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ERC-7540

Asynchronous ERC-4626 Tokenized Vaults

Extension of ERC-4626 with asynchronous deposit and redemption support
ReviewStandards Track: ERC
创建时间: 2023-10-18
关联 EIP: EIP-20, EIP-165, EIP-4626
Jeroen Offerijns (@hieronx), Alina Sinelnikova (@ilinzweilin), Vikram Arun (@vikramarun), Joey Santoro (@joeysantoro), Farhaan Ali (@0xfarhaan)
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Abstract

The following standard extends ERC-4626 by adding support for asynchronous deposit and redemption flows. The async flows are called Requests.

New methods are added to asynchronously Request a deposit or redemption, and view the pending status of the Request. The existing deposit, mint, withdraw, and redeem ERC-4626 methods are used for executing Claimable Requests.

Implementations can choose to whether to add asynchronous flows for deposits, redemptions, or both.

Motivation

The ERC-4626 Tokenized Vaults standard has helped to make yield-bearing tokens more composable across decentralized finance. The standard is optimized for atomic deposits and redemptions up to a limit. If the limit is reached, no new deposits or redemptions can be submitted.

This limitation does not work well for any smart contract system with asynchronous actions or delays as a prerequisite for interfacing with the Vault (e.g. real-world asset protocols, undercollateralized lending protocols, cross-chain lending protocols, liquid staking tokens, or insurance safety modules).

This standard expands the utility of ERC-4626 Vaults for asynchronous use cases. The existing Vault interface (deposit/withdraw/mint/redeem) is fully utilized to claim asynchronous Requests.

Specification

Definitions:

The existing definitions from ERC-4626 apply. In addition, this spec defines:

  • Request: a function call that initiates an asynchronous deposit/redemption flow
  • Pending: the state where a Request has been made but is not yet Claimable
  • Claimable: the state where a Request is processed by the Vault enabling the user to claim corresponding shares (for async deposit) or assets (for async redeem)
  • Claimed: the state where a Request is finalized by the user and the user receives the output token (e.g. shares for a deposit Request)
  • Claim function: the corresponding Vault method to bring a Request to Claimed state (e.g. deposit or mint claims shares from requestDeposit). Lower case claim always describes the verb action of calling a Claim function.
  • operator: the account specified by the sender of the Request which has the right to claim a given Request once it is Claimable
  • asynchronous deposit Vault: a Vault that implements asynchronous Requests for deposit flows
  • asynchronous redemption Vault: a Vault that implements asynchronous redemption flows
  • fully asynchronous Vault: a Vault that implements asynchronous Requests for both deposit and redemption

Request Flows

ERC-7540 Vaults MUST implement one or both of asynchronous deposit and redemption Request flows. If either flow is not implemented in a Request pattern, it MUST use the ERC-4626 standard synchronous interaction pattern.

All ERC-7540 asynchronous tokenized Vaults MUST implement ERC-4626 with overrides for certain behavior described below.

Asynchronous deposit Vaults MUST override the ERC-4626 specification as follows:

  1. The deposit and mint methods do not transfer asset to the Vault, because this already happened on requestDeposit.
  2. previewDeposit and previewMint MUST revert for all callers and inputs.

Asynchronous redeem Vaults MUST override the ERC-4626 specification as follows:

  1. The redeem and withdraw methods do not transfer shares to the Vault, because this already happened on requestRedeem.
  2. The owner/operator field of redeem and withdraw MUST be msg.sender to prevent the theft of requested redemptions by a non-owner/operator.
  3. previewRedeem and previewWithdraw MUST revert for all callers and inputs.

Request Lifecycle

After submission, Requests go through Pending, Claimable, and Claimed stages. An example lifecycle for a deposit Request is visualized in the table below.

StateUserVault
PendingrequestDeposit(assets, operator)asset.transferFrom(msg.sender, vault, assets); pendingDepositRequest[operator] += assets
ClaimableInternal Request fulfillment: pendingDepositRequest[msg.sender] -= assets; maxDeposit[operator] += assets
Claimeddeposit(assets, receiver)maxDeposit[msg.sender] -= assets; vault.balanceOf[receiver] += shares

An important Vault inequality is that following a Request(s), the cumulative requested quantity MUST be more than pendingDepositRequest + maxDeposit - claimed. The inequality may come from fees or other state transitions outside implemented by Vault logic such as cancellation of a Request, otherwise this would be a strict equality.

Requests MUST NOT skip or otherwise short-circuit the Claim state. In other words, to initiate and claim a Request, a user MUST call both request* and the corresponding Claim function separately, even in the same block. Vaults MUST NOT "push" tokens onto the user after a Request, users MUST "pull" the tokens via the Claim function.

For asynchronous Vaults, the exchange rate between shares and assets including fees and yield is up to the Vault implementation. In other words, pending redemption Requests MAY NOT be yield bearing and MAY NOT have a fixed exchange rate.

Methods

requestDeposit

Transfers assets from msg.sender into the Vault and submits a Request for asynchronous deposit/mint. This places the Request in Pending state, with a corresponding increase in pendingDepositRequest for the amount assets.

When the Request is Claimable, maxDeposit and maxMint will be increased for the case where the receiver input is the operator. deposit or mint can subsequently be called by operator to receive shares. A Request MAY transition straight to Claimable state but MUST NOT skip the Claimable state.

The shares that will be received on deposit or mint MAY NOT be equivalent to the value of convertToShares(assets) at the time of Request, as the price can change between Request and Claim.

MUST support ERC-20 approve / transferFrom on asset as a deposit Request flow.

MUST revert if all of assets cannot be requested for deposit/mint (due to deposit limit being reached, slippage, the user not approving enough underlying tokens to the Vault contract, etc).

Note that most implementations will require pre-approval of the Vault with the Vault's underlying asset token.

MUST emit the RequestDeposit event.

- name: requestDeposit type: function stateMutability: nonpayable inputs: - name: assets type: uint256 - name: operator type: address

pendingDepositRequest

The amount of requested assets in Pending state for the operator to deposit or mint.

MUST NOT include any assets in Claimable state for deposit or mint.

MUST NOT show any variations depending on the caller.

MUST NOT revert unless due to integer overflow caused by an unreasonably large input.

- name: pendingDepositRequest type: function stateMutability: view inputs: - name: operator type: address outputs: - name: assets type: uint256

requestRedeem

Assumes control of shares from owner and submits a Request for asynchronous redeem/withdraw. This places the Request in Pending state, with a corresponding increase in pendingRedeemRequest for the amount shares.

MAY support either a locking or a burning mechanism for shares depending on the Vault implemention.

If a Vault uses a locking mechanism for shares, those shares MUST be burned from the Vault balance before or upon claiming the Request.

MUST support a redeem Request flow where the control of shares is taken from owner directly where msg.sender has ERC-20 approval over the shares of owner.

When the Request is Claimable, maxRedeem and maxWithdraw will be increased for the case where the owner input is the operator. redeem or withdraw can subsequently be called by operator to receive assets. A Request MAY transition straight to Claimable state but MUST NOT skip the Claimable state.

The assets that will be received on redeem or withdraw MAY NOT be equivalent to the value of convertToAssets(shares) at time of Request, as the price can change between Pending and Claimed.

SHOULD check msg.sender can spend owner funds using allowance.

MUST revert if all of shares cannot be requested for redeem / withdraw (due to withdrawal limit being reached, slippage, the owner not having enough shares, etc).

MUST emit the RequestRedeem event.

- name: requestRedeem type: function stateMutability: nonpayable inputs: - name: shares type: uint256 - name: operator type: address - name: owner type: address

pendingRedeemRequest

The amount of requested shares in Pending state for the operator to redeem or withdraw.

MUST NOT include any shares in Claimable state for redeem or withdraw.

MUST NOT show any variations depending on the caller.

MUST NOT revert unless due to integer overflow caused by an unreasonably large input.

- name: pendingRedeemRequest type: function stateMutability: view inputs: - name: operator type: address outputs: - name: shares type: uint256

Events

DepositRequest

sender has locked assets in the Vault to Request a deposit. operator controls this Request.

MUST be emitted when a deposit Request is submitted using the requestDeposit method.

- name: DepositRequest type: event inputs: - name: sender indexed: true type: address - name: operator indexed: true type: address - name: assets indexed: false type: uint256

RedeemRequest

sender has locked shares, owned by owner, in the Vault to Request a redemption. operator controls this Request.

MUST be emitted when a redemption Request is submitted using the requestRedeem method.

- name: RedeemRequest type: event inputs: - name: sender indexed: true type: address - name: operator indexed: true type: address - name: owner indexed: true type: address - name: assets indexed: false type: uint256

ERC-165 support

Smart contracts implementing this standard MUST implement the ERC-165 supportsInterface function.

Asynchronous deposit Vaults MUST return the constant value true if 0xea446681 is passed through the interfaceID argument.

Asynchronous redemption Vaults MUST return the constant value true if 0x2e9dd5bd is passed through the interfaceID argument.

Rationale

Symmetry and Non-inclusion of requestWithdraw and requestMint

In ERC-4626, the spec was written to be fully symmetrical with respect to converting assets and shares by including deposit/withdraw and mint/redeem.

Due to the asynchronous nature of Requests, the Vault can only operate with certainty on the quantity that is fully known at the time of the Request (assets for deposit and shares for redeem). The deposit Request flow cannot work with a mint call, because the amount of assets for the requested shares amount may fluctuate before the fulfillment of the Request. Likewise, the redemption Request flow cannot work with a withdraw call.

Optionality of flows

Certain use cases are only asynchronous on one flow but not the other between Request and redeem. A good example of an asynchronous redemption Vault is a liquid staking token. The unstaking period necessitates support for asynchronous withdrawals, however, deposits can be fully synchronous.

Non Inclusion of a Request Cancelation Flow

In many cases, canceling a Request may not be straightforward or even technically feasible. The state transition of cancelations could be synchronous or asynchronous, and the way to claim a cancelation interfaces with the remaining Vault functionality in complex ways.

A separate EIP should be developed to standardize behavior of cancelling a pending Request. Defining the cancel flow is still important for certain classes of use cases for which the fulfillment of a Request can take a considerable amount of time.

Request Implementation flexibility

The standard is flexible enough to support a wide range of interaction patterns for Request flows. Pending Requests can be handled via internal accounting, globally or on per-user levels, use ERC-20 or ERC-721, etc.

Likewise yield on redemption Requests can accrue or not, and the exchange rate of any Request may be fixed or variable depending on the implementation.

Not allowing short-circuiting for claims

If claims can short circuit, this creates ambiguity for integrators and complicates the interface with overloaded behavior on Request functions.

An example of a short-circuiting Request flow could be as follows: user triggers a Request which enters Pending state. When the Vault fulfills the Request, the corresponding assets/shares are pushed straight to the user. This requires only 1 step on the user's behalf.

This approach has a few issues:

  • cost/lack of scalability: as the number of vault users grows it can become intractably expensive to offload the Claim costs to the Vault operator
  • hinders integration potential: Vault integrators would need to handle both the 2-step and 1-step case, with the 1-step pushing arbitrary tokens in from an unknown Request at an unknown time. This pushes complexity out onto integrators and reduces the standard's utility.

The 2-step approach used in the standard may be abstracted into a 1-step approach from the user perspective through the use of routers, relayers, message signing, or account abstraction.

In the case where a Request may become Claimable immediately in the same block, there can be router contracts which atomically check for Claimable amounts immediately upon Request. Frontends can dynamically route Requests in this way depending on the state and implementation of the Vault to handle this edge case.

Operator function parameter on requestDeposit and requestRedeem

To support flows where a smart contract manages the Request lifecycle on behalf of a user, the operator parameter is included in the requestDeposit and requestRedeem functions. This is not called owner because the assets or shares are not transferred from this account on Request submission, unlike the behaviour of an owner on redeem. It is also not called receiver because the shares or assets are not necessarily transferred on claiming the Request, this can be chosen by the operator when they call deposit, mint, redeem, or withdraw.

No Outputs for Request functions

requestDeposit and requestRedeem may not have a known exchange rate that will happen when the Request becomes Claimed. Returning the corresponding assets or shares could not work in this case.

The Requests could also output a timestamp representing the minimum amount of time expected for the Request to become Claimable, however not all Vaults will be able to return a reliable timestamp.

No Event for Claimable state

The state transition of a Request from Pending to Claimable happens at the Vault implementation level and is not specified in the standard. Requests may be batched into the Claimable state, or the state may transition automatically after a timestamp has passed. It is impractical to require an event to emit after a Request becomes Claimable at the user or batch level.

Reversion of Preview Functions in Async Request Flows

The preview functions do not take an address parameter, therefore the only way to discriminate discrepancies in exchange rate are via the msg.sender. However, this could lead to integration/implementation complexities where support contracts cannot determine the output of a claim on behalf of an operator.

In addition, there is no on-chain benefit to previewing the Claim step as the only valid state transition is to Claim anyway. If the output of a Claim is undesirable for any reason, the calling contract can revert on the output of that function call.

It reduces code and implementation complexity at little to no cost to simply mandate reversion for the preview functions of an async flow.

Mandated Support for ERC-165

Implementing support for ERC-165 is mandated because of the optionality of flows. Integrations can use the supportsInterface method to check whether a vault is fully asynchronous, partially asynchronous, or fully synchronous, and use a single contract to support all cases.

Not Allowing Pending Claims to be Fungible

The async pending claims represent a sort of semi-fungible intermediate share class. Vaults can elect to wrap these claims in any token standard they like, for example ERC-20, ERC-1155 or ERC-721 depending on the use case. This is intentionally left out of the spec to provide flexibility to implementers.

Backwards Compatibility

The interface is fully backwards compatible with ERC-4626. The specification of the deposit, mint, redeem, and withdraw methods is different as described in Specification.

Reference Implementation

WIP

Security Considerations

The methods pendingDepositRequest and pendingRedeemRequest are estimates useful for display purposes, and can be outdated due to the asynchronicity.

In general, asynchronicity concerns make state transitions in the Vault much more complex and vulnerable to security risks. Access control on Vault operations, clear documentation of state transitioning, and invariant checks should all be performed to mitigate these risks.

In particular, shares or assets locked for Requests can be stuck in the Pending state. Vaults may elect to allow for fungibility of pending claims or implement some cancellation functionality to protect users.

Moreover, users might not know what the final exchange rate will be on any Request due to the asynchronicity. Users therefore trust the implementation of the asynchronous Vault in the computation of the exchange rate and fulfillment of their Request.

It is worth highlighting again here that the Claim functions for any asynchronous flows MUST enforce that msg.sender == operator/owner to prevent theft of Claimable assets or shares

Copyright and related rights waived via CC0.

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