Integrate Oracles

This guide walks through integrating Mento's on-chain price feeds into your application. The SortedOracles contract aggregates FX rates from multiple oracle providers (Chainlink, Redstone) to provide reliable exchange rates for stable asset pairs. Mento relayers continuously push these rates on-chain to ensure fresh pricing data.

Common Use Cases

DeFi Protocols

Use real-time FX rates for cross-currency calculations, collateral valuation, and liquidation thresholds.

Price Feed Aggregators

Aggregate Mento's FX rates with other sources to provide comprehensive currency data.

Cross-Border Applications

Build remittance calculators, multi-currency wallets, or international payment systems using accurate FX rates.

Stable Asset Pricing

Understand how Mento pools price swaps between stable assets using these oracle rates.

Integration Steps

Step 1: Connect to SortedOracles Contract

Mento oracle data is available through the SortedOracles contract:

// Mainnet SortedOracles address
address constant SORTED_ORACLES = 0xefB84935239dAcdecF7c5bA76d8dE40b077B7b33;

// Interface
interface ISortedOracles {
  function medianRate(address rateFeedId) external view returns (uint256, uint256);
  function numRates(address rateFeedId) external view returns (uint256);
}

See Smart Contracts > Deployments for testnet addresses.

Step 2: Query Exchange Rates

Get FX rates from the sorted oracles contract. Note that rate feed IDs can be either:

  • Stable token addresses (legacy format, e.g., cUSD address for CELO/USD)

  • Derived addresses for specific currency pairs (e.g., keccak256("NGNUSD") for NGN/USD)

Note: For the full list of rate feed IDs check out the Oracles & Price Feeds page

// Example: Get CELO/USD rate using cUSD address as rate feed ID
ISortedOracles oracles = ISortedOracles(SORTED_ORACLES);
address rateFeedId = cUSD_ADDRESS; // Rate feed ID for CELO/USD

(uint256 numerator, uint256 denominator) = oracles.medianRate(rateFeedId);

// Rate represents how much CELO equals 1 USD (with 24 decimal precision)
uint256 celoPerUsd = numerator / denominator;

For cross-currency rates:

// Example: Get NGN/USD rate for cUSD/cNGN pool pricing
address ngnUsdFeedId = 0xC13D42556f1baeab4a8600C735afcd5344048d3C; // keccak256("relayed:NGNUSD")

// This gives USD per 1 NGN
(uint256 ngnUsdRate, uint256 denominator) = oracles.medianRate(ngnUsdFeedId);

Step 3: Implement Rate Conversion

Use the FX rates for currency conversions. Note that rates in SortedOracles represent various currency pairs:

// Example: Using NGN/USD rate for conversions
function ngnToUsd(uint256 ngnAmount) public view returns (uint256) {
 address ngnUsdFeedId = 0xC13D42556f1baeab4a8600C735afcd5344048d3C;
  (uint256 rate, uint256 denominator) = oracles.medianRate(ngnUsdFeedId);
  
  // rate represents USD per 1 NGN (e.g., 0.000654)
  return ngnAmount.mul(rate).div(denominator);
}

function usdToNgn(uint256 usdAmount) public view returns (uint256) {
  address ngnUsdFeedId = 0xC13D42556f1baeab4a8600C735afcd5344048d3C;
  (uint256 rate, uint256 denominator) = oracles.medianRate(ngnUsdFeedId);
  
  // To convert USD to NGN, we need to invert the rate
  return usdAmount.mul(denominator).div(rate);
}

These rates are crucial for Mento's pools - they determine the exchange ratios between stable assets (e.g., cUSD/cNGN uses the NGN/USD rate).

Step 4: Check Oracle Health

Verify that oracle data is fresh and reliable:

// Check report freshness (rates are relayed, so typically only 1 report)
(bool isExpired, ) = oracles.isOldestReportExpired(rateFeedId);
require(!isExpired, "Oracle rate is expired");

// Optional: Check the median timestamp for additional validation
uint256 medianTime = oracles.medianTimestamp(rateFeedId);
require(block.timestamp - medianTime < MAX_AGE, "Rate too old");

Working with Oracle Data

Rate Format

  • Rates use 24 decimal fixed-point precision (1e24)

  • numerator / denominator gives the exchange rate

  • For FX pairs, the rate represents Quote per 1 Base:

    • NGN/USD: How much USD per 1 NGN (e.g., 0.000654 USD = 1 NGN)

    • GBP/USD: How much USD per 1 GBP

    • EUR/XOF: How much XOF per 1 EUR

  • For CELO pairs (legacy format), the rate represents Base per 1 Quote:

    • CELO/USD: How much CELO per 1 USD

Report Expiry

Oracle reports expire after a configured time period. Expired reports are automatically excluded from median calculations.

Best Practices

Rate Validation

Always validate oracle data before use:

  • Verify rate is within expected bounds

  • Consider implementing circuit breakers for extreme values

Gas Optimization

Cache oracle results when multiple operations use the same rate within a transaction.

Error Handling

Handle cases where:

  • No oracle reports exist for a token

  • All reports have expired

  • Rate calculation results in zero denominator

Advanced Usage

Historical Data

While the contract stores current reports, historical data analysis requires:

  • Monitoring MedianUpdated events

  • Maintaining off-chain data stores

  • Using indexing services like The Graph

JavaScript Integration

Using ethers

import { ethers } from 'ethers';

const provider = new ethers.providers.JsonRpcProvider(RPC_URL);
const sortedOraclesABI = [...]; // Contract ABI
const sortedOracles = new ethers.Contract(SORTED_ORACLES_ADDRESS, sortedOraclesABI, provider);

// Get exchange rate
const rateFeedId = '0xC13D42556f1baeab4a8600C735afcd5344048d3C'; // NGN/USD
const result = await sortedOracles.medianRate(rateFeedId);
const rate = result.numerator.div(result.denominator);

Using viem

import { createPublicClient, http } from 'viem';
import { celo } from 'viem/chains';

const client = createPublicClient({
  chain: celo,
  transport: http(RPC_URL),
});

const sortedOraclesABI = [...]; // Contract ABI

// Get exchange rate
const rateFeedId = '0xC13D42556f1baeab4a8600C735afcd5344048d3C'; // NGN/USD
const result = await client.readContract({
  address: SORTED_ORACLES_ADDRESS,
  abi: sortedOraclesABI,
  functionName: 'medianRate',
  args: [rateFeedId],
});

const rate = Number(result.numerator) / Number(result.denominator);

Support

Next Steps

PreviousIntegrate the BrokerNextMento SDK

Last updated