twap

README

TWAP (Time Weighted Average Price)

The TWAP package is responsible for being able to serve TWAPs for every AMM pool.

A time weighted average price is a function that takes a sequence of (time, price) pairs, and returns a price representing an 'average' over the entire time period. The method of averaging can vary from the classic arithmetic mean, (such as geometric mean, harmonic mean), however we currently only implement arithmetic mean.

Arithmetic mean TWAP

Using the arithmetic mean, the TWAP of a sequence (t_i, p_i), from t_0 to t_n, indexed by time in ascending order, is: $$\frac{1}{t_n - t_0}\sum_{i=0}^{n-1} p_i (t_{i+1} - t_i)$$ Notice that the latest price p_n isn't used, as it has lasted for a time interval of 0 seconds in this range!

To illustrate with an example, given the sequence: (0s, $1), (4s, $6), (5s, $1), the arithmetic mean TWAP is: $$\frac{$1 * (4s - 0s) + $6 * (5s - 4s)}{5s - 0s} = \frac{$10}{5} = $2$$

Computation via accumulators method

The prior example for how to compute the TWAP takes linear time in the number of time entries in a range, which is too inefficient. We require TWAP operations to have constant time complexity (in the number of records).

This is achieved by using an accumulator. In the case of an arithmetic TWAP, we can maintain an accumulator from a_n, representing the numerator of the TWAP expression for the interval t_0...t_n, namely $$a_n = \sum_{i=0}^{n-1} p_i (t_{i+1} - t_i)$$ If we maintain such an accumulator for every pool, with t_0 = pool_creation_time to t_n = current_block_time, we can easily compute the TWAP for any interval. The TWAP for the time interval of price points t_i to t_j is then $twap = \frac{a_j - a_i}{t_j - t_i}$, which is constant time given the accumulator values.

In Mokita, we maintain accumulator records for every pool, for the last 48 hours. We also maintain within each accumulator record in state, the latest spot price. This allows us to interpolate accumulation records between times. Namely, if I want the twap from t=10s to t=15s, but the time records are at 9s, 13s, 17s, this is fine. Using the latest spot price in each record, we create the accumulator value for t=10 by computing a_10 = a_9 + a_9_latest_spot_price * (10s - 9s), and a_15 = a_13 + a_13_latest_spot_price * (15s - 13s). Given these interpolated accumulation values, we can compute the TWAP as before.

Module API

The primary intended API is GetArithmeticTwap, which is documented below, and has a similar cosmwasm binding.

// GetArithmeticTwap returns an arithmetic time weighted average price.
// The returned twap is the time weighted average price (TWAP), using the arithmetic mean of:
// * the base asset, in units of the quote asset (1 unit of base = x units of quote)
// * from (startTime, endTime),
// * as determined by prices from AMM pool `poolId`.
//
// startTime and endTime do not have to be real block times that occurred,
// the state machine will interpolate the accumulator values for those times
// from the latest Twap accumulation record prior to the provided time.
//
// startTime must be within 48 hours of ctx.BlockTime(), if you need older TWAPs,
// you will have to maintain the accumulator yourself.
//
// endTime will be set in the function ArithmeticTwap() to ctx.BlockTime() which calls GetArithmeticTwap function if:
// * it is not provided externally
// * it is set to current time
//
// This function will error if:
// * startTime > endTime
// * endTime in the future
// * startTime older than 48 hours OR pool creation
// * pool with id poolId does not exist, or does not contain quoteAssetDenom, baseAssetDenom
// * there were some computational errors during computing arithmetic twap within the time range of  
//   startRecord, endRecord - including the exact record times, which indicates that the result returned could be faulty

// N.B. If there is a notable use case, the state machine could maintain more historical records, e.g. at one per hour.
func (k Keeper) GetArithmeticTwap(ctx sdk.Context,
	poolId uint64,
	baseAssetDenom string, quoteAssetDenom string,
	startTime time.Time, endTime time.Time) (sdk.Dec, error) { ... }

There are convenience methods for GetArithmeticTwapToNow which sets endTime = ctx.BlockTime(), and has minor gas reduction. For users who need TWAPs outside the 48 hours stored in the state machine, you can get the latest accumulation store record from GetBeginBlockAccumulatorRecord.

Code layout

api.go is the main file you should look at as a user of this module.

logic.go is the main file you should look at for how the TWAP implementation works.

• client/* - Implementation of GRPC and CLI queries
• types/* - Implement TwapRecord, GenesisState. Define AMM interface, and methods to format keys.
• twapmodule/module.go - SDK AppModule interface implementation.
• api.go - Public API, that other users / modules can/should depend on
• listeners.go - Defines hooks & calls to logic.go, for triggering actions on
• keeper.go - generic SDK boilerplate (defining a wrapper for store keys + params)
• logic.go - Implements all TWAP module 'logic'. (Arithmetic, defining what to get/set where, etc.)
• store.go - Managing logic for getting and setting things to underlying stores

Store layout

We maintain TWAP accumulation records for every AMM pool on Mokita.

Because Mokita supports multi-asset pools, a complicating factor is that we have to store a record for every asset pair in the pool. For every pool, at a given point in time, we make one twap record entry per unique pair of denoms in the pool. If a pool has k denoms, the number of unique pairs is k * (k - 1) / 2. All public API's for the module will sort the input denoms to the canonical representation, so the caller does not need to worry about this. (The canonical representation is the denoms in lexicographical order)

Example of historical TWAP time index records for a pool containing 3 assets.

• Number of records per time: 3 * (3 - 1) / 2 = 3

• Records are in a format: HistoricalTWAPTimeIndexPrefix | time | pool id | denom1 | denom2

  For our pool with Id = 1 and 3 assets: denomA, denomB and denomC:

  historical_time_index|2009-11-10T23:00:00.000000000|1|denomA|denomB
  historical_time_index|2009-11-10T23:00:00.000000000|1|denomA|denomC
  historical_time_index|2009-11-10T23:00:00.000000000|1|denomB|denomC

Each twap record stores (source):

• last spot price of base asset A in terms of quote asset B
• last spot price of base asset B in terms of quote asset A
• Accumulation value of base asset A in terms of quote asset B
• Accumulation value of base asset B in terms of quote asset A

important for calculation of arthmetic twap.

Besides those values, TWAP records currently hold: poolId, Asset0Denom, Asset1Denom, Height (for debugging purposes), Time and
Last error time - time in which the last spot price error occured. This will allert the caller if they are getting a potentially erroneous TWAP.

All TWAP records are indexed in state by the time of write.

A new TWAP record is created in two situations:

• When a pool is created
• In the EndBlock, if the block contains any potentially price changing event for the pool. (Swap, LP, Exit)

When a pool is created, records are created with the current spot price of the pool.

During EndBlock, new records are created, with:

• The accumulator's value is updated based upon the most recent prior accumulator's stored last spot price
• The LastSpotPrice value is equal to the EndBlock spot price.

In the event that a pool is created, and has a swap in the same block, the record entries are over written with the end block price.

Error handling during records creation/updating:

• If there are issues with creating a record after pool creation, the creation of a pool will be aborted.
• Whereas, if there is an issue with updating records for a pool with potentially price changing events, existing errors will be ignored and the records will not be updated.

Tracking spot-price changing events in a block

The flow by which we currently track spot price changing events in a block is as follows:

• AMM hook triggers for Swapping, LPing or Exiting a pool
• TWAP listens for this hook, and adds this pool ID to a local tracker
• In end block, TWAP iterates over every changed pool in that block, based on the local tracker, and updates their TWAP records
• After execution in end block, when the block is committed, Transient Store that will hold the changed pool "list" within - will be cleared. This guarantees us that there are no changed pool IDs remaining by for processing in the next block.

The mechanism by which we maintain this changed pool list, is the SDK Transient Store. The transient store is a KV store in the SDK, that stores entries in memory, for the duration of a block, and then clears on the block committing. This is done to save on gas (and I/O for the state machine).

Pruning

To avoid infinite growth of the state with the TWAP records, we attempt to delete some old records after every epoch. Essentially, records older than a configurable parameter RecordHistoryKeepPeriod are pruned away. Currently, this parameter is set to 48 hours. Therefore, at the end of an epoch, records older than 48 hours before the current block time are pruned away.
This could potentially leave the store with only one record - or no records at all within the "keep" period, so the pruning mechanism keeps the newest record that is older than the pruning time. This record is necessary to enable us interpolating from and getting TWAPs from the "keep" period. Such record is preserved for each pool.

TWAP - storing records and pruning process flow

Testing Methodology

The pre-release testing methodology planned for the twap module is:

• Using table driven unit tests to test all foreseen states of the module
  • hook testing
    • All swaps correctly trigger twap record updates
    • Create pools cause records to be created
  • store
    • EndBlock triggers all relevant twaps to be saved correctly
    • Block commit wipes temporary stores
  • logic
    • Make tables of expected input / output cases for:
      • getMostRecentRecord
      • getInterpolatedRecord
      • updateRecord
      • computeArithmeticTwap
    • Test overflow handling in all relevant arithmetic
    • Complete testing code coverage (up to return err lines) for logic.go file
  • API
    • Unit tests for the public API, under foreseeable setup conditions
• End to end migration tests
  • Tests that migration of Mokita pools created prior to the TWAP upgrade, get TWAPs recorded starting at the v11 upgrade.
• Integration into the Mokita simulator
  • The mokita simulator, simulates building up complex state machine states, in random ways not seen before. We plan on, in a property check, maintaining expected TWAPs for short time ranges, and seeing that the keeper query will return the same value as what we get off of the raw price history for short history intervals.
  • Not currently deemed release blocking, but planned: Integration for gas tracking, to ensure gas of reads/writes does not grow with time.
• Mutation testing usage
  • integration of the TWAP module into go mutation testing:
    • We've seen with the tokenfactory module that it succeeds at surfacing behavior for untested logic. e.g. if you delete a line, or change the direction of a conditional, mutation tests show if regular Go tests catch it.
    • We expect to get this to a state, where after mutation testing is ran, the only items it mutates, that is not caught in a test, is: Deleting return err, or panic lines, in the situation where that error return or panic isn't reachable.

Documentation

Index

• type Keeper
  • func NewKeeper(storeKey sdk.StoreKey, transientKey *sdk.TransientStoreKey, ...) *Keeper
  • func (k Keeper) EndBlock(ctx sdk.Context)
  • func (k Keeper) EpochHooks() epochtypes.EpochHooks
  • func (k Keeper) ExportGenesis(ctx sdk.Context) *types.GenesisState
  • func (k Keeper) GammHooks() types.GammHooks
  • func (k Keeper) GetArithmeticTwap(ctx sdk.Context, poolId uint64, baseAssetDenom string, quoteAssetDenom string, ...) (sdk.Dec, error)
  • func (k Keeper) GetArithmeticTwapToNow(ctx sdk.Context, poolId uint64, baseAssetDenom string, quoteAssetDenom string, ...) (sdk.Dec, error)
  • func (k Keeper) GetBeginBlockAccumulatorRecord(ctx sdk.Context, poolId uint64, asset0Denom string, asset1Denom string) (types.TwapRecord, error)
  • func (k Keeper) GetParams(ctx sdk.Context) (params types.Params)
  • func (k Keeper) InitGenesis(ctx sdk.Context, genState *types.GenesisState)
  • func (k Keeper) MigrateExistingPools(ctx sdk.Context, latestPoolId uint64) error
  • func (k *Keeper) PruneEpochIdentifier(ctx sdk.Context) string
  • func (k *Keeper) RecordHistoryKeepPeriod(ctx sdk.Context) time.Duration
  • func (k Keeper) SetParams(ctx sdk.Context, params types.Params)

Types

type Keeper

type Keeper struct {
	// contains filtered or unexported fields
}

func NewKeeper

func NewKeeper(storeKey sdk.StoreKey, transientKey *sdk.TransientStoreKey, paramSpace paramtypes.Subspace, ammKeeper types.AmmInterface) *Keeper

func (Keeper) EndBlock

func (k Keeper) EndBlock(ctx sdk.Context)

func (Keeper) EpochHooks

func (k Keeper) EpochHooks() epochtypes.EpochHooks

func (Keeper) ExportGenesis

func (k Keeper) ExportGenesis(ctx sdk.Context) *types.GenesisState

ExportGenesis returns the twap module's exported genesis.

func (Keeper) GammHooks

func (k Keeper) GammHooks() types.GammHooks

func (Keeper) GetArithmeticTwap

func (k Keeper) GetArithmeticTwap(
	ctx sdk.Context,
	poolId uint64,
	baseAssetDenom string,
	quoteAssetDenom string,
	startTime time.Time,
	endTime time.Time,
) (sdk.Dec, error)

N.B. If there is a notable use case, the state machine could maintain more historical records, e.g. at one per hour.

func (Keeper) GetArithmeticTwapToNow

func (k Keeper) GetArithmeticTwapToNow(
	ctx sdk.Context,
	poolId uint64,
	baseAssetDenom string,
	quoteAssetDenom string,
	startTime time.Time,
) (sdk.Dec, error)

GetArithmeticTwapToNow returns GetArithmeticTwap on the input, with endTime being fixed to ctx.BlockTime() This function does not mutate records.

func (Keeper) GetBeginBlockAccumulatorRecord

func (k Keeper) GetBeginBlockAccumulatorRecord(ctx sdk.Context, poolId uint64, asset0Denom string, asset1Denom string) (types.TwapRecord, error)

GetBeginBlockAccumulatorRecord returns a TwapRecord struct corresponding to the state of pool `poolId` as of the beginning of the block this is called on. This uses the state of the beginning of the block, as if there were swaps since the block has started, these swaps have had no time to be arbitraged back. This accumulator can be stored, to compute wider ranged twaps.

func (Keeper) GetParams

func (k Keeper) GetParams(ctx sdk.Context) (params types.Params)

GetParams returns the total set of twap parameters.

func (Keeper) InitGenesis

func (k Keeper) InitGenesis(ctx sdk.Context, genState *types.GenesisState)

InitGenesis initializes the twap module's state from a provided genesis state.

func (Keeper) MigrateExistingPools

func (k Keeper) MigrateExistingPools(ctx sdk.Context, latestPoolId uint64) error

MigrateExistingPools iterates through all pools and creates state entry for the twap module.

func (*Keeper) PruneEpochIdentifier

func (k *Keeper) PruneEpochIdentifier(ctx sdk.Context) string

func (*Keeper) RecordHistoryKeepPeriod

func (k *Keeper) RecordHistoryKeepPeriod(ctx sdk.Context) time.Duration

func (Keeper) SetParams

func (k Keeper) SetParams(ctx sdk.Context, params types.Params)

SetParams sets the total set of twap parameters.