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.

  • ERC1967Utils: 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 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.

The current implementation of this security mechanism uses EIP1822 to detect the storage slot used by the implementation. A previous implementation, now deprecated, relied on a rollback check. It is possible to upgrade from a contract using the old mechanism to a new one. The inverse is however not possible, as old implementations (before version 4.5) did not include the ERC1822 interface.

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.

Functions

_delegate(address implementation) internal

Delegates the current call to implementation.

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

_implementation() → address internal

This is a virtual function that should be overridden 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 internal 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.

_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 overridden 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.

Functions
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 initializing the storage of the proxy like a Solidity constructor.

_implementation() → address impl internal

Returns the current implementation address.

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

ERC1967Utils 

import "@openzeppelin/contracts/proxy/ERC1967/ERC1967Utils.sol";

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

Functions
Events

getImplementation() → address internal

Returns the current implementation address.

upgradeTo(address newImplementation) internal

Perform implementation upgrade

Emits an IERC1967.Upgraded event.

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

Perform implementation upgrade with additional setup call.

Emits an IERC1967.Upgraded event.

getAdmin() → address internal

Returns the current admin.

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

changeAdmin(address newAdmin) internal

Changes the admin of the proxy.

Emits an IERC1967.AdminChanged event.

getBeacon() → address internal

Returns the current beacon.

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

Change the beacon and trigger a setup call.

Emits an IERC1967.BeaconUpgraded event.

Invoking this function has no effect on an instance of BeaconProxy since v5, since it uses an immutable beacon without looking at the value of the ERC-1967 beacon slot for efficiency.

Upgraded(address indexed implementation) event

Emitted when the implementation is upgraded.

AdminChanged(address previousAdmin, address newAdmin) event

Emitted when the admin account has changed.

BeaconUpgraded(address indexed beacon) event

Emitted when the beacon is changed.

Transparent Proxy

TransparentUpgradeableProxy 

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

This contract implements a proxy that is upgradeable by an immutable 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 indicating the proxy admin cannot fallback to the target implementation.

These properties mean that the admin account can only be used for upgrading the proxy, 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, which extends from the Ownable contract to allow for changing the proxy’s admin owner.

The real interface of this proxy is that defined in ITransparentUpgradeableProxy. This contract does not inherit from that interface, and instead the admin functions are implicitly implemented using a custom dispatch mechanism in _fallback. Consequently, the compiler will not produce an ABI for this contract. This is necessary to fully implement transparency without decoding reverts caused by selector clashes between the proxy and the implementation.
This contract avoids unnecessary storage reads by setting the admin only during construction as an immutable variable, preventing any changes thereafter. However, the admin slot defined in ERC-1967 can still be overwritten by the implementation logic pointed to by this proxy. In such cases, the contract may end up in an undesirable state where the admin slot is different from the actual admin.
It is not recommended to extend this contract to add additional external functions. If you do so, the compiler will not check that there are no selector conflicts, due to the note above. A selector clash between any new function and the functions declared in ITransparentUpgradeableProxy will be resolved in favor of the new one. This could render the admin operations inaccessible, which could prevent upgradeability. Transparency may also be compromised.
Functions
ERC1967Proxy
Proxy

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.

_fallback() internal

If caller is the admin process the call internally, otherwise transparently fallback to the proxy behavior

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.

Functions
Ownable
Events
Ownable

constructor(address initialOwner) public

Sets the initial owner who can perform upgrades.

upgrade(contract ITransparentUpgradeableProxy proxy, address implementation) public

Upgrades proxy to implementation. See {TransparentUpgradeableProxy-upgradeTo}.

Requirements:

  • This contract must be the admin of proxy.

upgradeAndCall(contract ITransparentUpgradeableProxy 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 an UpgradeableBeacon.

The beacon address can only be set once during construction, and cannot be changed afterwards. It is stored in an immutable variable to avoid unnecessary storage reads, and also in the beacon storage slot specified by EIP1967 so that it can be accessed externally.

Since the beacon address can never be changed, you must ensure that you either control the beacon, or trust the beacon to not upgrade the implementation maliciously.
Do not use the implementation logic to modify the beacon storage slot. Doing so would leave the proxy in an inconsistent state where the beacon storage slot does not match the beacon address.
Functions
Proxy

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 initializing the storage of the proxy like a Solidity constructor.

Requirements:

  • beacon must be a contract with the interface IBeacon.

_implementation() → address internal

Returns the current implementation address of the associated beacon.

_getBeacon() → address internal

Returns the beacon.

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.

Functions
Ownable
Events
Ownable

constructor(address implementation_, address initialOwner) public

Sets the address of the initial implementation, and the initial 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 indexed 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.

Functions

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 proxied contracts do not make use of 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.

The initialization functions use a version number. Once a version number is used, it is consumed and cannot be reused. This mechanism prevents re-execution of each "step" but allows the creation of new initialization steps in case an upgrade adds a module that needs to be initialized.

For example:

contract MyToken is ERC20Upgradeable {
    function initialize() initializer public {
        __ERC20_init("MyToken", "MTK");
    }
}

contract MyTokenV2 is MyToken, ERC20PermitUpgradeable {
    function initializeV2() reinitializer(2) public {
        __ERC20Permit_init("MyToken");
    }
}
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.

Avoid leaving a contract uninitialized.

An uninitialized contract can be taken over by an attacker. This applies to both a proxy and its implementation contract, which may impact the proxy. To prevent the implementation contract from being used, you should invoke the _disableInitializers function in the constructor to automatically lock it when it is deployed:

/// @custom:oz-upgrades-unsafe-allow constructor
constructor() {
    _disableInitializers();
}
Modifiers
Functions
Events

initializer() modifier

A modifier that defines a protected initializer function that can be invoked at most once. In its scope, onlyInitializing functions can be used to initialize parent contracts.

Similar to reinitializer(1), except that functions marked with initializer can be nested in the context of a constructor.

Emits an Initialized event.

reinitializer(uint8 version) modifier

A modifier that defines a protected reinitializer function that can be invoked at most once, and only if the contract hasn’t been initialized to a greater version before. In its scope, onlyInitializing functions can be used to initialize parent contracts.

A reinitializer may be used after the original initialization step. This is essential to configure modules that are added through upgrades and that require initialization.

When version is 1, this modifier is similar to initializer, except that functions marked with reinitializer cannot be nested. If one is invoked in the context of another, execution will revert.

Note that versions can jump in increments greater than 1; this implies that if multiple reinitializers coexist in a contract, executing them in the right order is up to the developer or operator.

setting the version to 255 will prevent any future reinitialization.

Emits an Initialized event.

onlyInitializing() modifier

Modifier to protect an initialization function so that it can only be invoked by functions with the initializer and reinitializer modifiers, directly or indirectly.

_disableInitializers() internal

Locks the contract, preventing any future reinitialization. This cannot be part of an initializer call. Calling this in the constructor of a contract will prevent that contract from being initialized or reinitialized to any version. It is recommended to use this to lock implementation contracts that are designed to be called through proxies.

Emits an Initialized event the first time it is successfully executed.

_getInitializedVersion() → uint8 internal

Returns the highest version that has been initialized. See reinitializer.

_isInitializing() → bool internal

Returns true if the contract is currently initializing. See onlyInitializing.

Initialized(uint8 version) event

Triggered when the contract has been initialized or reinitialized.

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.

Modifiers
Functions

onlyProxy() modifier

Check that the execution is being performed through a delegatecall call and that the execution context is a proxy contract with an implementation (as defined in ERC1967) pointing to self. This should only be the case for UUPS and transparent proxies that are using the current contract as their implementation. Execution of a function through ERC1167 minimal proxies (clones) would not normally pass this test, but is not guaranteed to fail.

notDelegated() modifier

Check that the execution is not being performed through a delegate call. This allows a function to be callable on the implementing contract but not through proxies.

proxiableUUID() → bytes32 external

Implementation of the ERC1822 proxiableUUID function. This returns the storage slot used by the implementation. It is used to validate the implementation’s compatibility when performing an upgrade.

A proxy pointing at a proxiable contract should not be considered proxiable itself, because this risks bricking a proxy that upgrades to it, by delegating to itself until out of gas. Thus it is critical that this function revert if invoked through a proxy. This is guaranteed by the notDelegated modifier.

upgradeTo(address newImplementation) public

Upgrade the implementation of the proxy to newImplementation.

Calls _authorizeUpgrade.

Emits an Upgraded event.

upgradeToAndCall(address newImplementation, bytes data) public

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

Calls _authorizeUpgrade.

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  onlyOwner {}
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