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Oracle Relayer

The glue between price feeds and the SAFE Engine

1. Summary

The OracleRelayer functions as an interface contract between FSMs and the SAFEEngine and only stores the current collateralType list as well as the current redemptionPrice and redemptionRate. The relayer will depend on governance to set each collateral's safety and liquidation ratios and might also depend on an external feedback mechanism to update the redemptionRate which affects the redemptionPrice.

2. Contract Variables & Functions

Variables
  • contractEnabled - settlement flag (1 or 0).
  • authorizedAccounts[usr: address] - addresses allowed to call modifyParameters() and disableContract().
  • collateralTypes[collateralType: bytes32] - mapping of each collateral type
  • cdpEngine - address of the CDPEngine contract
  • redemptionRate - the current redemption rate that reprices the system coin internally and changes user incentives
  • _redemptionPrice - virtual variable that does not reflect the latest redemptionPrice
  • redemptionPriceUpdateTime - last time when the redemption price was updated
  • redemptionRateUpperBound - maximum value that the redemptionRate can have
  • redemptionRateLowerBound - minimum value that the redemptionRate can have
  • RAY - number with 27 decimals
Data Structures
  • CollateralType - struct with data about each collateral type
    • orcl - the address of a price feed, usually an OSM
    • safetyCRatio - the collateralization ratio used to compute the safetyPrice of a collateral type
    • liquidationCRatio - the collateralization ratio used to compute the liquidationPrice of a collateral type
Modifiers
  • isAuthorized **** - checks whether an address is part of authorizedAddresses (and thus can call authed functions).
Functions
  • modifyParameters(parameter: bytes32, data: uint256) - update a uint256 parameter.
  • modifyParameters(parameter: bytes32, data: uint256) - update a collateral related parameter.
  • modifyParameters(collateralType: bytes32, parameter: bytes32, data: address) - update an address parameter.
  • addAuthorization(usr: address) - add an address to authorizedAddresses.
  • removeAuthorization(usr: address) - remove an address from authorizedAddresses.
  • updateRedemptionPrice() - internal function used to update the redemption price using the redemptionRate
  • redemptionPrice() external view returns (uint256) - getter function that updates and retrieves the virtual _redemptionPrice
  • updateCollateralPrice(collateralType: bytes32) - update the safety and liquidation prices of a collateral price and store them in the CDPEngine
  • disableContract() - disables the relayer
  • safetyCRatio(collateralType: bytes32) external view returns (uint256) - getter for a collateral's safety CRatio
  • liquidationCRatio(collateralType: bytes32) external view returns (uint256) - getter for a collateral's liquidation CRatio
  • orcl(collateralType: bytes32) external view returns (address) - getter for a collateral type's oracle
Events
  • AddAuthorization - emitted when a new address becomes authorized. Contains:
    • account - the new authorized account
  • RemoveAuthorization - emitted when an address is de-authorized. Contains:
    • account - the address that was de-authorized
  • DisableContract - emitted when the contract is disabled.
  • ModifyParameters - emitted when a parameter is updated.
  • UpdateRedemptionPrice - emitted when the redemption price is updated. Contains:
    • redemptionPrice - the latest redemption price
  • UpdateCollateralPrice - emitted when the safety and liquidation prices of a specific collateral price are updated. Contains:
    • collateralType - the collateral type whose prices are updated
    • priceFeedValue - the new price feed coming from the collateral's oracle
    • safetyPrice - the price computed by dividing the feed value by the redemptionPrice and then dividing the result again by the collateral's safetyCRatio
    • liquidationPrice - the price computed by dividing the feed value by the redemptionPrice and then dividing the result again by the collateral's liquidationCRatio

3. Walkthrough

UpdateCollateralPrice

updateCollateralPrice is a non-authenticated function. The function takes in a bytes32 representing a collateralType whose (safety and liquidation) prices need to be updated. updateCollateralPrice has three stages:
  1. 1.
    getResultWithValidity - interacts with the collateralType's orcl and returns a value and whether it isValid (a boolean which is false if the price is invalid). The second external call only happens if isValid == true.
  2. 2.
    When calculating the safetyPrice and the liquidationPrice, the _redemptionPrice is crucial as it defines the relationship between the system coin and one unit of collateral. The value from the OSM is divided by the (updated) redemptionPrice (to get a ratio of collateral value to system coins) and then the result is divided again by the collateralType.safetyCRatio (when calculating the safetyPrice) and by the collateralType.liquidationCRatio (when calculating the liquidationPrice).
  3. 3.
    cdpEngine.modifyParameters is then called to update the collateral's prices inside the system.

Redemption Price

Every time someone wants to read the _redemptionPrice its value will first be updated using the redemptionRate and then the output will be returned. We chose this design in order to ensure a smooth redemptionPrice pro-ration (using the virtual variable + a state modifying getter).

Updating the Redemption Rate

Every time the redemptionRate is updated, the contract makes sure to bound the value that is can be set to.

4. Gotchas

The methods in the oracleRelayer are relatively basic compared to most other portions of geb. There is not much room for user error in the single unauthed method updateCollateralPrice. If an incorrect bytes32 is supplied the call will fail.
Any module that is authed against the oracleRelayer has full root access, and can, therefore, add and remove which collateralTypes can be "updateCollateralPrice"'d. While not completely breaking the system, this could cause considerable risk. An authed caller can also update the redemptionRate and redemptionPrice, causing considerable impact depending on the values used.

5. Failure Modes

Coding Error

A bug in oracleRelayer would most likely result in the prices for collaterals and the redemptionRate not being updated anymore. In this case, the system would need to authorize a new oracleRelayer which would then be able to update the prices/rate. Overall this is not a catastrophic failure as this would only pause all price fluctuation for some period.

Feeds

The oracleRelayer relies upon a set of trusted oracles to provide price data. Should these price feeds fail, it would become possible for unbacked Coin to be minted, or Safes could be unfairly liquidated.

Spot Price Becoming Stale

When updateCollateralPrice is not called frequently enough, the SAFE's safetyPrice price will become stale. This could arise for a few reasons including tragedy of the commons or miner collusion and could lead to negative outcomes such as inappropriate liquidations, or the prevention of liquidations that should be possible.
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