Integrating with the Optimistic Oracle

This demo will set up an OptimisticDepositBox contract which custodies a user’s ERC-20 token balance.

On a local testnet blockchain, the user will deposit wETH (Wrapped Ether) into the contract and withdraw wETH denominated in USD. For example, if the user wants to withdraw $10,000 USD of wETH, and the ETH/USD exchange rate is $2,000, they would withdraw 5 wETH.

  • The user links the OptimisticDepositBox with one of the price identifiers enabled on the DVM.

  • The user deposits wETH into the OptimisticDepositBox and register it with the ETH/USD price identifier.

  • The user can now withdraw a USD-denominated amount of wETH from their DepositBox via smart contract calls, with the Optimistic Oracle enabling optimistic on-chain pricing.

In this example, the user would not have been able to transfer USD-denominated amounts of wETH without referencing an off-chain ETH/USD price feed. The Optimistic Oracle therefore enables the user to "pull" a reference price.

Unlike price requests to the DVM, a price request to the Optimistic Oracle can be resolved within a specified liveness window if there are no disputes, which can be significantly shorter than the DVM voting period. The liveness window is configurable, but is typically two hours, compared to 2-3 days for settlement via the DVM.

The price requestor is not currently required to pay fees to the DVM. The requestor can offer a reward for the proposer who responds to a price request, but the reward value is set to 0 in this example.

The price proposer posts a bond along with their price, which will be refunded if the price is not disputed, or if a dispute is resolved in the proposer's favor. Otherwise, this bond is used to pay the final fee to the DVM and pay a reward to a successful disputer.

In the demo, the requestor does not require an additional bond from the price proposer, so the total bond posted is equal to the wETH final fee, currently 0.2 wETH. See the proposePriceFor function in the OptimisticOracle contract for implementation details.

Running the Demo#

  1. Ensure that you have followed all the prerequisite setup steps here.
  2. Run a local Ganache instance (i.e. not Kovan/Ropsten/Rinkeby/Mainnet) with yarn ganache-cli --port 9545
  3. In another window, migrate the contracts by running the following command:
yarn truffle migrate --reset --network test
  1. To deploy the OptimisticDepositBox contract and go through a simple user flow, run the following demo script from the root of the repo:
yarn truffle exec ./packages/core/scripts/demo/OptimisticDepositBox.js --network test

You should see the following output:

1. Deploying new OptimisticDepositBox
- Using wETH as collateral token
- Pricefeed identifier for ETH/USD is whitelisted
- Collateral address for wETH is whitelisted
- Deployed an OptimisticOracle
- Deployed a new OptimisticDepositBox
2. Minting ERC20 to user and giving OptimisticDepositBox allowance to transfer collateral
- Converted 10 ETH into wETH
- User's wETH balance: 10
- Increased OptimisticDepositBox allowance to spend wETH
- Contract's wETH allowance: 10
3. Depositing ERC20 into the OptimisticDepositBox
- Deposited 10 wETH into the OptimisticDepositBox
- User's deposit balance: 10
- Total deposit balance: 10
- User's wETH balance: 0
4. Withdrawing ERC20 from OptimisticDepositBox
- Submitted a withdrawal request for 10000 USD of wETH
- Proposed a price of 2000000000000000000000 ETH/USD
- Fast-forwarded the Optimistic Oracle and Optimistic Deposit Box to after the liveness window so we can settle.
- New OO time is [fast-forwarded timestamp]
- New ODB time is [fast-forwarded timestamp]
- Executed withdrawal. This also settles and gets the resolved price within the withdrawal function.
- User's deposit balance: 5
- Total deposit balance: 5
- User's wETH balance: 5

Explaining the Contract Functions#

It is worthwhile to examine the OptimisticDepositBox contract code to see how it interacts with the UMA protocol.

The constructor function includes a _finderAddress argument for the UMA Finder contract, which maintains a registry of the OptimisticOracle address, approved collateral and price identifier whitelists, and other important contract addresses.

This allows the constructor to check that the collateral type and price identifier are valid, and allows the OptimisticDepositBox to find and interact with the OptimisticOracle later.

The requestWithdrawal function includes an internal call to the OptimisticOracle requesting the ETH/USD price. Once it's returned, the user can call executeWithdrawal to complete the withdrawal.

There is much more information and explanation in the code comments, so please take a look if you're interested in learning more!

Extending the Example Contract#

While the example contract uses wETH as the currency for deposits to the OptimisticDepositBox and ETH/USD as the price identifier, the code supports use of any approved ERC-20 token as the currency for deposits.

The code can also be deployed with any other price identifier approved by UMA governance. For example, if users wanted to denominate their withdrawals in BTC instead of USD, the price identifier could be set to ETH/BTC.

This example is intended to serve as a basic technical tutorial for how to integrate the Optimistic Oracle into a new contract. Some interesting ideas for extending the example contract include:

  • Limiting withdrawals unless ETH is above a certain USD price, which would enforce a "HODL" strategy for depositors.
  • Allowing a user to request a price and propose a price in the same transaction.
  • Only allowing withdrawals after a specified future timestamp.
  • Allowing partial withdrawals denominated in USD that must be spaced out over time.

The Optimistic Oracle is a powerful tool for smart contracts that need quick and secure resolution of any kind of off-chain data, and this tutorial is only scratching the surface.

To learn more about the Optimistic Oracle, see the full contract documentation.

If you'd like to test your contract on Kovan or Ethereum mainnet, you can find deployed addresses here.

If you have ideas or questions about something you'd like to build, join our community on Discord.

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