BSIP73: Match force-settlement orders with margin calls and limit orders

bsip-0073.md

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+    BSIP: 0073
+    Title: Match force-settlement orders with margin calls and limit orders
+    Author: Abit More <https://github.com/abitmore>
+    Status: Draft
+    Type: Protocol
+    Created: 2019-06-09
+    Discussion: https://github.com/bitshares/bsips/issues/181
+    Worker: TBD
+
+# Abstract
+
+This BSIP proposes a protocol change to improve user experience (UX) of
+force-settlements by trying to fill force-settlement orders
+(*settle orders* in short) at a better price
+and optionally fill them before expiration when certain conditions are met.
+
+# Motivation
+
+Force-settlements were designed for debt asset holders to convert an amount
+of the debt asset into corresponding collateral asset at a fair price *when
+there is nobody willing to buy back the debt at a fair price*.
+
+To mitigate malious behavior and market manipulation, a *delay* and a *price
+offset* were designed. But the mechanism has flaws.
+* If a positive *offset* is configured in the debt asset's options,
+  force-settlement requesters will always "buy expensive" even when there are
+  orders "selling low". This also leads to occasional spikes in the market
+  history charts.
+* Force-settlement requesters have to wait for the *delay* even when there
+  appear traders buying the debt asset at a fair price in the market.
+
+These flaws have led to certain confusion and anger among market participants.
+
+# Rationale
+
+Actually, when a margin call appears, it means there is somebody
+willing to buy back the debt. Thus, it makes sense to fill the margin calls
+with the settle orders.
+
+Similarly, if there are limit orders selling the debt asset
+below the feed price,
+it makes sense to match the limit orders with the settle orders as well.
+
+When one of these opportunities appears, it makes sense to fill certain
+settle orders immediately if it's desired by the owners of the settle orders.
+
+These changes would improve user experience (UX).
+
+# Specifications
+
+## Hard fork
+
+Since this is a protocol change, a hard fork is needed. A time will need to be
+scheduled for applying the change, which is commonly known as "hard fork time".
+In this document, terms "before the hard fork", "at the hard fork" and
+"after the hard fork" may or may not be used to indicate things happen before
+the scheduled time, at the scheduled time and after the scheduled time.
+
+## `settle_order_object`
+
+The `settle_order_object` stores current status of a force-settlement order.
+
+A new field is needed within it:
+
+* `bool fill_asap;`
+
+By default this field is set to `false`, which means the order will be
+processed after the *delay* defined in the asset's options, which is current
+behavior.
+
+If this field is set to `true`, the order will be filled or partially filled
+when a debt position enters margin call territory. It will also be filled or
+partially filled when someone places a limit order selling the collateral
+asset below the feed price.
+
+If multiple settle orders with this field as `true` exist in the market,
+when filling before the *delay*, the order which was created first will be
+filled first.
+
+## `asset_settle_operation`
+
+The `asset_settle_operation` is used to request a force-settlement. It has
+an `extensions` field:
+
+* `extensions_type     extensions;`
+
+The data type of this field needs to be overridden so that it can include the
+new `fill_asap` option.
+
+## `asset_settle_evaluator`
+
+The `asset_settle_evaluator` is used to evaluate and apply the
+`asset_settle_operation`. New logic is needed:
+* only allow `fill_asap` option to be set after the hard fork;
+* if the `fill_asap` option is specified in an `asset_settle_operation`, when
+  creating a `settle_order_object` (note: it implies some conditions are met,
+  E.G. the asset is not globally settled), assign the value of the operation's
+  `fill_asap` field to the object's `fill_asap` field.
+* if the `fill_asap` option is set to `true`, and if the feed price is valid,
+  after creation of the settle order object, try to match it against the order
+  book immediately. In other words, treat it as a taker limit order buying
+  at the feed price.
+
+## `proposal_create_evaluator`
+
+The `proposal_create_evaluator` is used to evaluate and apply the
+`proposal_create_operation`, which can contain zero or more
+`asset_settle_operation` objects. New logic is needed:
+* only allow `fill_asap` to be set after the hard fork.
+
+## Matching and filling settle orders whose `fill_asap` field is `true`
+
+### When a new limit order is created
+
+If the new limit order is selling the collateral asset for the debt asset, and
+its price is below the feed price (which implies the feed price is valid),
+* current logic is to only match it with the limit orders on the opposite side
+  of the market;
+* after the hard fork, the new logic would be:
+  * firstly match it with the limit orders on the opposite side whose buy
+    prices are higher than the feed price;
+  * secondly match it with the settle orders whose `fill_asap` options were
+    set to `true`. The matching price would be the feed price. In other words,
+    treat the settle orders as maker limit orders buying at the feed price;
+  * lastly match it with remaining limit orders on the opposite side.
+
+### When a debt position enters margin call territory
+
+If a margin call order appears, either due to the feed price changing, or due
+to the amount of collateral or the amount of debt changing (which implies the
+feed price is valid),
+* current logic is to only match it with the limit orders on the opposite
+  side of the market and conditionally trigger a black swan event;
+* after the hard fork, the new logic would be:
+  * firstly match it with the limit orders on the opposite side whose buy
+    prices are higher than the feed price; if this step triggers a black swan
+    event, apply the corresponding black swan processing logic to the market;
+  * secondly match it with the settle orders whose `fill_asap` options were
+    set to `true`. The matching price would be the feed price. In other words,
+    treat the settle orders as maker limit orders buying at the feed price;
+  * lastly match it with remaining limit orders on the opposite side (which
+    would possibly trigger a black swan event as well, but it's out of this
+    document's scope).
+
+### When the feed price changes
+
+When the feed price changes, either due to a new price being published, or due
+to an old price expiring, or due to the asset's options changing, if the new
+feed price is valid, and if the new feed price in the direction of "X debt
+asset per collateral asset" is higher than the old feed price or the old feed
+price was invalid,
+* current logic doesn't handle settle orders,
+* after the hard fork, the new logic would be:
+  * if there are limit orders selling the collateral asset below the new feed
+    price, and there are settle orders whose `fill_asap` options were `true`,
+    match the settle orders with the limit orders. In other words, treat
+    those settle orders as taker limit orders buying at the new feed price.
+
+### Do not update `force_settled_volume` when filling the `fill_asap` orders
+
+When a settle order is filled or partially filled due to having the
+`fill_asap` option set to `true`,
+it doesn't affect `force_settled_volume` (which indicates how much of the debt
+asset has been force-settled in the current maintenance interval).
+
+## Processing settle orders after the delay
+
+Currently, when filling a settle order after the delay (note: it implies
+some conditions are met, E.G. the feed price is valid and total settled volume
+in the current maintenance interval doesn't exceed the maximum allowed volume),
+the settle order will be matched against the debt position with the least
+collateral ratio, the fill price in the direction of "X debt asset
+per collateral asset" would be
+`fill_price = feed_price * (1 + foce_settlement_offset)`.
+
+After the hard fork, when processing a settle order after the delay,
+* firstly try to match it with the margin calls and the limit orders selling
+  below `fill_price`. In other words, treat it as a taker limit order buying
+  at `fill_price`.
+  Note: this step doesn't affect `force_settled_volume`.
+* if the settle order still exists, process it with the logic before the
+  hard fork.
+  Note: this step does affect `force_settled_volume` as before.
+
+## Logic related to BSIP 71 (Prevent Global Settlement)
+
+The behaviors proposed in this BSIP would be impacted by BSIP 71.
+
+In general, this BSIP proposes that
+* the settle orders whose `fill_asap` field is `true` will be treated as
+  limit orders buying at feed price when the market engine processing
+  limit orders or margin calls, and
+* when processing a settle order after the delay, it will "buy the way up"
+  before being matched with a debt position that need to apply
+  `force_settlement_offset`.
+
+## API
+
+APIs which return `settle_order_object` need to return the new `fill_asap`
+field in the `settle_order_object`.
+
+APIs which return a combined order book can combine settle orders whose
+`fill_asap` is true with the limit orders on the same side of the market.
+
+## CLI
+
+Currently there is a `settle_asset` command in CLI which can be used to
+create force-settlement orders. After the hard fork, we need a command in
+CLI for users to create force-settlement orders with the new `fill_asap`
+option.
+
+One option is to add an optional boolean parameter to the parameter list of
+the existing `settle_asset` command, if it doesn't break existing client
+applications which rely on that wallet API. Otherwise, a new command is
+needed, E.G. `settle_asset_ext`.
+
+## GUI/UX
+
+Note: GUI changes here are only suggestions, formally they're not part of the
+specification.
+
+The new `fill_asap` option needs to be presented and can be used in GUI after
+the hard fork.
+
+When there are settle orders with `fill_asap` option set to `true`, the UI
+can show them
+as special buy orders which are buying at the feed price in the order book.
+
+# Discussion
+
+* With this BSIP, we provided a tool that can be used by debt asset holders to
+convert their debt asset holdings into corresponding collateral asset more
+conveniently and flexibly.
+However, it's not guaranteed that a settle order will be filled at a better
+price if the owner choose to fill it "as soon as possible". Market paticipants
+should always make their own decisions on whether to use the new tool.
+
+* From the perspective of the debt asset owners (note: "asset owners" and
+"asset holders" have different meanings), for the performance of the debt
+assets, it could be more desirable to execute force-settlements against margin
+calls than against market orders, because it would improve the overall
+collateral ratio of the asset (although it's in the price of worse experience
+for force-settlement requesters). In that sense it contradicts the goals of
+BSIP 71 (Prevent Global Settlement).
+
+* The proposed functionality is mostly convenience for users. Manual selling on
+the market is (almost) always available when it would be used automatically
+by this proposal (exception: new incoming limit orders).
+
+* The proposed functionality complicates the market engine and possibly the
+`get_order_book` call (see [bitshares/bitshares-core#1958](
+https://github.com/bitshares/bitshares-core/issues/1958)).
+It is probably harmful for performance in the long run, which is particularly
+undesirable for functionality that can mostly be emulated client-side.
+
+# Summary for Shareholders
+
+TBD
+
+# Copyright
+
+This document is placed in the public domain.
+
+# See Also
+
+* https://github.com/bitshares/bsips/issues/181
+* https://github.com/bitshares/bitshares-ui/issues/1711