makerdao / dss-cdp-manager

1. Introduction (Summary)

Summary: The DssCdpManager (aka manager) was created to enable a formalized process for CDPs to be transferred between owners. In short, the manager works by having a dss wrapper that allows users to interact with their CDPs in an easy way, treating them as non-fungible tokens (NFTs).

2. Contract Details

Key Functionalities (as defined in the smart contract)

• cdpAllow(uint cdp, address usr, uint ok): Allow/Disallow (ok) usr to manage cdp.
• urnAllow(address usr, uint ok): Allow/Disallow (ok) usr to access msg.sender space (for sending a position in quit).
• open(bytes32 ilk, address usr): Opens a new CDP for usr to be used for an ilk collateral type.
• give(uint cdp, address dst): Transfers cdp to dst.
• frob(uint cdp, int dink, int dart): Increments/decrements the dink amount of collateral locked and increments/decrements the dart amount of debt in the cdp depositing the generated DAI or collateral freed in the cdp address.
• flux(bytes32 ilk, uint cdp, address dst, uint wad): Moves wad (precision 18) amount of collateral ilk from cdp to dst.
• flux(uint cdp, address dst, uint wad): Moves wad amount of cdp collateral from cdp to dst.
• move(uint cdp, address dst, uint rad): Moves rad (precision 45) amount of DAI from cdp to dst.
• quit(uint cdp, address dst): Moves the collateral locked and debt generated from cdp to dst.
• enter(address src, uint cdp): Moves the collateral locked and debt generated from src to cdp.
• shift(uint cdpSrc, uint cdpDst): Moves the collateral locked and debt generated from cdpSrc to cdpDst.

Note: dst refers to the destination address.

Storage Layout

• vat : core contract address that holds the CDPs.
• cdpi: Auto incremental id.
• urns: Mapping CDPId => UrnHandler
• list: Mapping CDPId => Prev & Next CDPIds (double linked list)
• owns: Mapping CDPId => Owner
• ilks: Mapping CDPId => Ilk (collateral type)
• first : Mapping Owner => First CDPId
• last: Mapping Owner => Last CDPId
• count: Mapping Owner => Amount of CDPs
• cdpCan: Mapping Owner => CDPId => Allowed Addr => True/False
• urnCan: Mapping Urn => Allowed Addr => True/False

3. Key Mechanisms & Concepts

Summary

The CDP manager was created as a way to enable CDPs to be treated more like assets that can be exchanged as non-fungible tokens (NFT) would. Originally when created, the dss core contracts did not have the functionality to enable the transfer of CDP positions, hence the CDP manager was created to wrap this functionality and enable transferring between users. Since then, the core contracts have also implemented a native transfer functionality called fork which allows the transferring of a CDP to another address. However, there is a restriction, which is that the address owner that will be receiving the CDP needs to provide authorization that they do in fact want to receive it. This was created for the situation when a user is transferring the collateral that is locked as well as the debt generated. If you are simply moving collateral to another address, there is no issue but in the case that you are also transferring the debt generated, there is a chance of putting a perfectly safe CDP in a risky position. This makes the contract functionality a little more restrictive. Therefore, the CDP manager is a good option to keep a simple way of transferring CDPs and recognizing them via a numeric Id.

High-level Purpose

• The manager receives the vat address in its creation and acts as an interface contract between it and the users.
• The manager keeps an internal registry of id => owner and id => urn allowing for the owner to execute vat functions for their urn via the manager.
• The manager keeps a double linked list structure that allows the retrieval of all the CDPs that an owner has via on-chain calls.
  • In short, this is what the GetCdps is for. This contract is a helper contract that allows the fetching of all the CDPs in just one call.

CDP Manager Usage Example (common path):

• A User executes open and gets a CDPId in return.
• After this, the CDPId gets associated with an urn with manager.urns(cdpId) and then join's collateral to it.
• After the user executes frob, the generated DAI will remain in the CDP's urn. Then the user can move it at a later point in time.
  • Note that this is the same process for collateral that is freed after frob. The user can flux it to another address at a later time.
• In the case where a user wants to abandon the manager, they can use quit as a way to migrate their position of their CDP to another dst address.

4. Gotchas (Potential source of user error)

• For the developers who want to integrate with the manager, they will need to understand that the CDP actions are still in the urn environment. Regardless of this, the manager tries to abstract the urn usage by a CDPId. This means that developers will need to get the urn (urn = manager.urns(cdpId)) to allow the joining of collateral to that CDP.
• As the manager assigns a specific ilk per CDPId and doesn't allow others to use it for theirs, there is a second flux function which expects an ilk parameter. This function has the simple purpose of taking out collateral that was wrongly sent to a CDP that can't handle it/is incompatible.
• Frob Function:
  • When you frob in the CDP manager, you generate new DAI in the vat via the CDP manager which is then deposited in the urn that the CDP manager manages.
  • You would need to manually use the flux or move functions to get the DAI or collateral out.

5. Failure Modes (Bounds on Operating Conditions & External Risk Factors)

Potential Issues around Chain Reorganization

When open is executed, a new urn is created and a cdpId is assigned to it for a specific owner. If the user uses join to add collateral to the urn immediately after the transaction is mined, there is a chance that a reorganization of the chain occurs. This would result in the user losing the ownership of that cdpId/urn pair, therefore losing their collateral. However, this issue can only arise when avoiding the use of the proxy functions (https://github.com/makerdao/dss-proxy-actions) via a profile proxy (https://github.com/dapphub/ds-proxy) as the user will open the cdp and join collateral in the same transaction.