1167 is a standard for deploying minimal proxy contracts, also known as "clones".

To simply and cheaply clone contract functionality in an immutable way, this standard specifies a minimal bytecode implementation that delegates all calls to a known, fixed address.

The library includes functions to deploy a proxy using either create (traditional deployment) or create2 (salted deterministic deployment). It also includes functions to predict the addresses of clones deployed using the deterministic method.

Available since v3.4.

clone(address implementation) → address instance internal

Deploys and returns the address of a clone that mimics the behaviour of implementation.

This function uses the create opcode, which should never revert.

cloneDeterministic(address implementation, bytes32 salt) → address instance internal

Deploys and returns the address of a clone that mimics the behaviour of implementation.

This function uses the create2 opcode and a salt to deterministically deploy the clone. Using the same implementation and salt multiple time will revert, since the clones cannot be deployed twice at the same address.

predictDeterministicAddress(address implementation, bytes32 salt, address deployer) → address predicted internal

Computes the address of a clone deployed using Clones.cloneDeterministic.

predictDeterministicAddress(address implementation, bytes32 salt) → address predicted internal

Computes the address of a clone deployed using Clones.cloneDeterministic.

Utils

Initializable

import "@openzeppelin/contracts/proxy/utils/Initializable.sol";

This is a base contract to aid in writing upgradeable contracts, or any kind of contract that will be deployed behind a proxy. Since a proxied contract can’t have a constructor, it’s common to move constructor logic to an external initializer function, usually called initialize. It then becomes necessary to protect this initializer function so it can only be called once. The initializer modifier provided by this contract will have this effect.

To avoid leaving the proxy in an uninitialized state, the initializer function should be called as early as possible by providing the encoded function call as the _data argument to ERC1967Proxy.constructor.
When used with inheritance, manual care must be taken to not invoke a parent initializer twice, or to ensure that all initializers are idempotent. This is not verified automatically as constructors are by Solidity.
Modifiers

initializer() modifier

Modifier to protect an initializer function from being invoked twice.

UUPSUpgradeable

import "@openzeppelin/contracts/proxy/utils/UUPSUpgradeable.sol";

An upgradeability mechanism designed for UUPS proxies. The functions included here can perform an upgrade of an ERC1967Proxy, when this contract is set as the implementation behind such a proxy.

A security mechanism ensures that an upgrade does not turn off upgradeability accidentally, although this risk is reinstated if the upgrade retains upgradeability but removes the security mechanism, e.g. by replacing UUPSUpgradeable with a custom implementation of upgrades.

The _authorizeUpgrade function must be overridden to include access restriction to the upgrade mechanism.

Available since v4.1.

upgradeTo(address newImplementation) external

Upgrade the implementation of the proxy to newImplementation.

Emits an Upgraded event.

upgradeToAndCall(address newImplementation, bytes data) external

Upgrade the implementation of the proxy to newImplementation, and subsequently execute the function call encoded in data.

Emits an Upgraded event.

_authorizeUpgrade(address newImplementation) internal

Function that should revert when msg.sender is not authorized to upgrade the contract. Called by upgradeTo and upgradeToAndCall.

Normally, this function will use an access control modifier such as Ownable.onlyOwner.

function _authorizeUpgrade(address) internal override onlyOwner {}