cosmos-sdk/docs/spec/auth/vesting.md

Vesting

• Vesting
  • Intro and Requirements
  • Vesting Account Types
  • Vesting Account Specification
    • Determining Vesting & Vested Amounts
      • Continuously Vesting Accounts
      • Delayed/Discrete Vesting Accounts
    • Transferring/Sending
      • Keepers/Handlers
    • Delegating
      • Keepers/Handlers
    • Undelegating
      • Keepers/Handlers
  • Keepers & Handlers
  • Initializing at Genesis
  • Examples
    • Simple
    • Slashing
  • Glossary

Intro and Requirements

This specification describes the vesting account implementation for the Cosmos Hub. The requirements for this vesting account is that it should be initialized during genesis with a starting balance X and a vesting end time T.

The owner of this account should be able to delegate to and undelegate from validators, however they cannot send locked coins to other accounts until those coins have been fully vested.

In addition, a vesting account vests all of its coin denominations at the same rate. This may be subject to change.

Note: A vesting account could have some vesting and non-vesting coins. To support such a feature, the GenesisAccount type will need to be updated in order to make such a distinction.

Vesting Account Types

// VestingAccount defines an interface that any vesting account type must
// implement.
type VestingAccount interface {
    Account

    GetVestedCoins(Time) Coins
    GetVestingCoins(Time) Coins

    // Delegation and undelegation accounting that returns the resulting base
    // coins amount.
    TrackDelegation(Time, Coins)
    TrackUndelegation(Coins)
}

// BaseVestingAccount implements the VestingAccount interface. It contains all
// the necessary fields needed for any vesting account implementation.
type BaseVestingAccount struct {
    BaseAccount

    OriginalVesting  Coins // coins in account upon initialization
    DelegatedFree    Coins // coins that are vested and delegated
    DelegatedVesting Coins // coins that vesting and delegated

    EndTime  Time // when the coins become unlocked
}

// ContinuousVestingAccount implements the VestingAccount interface. It
// continuously vests by unlocking coins linearly with respect to time.
type ContinuousVestingAccount struct {
    BaseVestingAccount

    StartTime  Time // when the coins start to vest
}

// DelayedVestingAccount implements the VestingAccount interface. It vests all
// coins after a specific time, but non prior. In other words, it keeps them
// locked until a specified time.
type DelayedVestingAccount struct {
    BaseVestingAccount
}

In order to facilitate less ad-hoc type checking and assertions and to support flexibility in account usage, the existing Account interface is updated to contain the following:

type Account interface {
    // ...

    // Calculates the amount of coins that can be sent to other accounts given
    // the current time.
    SpendableCoins(Time) Coins
}

Vesting Account Specification

Given a vesting account, we define the following in the proceeding operations:

• OV: The original vesting coin amount. It is a constant value.
• V: The number of OV coins that are still vesting. It is derived by OV, StartTime and EndTime. This value is computed on demand and not on a per-block basis.
• V': The number of OV coins that are vested (unlocked). This value is computed on demand and not a per-block basis.
• DV: The number of delegated vesting coins. It is a variable value. It is stored and modified directly in the vesting account.
• DF: The number of delegated vested (unlocked) coins. It is a variable value. It is stored and modified directly in the vesting account.
• BC: The number of OV coins less any coins that are transferred (which can be negative or delegated). It is considered to be balance of the embedded base account. It is stored and modified directly in the vesting account.

Determining Vesting & Vested Amounts

It is important to note that these values are computed on demand and not on a mandatory per-block basis (e.g. BeginBlocker or EndBlocker).

Continuously Vesting Accounts

To determine the amount of coins that are vested for a given block time T, the following is performed:

1. Compute X := T - StartTime
2. Compute Y := EndTime - StartTime
3. Compute V' := OV * (X / Y)
4. Compute V := OV - V'

Thus, the total amount of vested coins is V' and the remaining amount, V, is vesting.

func (cva ContinuousVestingAccount) GetVestedCoins(t Time) Coins {
    // We must handle the case where the start time for a vesting account has
    // been set into the future or when the start of the chain is not exactly
    // known.
    if t <= va.StartTime {
        return ZeroCoins
    }

    x := t - cva.StartTime
    y := cva.EndTime - cva.StartTime

    return cva.OriginalVesting * (x / y)
}

func (cva ContinuousVestingAccount) GetVestingCoins(t Time) Coins {
    return cva.OriginalVesting - cva.GetVestedCoins(t)
}

Delayed/Discrete Vesting Accounts

Delayed vesting accounts are easier to reason about as they only have the full amount vesting up until a certain time, then all the coins become vested (unlocked). This does not include any unlocked coins the account may have initially.

func (dva DelayedVestingAccount) GetVestedCoins(t Time) Coins {
    if t >= dva.EndTime {
        return dva.OriginalVesting
    }

    return ZeroCoins
}

func (dva DelayedVestingAccount) GetVestingCoins(t Time) Coins {
    return dva.OriginalVesting - dva.GetVestedCoins(t)
}

Transferring/Sending

At any given time, a vesting account may transfer: min((BC + DV) - V, BC).

In other words, a vesting account may transfer the minimum of the base account balance and the base account balance plus the number of currently delegated vesting coins less the number of coins vested so far.

func (va VestingAccount) SpendableCoins(t Time) Coins {
    bc := va.GetCoins()
    return min((bc + va.DelegatedVesting) - va.GetVestingCoins(t), bc)
}

Keepers/Handlers

The corresponding x/bank keeper should appropriately handle sending coins based on if the account is a vesting account or not.

func SendCoins(t Time, from Account, to Account, amount Coins) {
    bc := from.GetCoins()

    if isVesting(from) {
        sc := from.SpendableCoins(t)
        assert(amount <= sc)
    }

    newCoins := bc - amount
    assert(newCoins >= 0)

    from.SetCoins(bc - amount)
    to.SetCoins(amount)

    // save accounts...
}

Delegating

For a vesting account attempting to delegate D coins, the following is performed:

1. Verify BC >= D > 0
2. Compute X := min(max(V - DV, 0), D) (portion of D that is vesting)
3. Compute Y := D - X (portion of D that is free)
4. Set DV += X
5. Set DF += Y
6. Set BC -= D

func (va VestingAccount) TrackDelegation(t Time, amount Coins) {
    x := min(max(va.GetVestingCoins(t) - va.DelegatedVesting, 0), amount)
    y := amount - x

    va.DelegatedVesting += x
    va.DelegatedFree += y
    va.SetCoins(va.GetCoins() - amount)
}

Keepers/Handlers

func DelegateCoins(t Time, from Account, amount Coins) {
    bc := from.GetCoins()
    assert(amount <= bc)

    if isVesting(from) {
        from.TrackDelegation(t, amount)
    } else {
        from.SetCoins(sc - amount)
    }

    // save account...
}

Undelegating

For a vesting account attempting to undelegate D coins, the following is performed:

1. Verify (DV + DF) >= D > 0 (this is simply a sanity check)
2. Compute X := min(DF, D) (portion of D that should become free, prioritizing free coins)
3. Compute Y := D - X (portion of D that should remain vesting)
4. Set DF -= X
5. Set DV -= Y
6. Set BC += D

func (cva ContinuousVestingAccount) TrackUndelegation(amount Coins) {
    x := min(cva.DelegatedFree, amount)
    y := amount - x

    cva.DelegatedFree -= x
    cva.DelegatedVesting -= y
    cva.SetCoins(cva.GetCoins() + amount)
}

Note: If a delegation is slashed, the continuous vesting account will end up with an excess DV amount, even after all its coins have vested. This is because undelegating free coins are prioritized.

Keepers/Handlers

func UndelegateCoins(to Account, amount Coins) {
    if isVesting(to) {
        if to.DelegatedFree + to.DelegatedVesting >= amount {
            to.TrackUndelegation(amount)
            // save account ...
        }
    } else {
        AddCoins(to, amount)
        // save account...
    }
}

Keepers & Handlers

The VestingAccount implementations reside in x/auth. However, any keeper in a module (e.g. staking in x/staking) wishing to potentially utilize any vesting coins, must call explicit methods on the x/bank keeper (e.g. DelegateCoins) opposed to SendCoins and SubtractCoins.

In addition, the vesting account should also be able to spend any coins it receives from other users. Thus, the bank module's MsgSend handler should error if a vesting account is trying to send an amount that exceeds their unlocked coin amount.

See the above specification for full implementation details.

Initializing at Genesis

To initialize both vesting and base accounts, the GenesisAccount struct will include an EndTime. Accounts meant to be of type BaseAccount will have EndTime = 0. The initChainer method will parse the GenesisAccount into BaseAccounts and VestingAccounts as appropriate.

type GenesisAccount struct {
    Address       sdk.AccAddress
    GenesisCoins  sdk.Coins
    EndTime       int64
    StartTime     int64
}

func initChainer() {
    for genAcc in GenesisAccounts {
        baseAccount := BaseAccount{
            Address: genAcc.Address,
            Coins:   genAcc.GenesisCoins,
        }

        if genAcc.StartTime != 0 && genAcc.EndTime != 0 {
            vestingAccount := ContinuousVestingAccount{
                BaseAccount:      baseAccount,
                OriginalVesting:  genAcc.GenesisCoins,
                StartTime:        RequestInitChain.Time,
                StartTime:        genAcc.StartTime,
                EndTime:          genAcc.EndTime,
            }

            AddAccountToState(vestingAccount)
        } else if genAcc.EndTime != 0 {
            vestingAccount := DelayedVestingAccount{
                BaseAccount:      baseAccount,
                OriginalVesting:  genAcc.GenesisCoins,
                EndTime:          genAcc.EndTime,
            }

            AddAccountToState(vestingAccount)
        } else {
            AddAccountToState(baseAccount)
        }
    }
}

Examples

Simple

Given a continuous vesting account with 10 vesting coins.

OV = 10
DF = 0
DV = 0
BC = 10
V = 10
V' = 0

1. Immediately receives 1 coin

   BC = 11
   

2. Time passes, 2 coins vest

   V = 8
   V' = 2
   

3. Delegates 4 coins to validator A

   DV = 4
   BC = 7
   

4. Sends 3 coins

   BC = 4
   

5. More time passes, 2 more coins vest

   V = 6
   V' = 4
   

6. Sends 2 coins. At this point the account cannot send anymore until further coins vest or it receives additional coins. It can still however, delegate.

   BC = 2
   

Slashing

Same initial starting conditions as the simple example.

1. Time passes, 5 coins vest

   V = 5
   V' = 5
   

2. Delegate 5 coins to validator A

   DV = 5
   BC = 5
   

3. Delegate 5 coins to validator B

   DF = 5
   BC = 0
   

4. Validator A gets slashed by 50%, making the delegation to A now worth 2.5 coins

5. Undelegate from validator A (2.5 coins)

   DF = 5 - 2.5 = 2.5
   BC = 0 + 2.5 = 2.5
   

6. Undelegate from validator B (5 coins). The account at this point can only send 2.5 coins unless it receives more coins or until more coins vest. It can still however, delegate.

   DV = 5 - 2.5 = 2.5
   DF = 2.5 - 2.5 = 0
   BC = 2.5 + 5 = 7.5
   

   Notice how we have an excess amount of DV.

Glossary

• OriginalVesting: The amount of coins (per denomination) that are initially part of a vesting account. These coins are set at genesis.
• StartTime: The BFT time at which a vesting account starts to vest.
• EndTime: The BFT time at which a vesting account is fully vested.
• DelegatedFree: The tracked amount of coins (per denomination) that are delegated from a vesting account that have been fully vested at time of delegation.
• DelegatedVesting: The tracked amount of coins (per denomination) that are delegated from a vesting account that were vesting at time of delegation.
• ContinuousVestingAccount: A vesting account implementation that vests coins linearly over time.
• DelayedVestingAccount: A vesting account implementation that only fully vests all coins at a given time.