@@ -5,32 +5,48 @@ NOTE: This document is better viewed at https://docs.openzeppelin.com/contracts/
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 xref:upgrades-plugins::proxies.adoc[Proxy Upgrade Pattern] page.
The abstract {Proxy} contract implements the core delegation functionality. If the concrete proxies that we provide below are not suitable, we encourage building on top of this base contract since it contains an assembly block that may be hard to get right.
Most of the proxies below are built on an abstract base contract.
{ERC1967Proxy} provides a simple, fully functioning, proxy. While this proxy is not by itself upgradeable, it includes an internal upgrade interface. For an upgrade interface exposed externally to an admin, we provide {TransparentUpgradeableProxy}. Both of these contracts use the storage slots specified in https://eips.ethereum.org/EIPS/eip-1967[EIP1967] to avoid clashes with the storage of the implementation contract behind the proxy.
- {Proxy}: Abstract contract implementing the core delegation functionality.
An alternative upgradeability mechanism is provided in <<Beacon>>. 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 {ERC1967Proxy}, but the address of a {UpgradeableBeacon} contract, which is where the implementation address is actually stored and retrieved from. The `upgrade` operations that change the implementation contract address are then sent to the beacon instead of to the proxy contract, and all proxies that follow that beacon are automatically upgraded.
In order to avoid clashes with the storage variables of the implementation contract behind a proxy, we use https://eips.ethereum.org/EIPS/eip-1967[EIP1967] storage slots.
The {Clones} library provides a way to deploy minimal non-upgradeable proxies for cheap. This can be useful for applications that require deploying many instances of the same contract (for example one per user, or one per task). These instances are designed to be both cheap to deploy, and cheap to call. The drawback being that they are not upgradeable.
- {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>>.
- {TransparentUpgradeableProxy}: A proxy with a built in admin and upgrade interface.
- {UUPSUpgradeable}: An upgradeability mechanism to be included in the implementation for an ERC1967 proxy.
CAUTION: 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 xref:upgrades-plugins::index.adoc[OpenZeppelin Upgrades Plugins] for Truffle and Hardhat.
== UUPS Design: Upgradeability as an Implementation Feature
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.
- {BeaconProxy}: A proxy that retreives its implementation from a beacon contract.
- {UpgradeableBeacon}: A beacon contract that can be upgraded.
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.
Upgradeable smart contracts rely on proxies to relay the calls in a way that is programmable. As discussed previously, we provide different proxy contracts that each come with a specific set of features. Other designs, not (yet) proposed as part of the OpenZeppelin products, also exist.
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.
The most simple, and common, design is known as Universal Upgradeable Proxy Standard (UUPS). This design is both lightweight and versatile and is proposed through the {ERC1967Proxy} and the {UUPSUpgradeable} contract.
- {Clones}: A library that can deploy cheap minimal non-upgradeable proxies.
While {UUPSUpgradeable} uses the same interface as {TransparentUpgradeableProxy}, in the first case the upgrade is handled by the implementation, and can eventually be removed. {TransparentUpgradeableProxy}, on the other hand, includes the upgrade logic in the proxy. This means {TransparentUpgradeableProxy} is more expensive to deploy. 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.
[[transparent-vs-uups]]
== Transparent vs UUPS Proxies
According to this design, the {ERC1967Proxy} is only capable of forwarding calls to an implementation contract. Unlike the more complex and expensive to deploy {TransparentUpgradeableProxy}, the {ERC1967Proxy} doesn't by itself provide any upgradeability mechanism. 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 complaint implementation.
The original proxies included in OpenZeppelin followed the https://blog.openzeppelin.com/the-transparent-proxy-pattern/[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 https://eips.ethereum.org/EIPS/eip-1822[EIP1822], which first documented the pattern.
By default, the upgrade mechanism 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 proxy forever. This security mechanism can however be bypassed, either by:
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.
- 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 upgrade again to another implementation without the upgrade mechanism.
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 {xref-UUPSUpgradeable-_authorizeUpgrade-address-}[`_authorizeUpgrade`] function with the relevant access control mechanism) will turn your contract into a UUPS compliant implementation.
When doing an upgrade, the second parameter of the {upgradeToAndCall} function allows for the atomic execution of an optional initialization/migration call.
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
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@@ -40,8 +56,6 @@ When doing an upgrade, the second parameter of the {upgradeToAndCall} function a
