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adr-004-split-denomination-keys.md

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ADR 004: Split Denomination Keys

Changelog

  • 2020-01-08: Initial version
  • 2020-01-09: Alterations to handle vesting accounts
  • 2020-01-14: Updates from review feedback
  • 2020-01-30: Updates from implementation

Glossary

  • denom / denomination key -- unique token identifier.

Context

With permissionless IBC, anyone will be able to send arbitrary denominations to any other account. Currently, all non-zero balances are stored along with the account in an sdk.Coins struct, which creates a potential denial-of-service concern, as too many denominations will become expensive to load & store each time the account is modified. See issues 5467 and 4982 for additional context.

Simply rejecting incoming deposits after a denomination count limit doesn't work, since it opens up a griefing vector: someone could send a user lots of nonsensical coins over IBC, and then prevent the user from receiving real denominations (such as staking rewards).

Decision

Balances shall be stored per-account & per-denomination under a denomination- and account-unique key, thus enabling O(1) read & write access to the balance of a particular account in a particular denomination.

Account interface (x/auth)

GetCoins() and SetCoins() will be removed from the account interface, since coin balances will now be stored in & managed by the bank module.

The vesting account interface will replace SpendableCoins in favor of LockedCoins which does not require the account balance anymore. In addition, TrackDelegation() will now accept the account balance of all tokens denominated in the vesting balance instead of loading the entire account balance.

Vesting accounts will continue to store original vesting, delegated free, and delegated vesting coins (which is safe since these cannot contain arbitrary denominations).

Bank keeper (x/bank)

The following APIs will be added to the x/bank keeper:

  • GetAllBalances(ctx Context, addr AccAddress) Coins
  • GetBalance(ctx Context, addr AccAddress, denom string) Coin
  • SetBalance(ctx Context, addr AccAddress, coin Coin)
  • LockedCoins(ctx Context, addr AccAddress) Coins
  • SpendableCoins(ctx Context, addr AccAddress) Coins

Additional APIs may be added to facilitate iteration and auxiliary functionality not essential to core functionality or persistence.

Balances will be stored first by the address, then by the denomination (the reverse is also possible, but retrieval of all balances for a single account is presumed to be more frequent):

var BalancesPrefix = []byte("balances")

func (k Keeper) SetBalance(ctx Context, addr AccAddress, balance Coin) error {
  if !balance.IsValid() {
    return err
  }

  store := ctx.KVStore(k.storeKey)
  balancesStore := prefix.NewStore(store, BalancesPrefix)
  accountStore := prefix.NewStore(balancesStore, addr.Bytes())

  bz := Marshal(balance)
  accountStore.Set([]byte(balance.Denom), bz)

  return nil
}

This will result in the balances being indexed by the byte representation of balances/{address}/{denom}.

DelegateCoins() and UndelegateCoins() will be altered to only load each individual account balance by denomination found in the (un)delegation amount. As a result, any mutations to the account balance by will made by denomination.

SubtractCoins() and AddCoins() will be altered to read & write the balances directly instead of calling GetCoins() / SetCoins() (which no longer exist).

trackDelegation() and trackUndelegation() will be altered to no longer update account balances.

External APIs will need to scan all balances under an account to retain backwards-compatibility. It is advised that these APIs use GetBalance and SetBalance instead of GetAllBalances when possible as to not load the entire account balance.

Supply module

The supply module, in order to implement the total supply invariant, will now need to scan all accounts & call GetAllBalances using the x/bank Keeper, then sum the balances and check that they match the expected total supply.

Status

Accepted.

Consequences

Positive

  • O(1) reads & writes of balances (with respect to the number of denominations for which an account has non-zero balances). Note, this does not relate to the actual I/O cost, rather the total number of direct reads needed.

Negative

  • Slightly less efficient reads/writes when reading & writing all balances of a single account in a transaction.

Neutral

None in particular.

References