MarketBuy/SellOrdersFillOrKill

[dorothy-zbornak]

MarketBuy/SellOrdersFillOrKill

Summary

Remove the old marketBuyOrders() and marketSellOrders() Exchange functions in 3.0 in favor of just marketBuyOrdersNoThrow() and marketSellOrdersNoThrow(). Add marketBuyOrdersFillOrKill() and marketSellOrdersFillOrKill() which wrap these functions.

Motivation

Our reasoning for this decision was that the typical user performing a market buy or sell would likely not care if any individual order in the operation fails.

One feature of marketBuy/SellOrdersNoThrow() is that it will not fail even if it buys or sells less than the amount of taker or maker asset requested. Thus, it’s useful when you’re simply trying to purchase or liquidate as much of an asset as possible. However, there are cases when more deterministic behavior is desirable, i.e., where it must fill no less than the amount requested or revert. This is the motivation for introducing marketSellOrdersFillOrKill() and marketBuyOrdersFillOrKill().

Specification

The behavior of these new functions would be nearly identical to marketBuyOrdersNoThrow() and marketSellOrdersNoThrow() but with a final check that the combined fill results indicate that the correct amount of assets were sold or bought:

• In the case of marketBuyOrdersNoThrow():
  • fillResults.makerAssetFilledAmount >= makerAssetFillAmount
• In the case of marketSellOrdersNoThrow():
  • fillResults.takerAssetFilledAmount >= takerAssetFillAmount

Implementation

The pseudocode for marketSellOrdersFillOrKill() would look like:

def marketSellOrdersFillOrKill(orders, taker_asset_to_sell, signatures):
    # Leaving out checks that orders are homogenous for simplicity.
    fill_results = FillResults()
    for order, signature in orders, signatures:
          fill_results += fillOrderNoThrow(
            order,
            taker_asset_to_sell - fill_results.taker_asset_filled,
            signature
          )
          # Stop if we've met our target
          if fill_results.taker_asset_filled >= taker_asset_to_sell:
            break
    # Revert if we did not spend all of taker_asset_fill_amount
    if fill_results.taker_asset_filled < taker_asset_to_sell:
        revert()
    return fill_results

The pseudocode for marketBuyOrdersFillOrKill() would look like:

def marketBuyOrdersFillOrKill(orders, maker_asset_to_buy, signatures):
    # Leaving out checks that orders are homogenous for simplicity.
    fill_results = FillResults()
    for order, signature in orders, signatures:
          fill_results += fillOrderNoThrow(
            order,
            # Calculate the amount of taker asset to sell given the
            # amount of remaining maker asset we need.
            (maker_asset_to_buy - fill_results.maker_asset_filled)
                * order.taker_asset_amount / order.maker_asset_amount,
            signature
          )
          # Stop if we've met our target
          if fill_results.maker_asset_filled >= maker_asset_to_buy:
            break
    # Revert if we did not spend all of taker_asset_fill_amount
    if fill_results.maker_asset_filled < maker_asset_to_buy:
        revert()
    return fill_results

Notes/Challenges

• In production, we would likely just wrap the existing marketBuyOrdersNoThrow() and marketSellOrdersNoThrow() then perform the fill check before returning.

• If you preferred the original versions because they asserted that all orders were valid, you can still detect invalid orders (as an EOA) by examining the Fill event logs. If an order hash does not appear in the logs, it was either unfillable or you hit your target without needing to fill that order.

[dorothy-zbornak]

@amaurer I think you had some concerns over the removal of marketBuy/SellOrders() in your usage. Do these new functions meet your needs?

[amaurer]

@amaurer I think you had some concerns over the removal of marketBuy/SellOrders() in your usage. Do these new functions meet your needs?

I have no issue with removing the generalized market buy/sell functions after reading the proposal. I think it's ideal to have explicit names. We're using the fillOrKillOrder and batchFillOrKillOrders functions. Thanks for checking!