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ERC-7540

Asynchronous ERC-4626 Tokenized Vaults

Extension of ERC-4626 with asynchronous deposit and redemption support
ReviewStandards Track: ERC
Created: 2023-10-18
Requires: 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 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 request to enter (requestDeposit) or exit (requestRedeem) the Vault
  • 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.
  • 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 field of redeem and withdraw MUST be msg.sender to prevent the theft of requested redemptions by a non-owner.
  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, receiver, owner, data)asset.transferFrom(owner, vault, assets); pendingDepositRequest[receiver] += assets
ClaimableInternal Request fulfillment: pendingDepositRequest[owner] -= assets; claimableDepositRequest[owner] += assets
Claimeddeposit(assets, receiver)claimableDepositRequest[owner] -= assets; vault.balanceOf[receiver] += shares

Note that maxDeposit increases and decreases in sync with claimableDepositRequest.

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.

Request Ids

The request ID (requestId) of a request is returned by the corresponding requestDeposit and requestRedeem functions.

Multiple requests may have the same requestId, so a given Request is discriminated by both the requestId and the owner.

Requests of the same requestId MUST be fungible with each other (except in the special case requestId == 0 described below). I.e. all Requests with the same requestId MUST transition from Pending to Claimable at the same time and receive the same exchange rate between assets and shares.

If a Request becomes partially claimable, all requests of the same requestId MUST become claimable at the same pro rata rate.

There are no assumptions or requirements of requests with different requestId. I.e. they MAY transition to Claimable at different times and exchange rates with no ordering or correlation enforced in any way.

When requestId==0, the Vault MUST use purely the owner to discriminate the request state. The Pending and Claimable state of multiple requests from the same owner would be aggregated. If a Vault returns 0 for the requestId of any request, it MUST return 0 for all requests.

Methods

requestDeposit

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

The output requestId is used to partially discriminate the request along with the receiver. See Request Ids section for more info.

If the length of data is not 0, the Request MUST send an onERC7540DepositReceived callback to receiver following the interface of ERC7540DepositReceiver described in Request Callbacks section. If the length of data is 0, the Request MUST NOT send a callback.

When the Request is Claimable, claimableDepositRequest will be increased for the receiver. deposit or mint can subsequently be called by receiver 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.

owner MUST equal msg.sender unless the owner has approved the msg.sender by some mechanism.

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: receiver type: address - name: owner type: address - name: data type: bytes outputs: - name: requestId type: uint256

pendingDepositRequest

The amount of requested assets in Pending state for the owner 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: owner type: address outputs: - name: assets type: uint256

claimableDepositRequest

The amount of requested assets in Claimable state for the owner to deposit or mint.

MUST NOT include any assets in Pending 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: claimableDepositRequest type: function stateMutability: view inputs: - name: owner type: address outputs: - name: assets type: uint256

requestRedeem

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

The output requestId is used to partially discriminate the request along with the receiver. See Request Ids section for more info.

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.

If the length of data is not 0, the Request MUST send an onERC7540RedeemReceived callback to receiver following the interface of ERC7540RedeemReceiver described in Request Callbacks section. If the length of data is 0, the Request MUST NOT send a callback.

When the Request is Claimable, claimableRedeemRequest will be increased for the receiver. redeem or withdraw can subsequently be called by receiver 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: receiver type: address - name: owner type: address - name: data type: bytes outputs: - name: requestId - type: uint256

pendingRedeemRequest

The amount of requested shares in Pending state for the owner 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: owner type: address outputs: - name: shares type: uint256

claimableRedeemRequest

The amount of requested shares in Claimable state for the owner to redeem or withdraw.

MUST NOT include any shares in Pending 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: claimableRedeemRequest type: function stateMutability: view inputs: - name: owner type: address outputs: - name: shares type: uint256

Events

DepositRequest

owner has locked assets in the Vault to Request a deposit with request ID requestId. receiver controls this Request. sender is the caller of the requestDeposit which may not be equal to the owner.

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

- name: DepositRequest type: event inputs: - name: receiver indexed: true type: address - name: owner indexed: true type: address - name: requestId indexed: true type: uint256 - name: sender indexed: false type: address - name: assets indexed: false type: uint256

RedeemRequest

sender has locked shares, owned by owner, in the Vault to Request a redemption. receiver controls this Request, but is not necessarily the owner.

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

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

Request Callbacks

All methods which initiate a request (including requestId==0) include a data parameter, which if nonzero length MUST send a callback to the receiver.

There are two interfaces, ERC7540DepositReceiver and ERC7540RedeemReceiver which each define the single callback method to be called.

ERC7540DepositReceiver

The interface to be called on requestDeposit.

operator is the msg.sender of the original requestDeposit call. owner is the owner of the requestDeposit. requestId is the output requestId of the requestDeposit and data is the data of the requestDeposit.

This function MUST return 0xe74d2a41 upon successful execution of the callback.

- name: onERC7540DepositReceived type: function inputs: - name: operator type: address - name: owner type: address - name: requestId type: uint256 - name: data type: bytes outputs: - name: interfaceId type: bytes4

ERC7540RedeemReceiver

The interface to be called on requestRedeem.

operator is the msg.sender of the original requestRedeem call. owner is the owner of the requestRedeem. requestId is the output requestId of the requestRedeem and data is the data of the requestRedeem.

This function MUST return 0x0102fde4 upon successful execution of the callback.

- name: onERC7540RedeemReceived type: function inputs: - name: operator type: address - name: owner type: address - name: requestId type: uint256 - name: data type: bytes outputs: - name: interfaceId type: bytes4

ERC-165 support

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

Asynchronous deposit Vaults MUST return the constant value true if 0x1683f250 is passed through the interfaceID argument.

Asynchronous redemption Vaults MUST return the constant value true if 0x0899cb0b is passed through the interfaceID argument.

Rationale

Including Request IDs but Not Including a Claim by ID method

Requests in an Asynchronous Vault have properties of NFTs or Semi-Fungible tokens due to their asynchronicity. However, trying to pigeonhole all ERC-7540 Vaults into supporting ERC-721 or ERC-1155 for Requests would create too much interface bloat.

Using both an id and address to discriminate Requests allows for any of these use cases to be developed at an external layer without adding too much complexity to the core interface.

Certain Vaults especially requestId==0 cases benefit from using the underlying ERC-4626 methods for claiming because there is no discrimination at the requestId level. This standard is written primarily with those use cases in mind. A future standard can optimize for nonzero request ID with support for claiming and transferring requests discriminated also with an requestId.

Callbacks

Callbacks on Request calls can be used among other things to allow Requests to become fully ERC-721 or ERC-1155 compatible in an external layer.

This can support flows where a smart contract manages the Request lifecycle on behalf of a user.

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.

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 owner.

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

// This code snippet is incomplete pseudocode used for example only and is no way intended to be used in production or guaranteed to be secure mapping(address => uint256) public pendingDepositRequest; mapping(address => uint256) public claimableDepositRequest; function requestDeposit(uint256 assets, address receiver, address owner, bytes calldata data) external returns (uint256 requestId) { require(assets != 0); require(owner == msg.sender); requestId = 0; // no requestId associated with this request asset.safeTransferFrom(owner, address(this), assets); // asset here is the Vault underlying asset pendingDepositRequest[owner] += assets; // Perform the callback if (data.length != 0) { require(ERC7540Receiver(receiver).onERC7540DepositReceived(msg.sender, owner, requestId, data) == ERC7540Receiver.onERC7540DepositReceived.selector, "receiver failed"); } emit DepositRequest(receiver, owner, requestId, msg.sender, assets); return requestId; } /** * Include some arbitrary transition logic here from Pending to Claimable */ function deposit(uint256 assets, address receiver) external returns (uint256 shares) { require(assets != 0); claimableDepositRequest[msg.sender] -= assets; // underflow would revert if not enough claimable assets shares = convertToShares(assets); // this naive example uses the instantaneous exchange rate. It may be more common to use the rate locked in upon Claimable stage. balanceOf[receiver] += shares; emit Deposit(msg.sender, receiver, assets, shares); }

Security Considerations

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. For example:

  • The view methods for viewing Pending and Claimable request states (e.g. pendingDepositRequest) are estimates useful for display purposes but can be outdated. The inability to know the final exchange rate will be on any Request requires users to trust the implementation of the asynchronous Vault in the computation of the exchange rate and fulfillment of their Request.
  • 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.

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

Copyright and related rights waived via CC0.

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