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Ruaridh e11b298c28 Add CDP Spec (#328 )

* add overview and basic structure

* add state and params

* add basic messages

* add state transitions

* add begin block state transitions

* add missing titles

* add concepts

* add events

* update state and concepts

* update for liquidator changes

* update events

* mention module accounts

* update begin block

* update params

* update page numbering

* add fee descriptions

* add broken link linter

* add broken link linter to CI

* move link check to end of CI

* update typo

Co-Authored-By: Kevin Davis <karzak@users.noreply.github.com>

* address review comments

* Update x/cdp/spec/06_params.md

Co-Authored-By: Kevin Davis <karzak@users.noreply.github.com>

* Update x/cdp/spec/README.md

Co-Authored-By: Kevin Davis <karzak@users.noreply.github.com>

* Update x/cdp/spec/README.md

Co-Authored-By: Kevin Davis <karzak@users.noreply.github.com>

Co-authored-by: Kevin Davis <karzak@users.noreply.github.com>

2020-01-23 14:09:36 -05:00

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State

For detail on the state tracked by the cdp module see the types package. In particular keys.go describes how state is stored in the key-value store.

Module Accounts

The cdp module account controls two module accounts:

CDP Account: Stores the deposited cdp collateral, and the debt coins for the debt in all the cdps.

Liquidator Account: Stores debt coins that have been seized by the system, and any stable asset that has been raised through auctions.

CDP

A CDP is a struct representing a debt position owned by one address. It has one collateral type and records the debt that has been drawn and how much fees should be repaid.

Only an owner is authorized to draw or repay debt, but anyone can deposit collateral to a CDP. Deposits are scoped per address and are recorded separately in Deposit types. Depositors are free to withdraw their collateral provided it does not put the CDP below the liquidation ratio.

The CDP's collateral always equal to the total of the deposits.

type CDP struct {
    ID              uint64
    Owner           sdk.AccAddress
    Collateral      sdk.Coins
    Principal       sdk.Coins
    AccumulatedFees sdk.Coins
    FeesUpdated     time.Time
}

CDPs are stored with a couple of database indexes for faster lookup:

by collateral ratio - to look up cdps that are close to the liquidation ratio
by owner index - to look up cdps that an address is the owner of

Deposit

A Deposit is a struct recording collateral added to a CDP by one address. The address only has authorization to change their deposited amount (provided it does not put the CDP below the liquidation ratio).

type Deposit struct {
    CdpID         uint64
    Depositor     sdk.AccAddress
    Amount        sdk.Coins
}

Params

Module parameters controlled by governance. See Parameters for details.

NextCDPID

A global counter used to create unique CDP ids.

DebtDenom

The name of the internal debt coin. Its value can be configured at genesis.

Total Principle

Sum of all non seized debt plus accumulated fees. This is used to calculate the new debt created every block due to the fee interest rate.

Previous Block Time

A record of the last block time used to calculate fees.

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