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Join - Detailed Documentation

1. Introduction (Summary)

Join consists of three smart contracts: GemJoin, ETHJoin, and dotBTCJoin: GemJoin - allows standard ERC20 tokens to be deposited for use with the system. ETHJoin - allows native Ether to be used with the system. dotBTCJoin - allows users to withdraw their dotBTC from the system into a standard ERC20 token.
Each join contract is created specifically to allow the given token type to be join'ed to the vat. Because of this, each join contract has slightly different logic to account for the different types of tokens within the system.

2. Contract Details:

Glossary (Join)

  • vat - storage of the Vat’s address.
  • ilk - id of the Ilk for which a GemJoin is created for.
  • gem - the address of the ilk for transferring.
  • dotBTC - the address of the dotBTC token.
  • one - a 10^27 uint used for math in dotBTCJoin.
  • live - an access flag for the join adapter.
  • dec - decimals for the Gem.
Every join contract has 4 public functions: a constructor, join, exit, and cage. The constructor is used on contract initialization and sets the core variables of that join contract. Join and exit are both true to their names. Join provides a mechanism for users to add the given token type to the vat. It has slightly different logic in each variation, but generally resolves down to a transfer and a function call in the vat. Exit is very similar, but instead allows the the user to remove their desired token from the vat. Cage allows the adapter to be drained (allows tokens to move out but not in).

3. Key Mechanisms & Concepts

The GemJoin contract serves a very specified and singular purpose which is relatively abstracted away from the rest of the core smart contract system. When a user desires to enter the system and interact with the dss contracts, they must use one of the join contracts. After they have finished with the dss contracts, they must call exit to leave the system and take out their tokens. When the GemJoin gets caged by an authed address, it can exit collateral from the Vat but it can no longer join new collateral.
User balances for collateral tokens added to the system via join are accounted for in the Vat as Gem according to collateral type Ilk until they are converted into locked collateral tokens (ink) so the user can draw dotBTC.
The dotBTCJoin contract serves a similar purpose. It manages the exchange of dotBTC that is tracked in the Vat and ERC-20 dotBTC that is tracked by DotBTC.sol. After a user draws dotBTC against their collateral, they will have a balance in Vat.dotBTC. This dotBTC balance can be exit' ed from the Vat using the dotBTCJoin contract which holds the balance of Vat.dotBTC and mint's ERC-20 dotBTC. When a user wants to move their dotBTC back into the Vat accounting system (to pay back debt, participate in auctions, pack bag's in the End, or utilize the DSR, etc), they must call dotBTCJoin.join. By calling dotBTCJoin.join this effectively burn's the ERC-20 dotBTC and transfers Vat.dotBTC from the dotBTCJoin's balance to the User's account in the Vat. Under normal operation of the system, the dotBTC.totalSupply should equal the Vat.dotBTC(dotBTCJoin) balance. When the dotBTCJoin contract gets cage'd by an auth'ed address, it can move dotBTC back into the Vat but it can no longer exit dotBTC from the Vat.

4. Gotchas (Potential source of user error)

The main source of user error with the Join contract is that Users should never transfer tokens directly to the contracts, they must use the join functions or they will not be able to retrieve their tokens.
There are limited sources of user error in the join contract system due to the limited functionality of the system. Barring a contract bug, should a user call join by accident they could always get their tokens back through the corresponding exit call on the given join contract.
The main issue to be aware of here would be a well-executed phishing attack. As the system evolves and potentially more join contracts are created, or more user interfaces are made, there is the potential for a user to have their funds stolen by a malicious join contract which does not actually send tokens to the vat, but instead to some other contract or wallet.

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

There could potentially be a vat upgrade that would require new join contracts to be created.
If a gem contract were to go through a token upgrade or have the tokens frozen while a user's collateral was in the system, there could potentially be a scenario in which the users were unable to redeem their collateral after the freeze or upgrade was finished. This scenario likely presents little risk though because the token going through this upgrade would more than likely want to work alongside the ABM community to be sure this was not an issue.