Architecture
Overall architecture
Mainnet:
Basic architecture: Multiple whitelisted-nodes independently fetching price data from multiple data sources to guard against potential node failure. Then the medianizer contract aggregates and picks the median data reported from whitelisted nodes.
There are two ways for nodes to perform price aggregation: median method and weighted-average method.
Median: When there are enough price origins, taking the median value can improve security more effectively.
Weighted-average: Based-on trading volume, liquidity depth, exchange reputation, etc., price will be fetch from different exchanges for different fungible tokens, applied with different weights.
Flow/USD price feed
It has 8 origins currently and is applied with the median method based on https://www.coingecko.com/en/coins/flow#markets:
Kraken Flow/USD
Coinbase Flow/USD
Binance Flow/USDT x (Latest USDT/USD On-Chain PriceOracle data)
OKX Flow/USDT x (Latest USDT/USD On-Chain PriceOracle data)
Gateio Flow/USDT x (Latest USDT/USD On-Chain PriceOracle data)
Huobi Flow/USDT x (Latest USDT/USD On-Chain PriceOracle data)
KuCoin Flow/USDT x (Latest USDT/USD On-Chain PriceOracle data)
Bybit Flow/USDT x (Latest USDT/USD On-Chain PriceOracle data)
USDT/USD price feed
It has 3 origins and is applied with the weighted-average method base on https://www.coingecko.com/en/coins/tether#markets:
* There're concerns and rumors about Bitfinex & Tether manipulation / non-transparency. So although Bitfinex's USDT/USD volume is on par with Coinbase's, it only has a weight of 15%.
Stablecoins' price feed
USDC/USD price feed has 5 price origins and is applied with the median method based on https://www.coingecko.com/en/coins/usd-coin#markets:
(* According to the recent SBV's collapse and Circle is holding a % of cash reserve in SBV, so USDC is not fixed to 1.0 face value any more since 03/11/2023.)
FUSD/USD price feed is applied with a constant 1.0. (FUSD is under-deprecation on Flow, and there's very little exposure to FUSD.)
Other fungible tokens' price feed:
BLT/USD price feed is applied with the weighted-average method based on https://www.coingecko.com/en/coins/blocto-token#markets:
stFlow/USD price feed is computed with on-chain data by:
(stFlow/Flow taken from LiquidStaking Contract) x (Flow/USD On-Chain PriceOracle data)
Testnet:
Basic architecture: A single node fetching from multiple data sources. (Very much close to the mainnet's arch, but to save our resources and costs.)
Emulator:
Basic architecture: A single node SimpleOracle fetching from a single data source (Coingecko api) is used on emulator for local bootstrapping and testing.
Price data is periodically fetched, and it gets updated whichever below conditions happen first:
windowSize: e.g. 1200 - every 1,200 secondsdeviation: e.g. 0.01 - once ฮ { fetchedData, lastData } > 0.01 * lastData
Contract architecture
For each different type of price feed, a separate PriceOracle contract is deployed to support it.
The PriceOracle contract defines two whitelists and two resources: PriceReader and PriceFeeder. PriceReader resource holder has the ability to access reported data as long as it's also in the reader whitelist. PriceFeeder resource holder has the ability to post price data and influence the medianized onchain price data as long as it's also in the feeder whitelist.
For developers to use the on-chain price data in your Cadence contracts there's no barrier to apply for the reader whitelist - we've made it free and public through the PublicPriceOracle contract with some Chainlink-like APIs. Please read Using Price Feeds for more details.
For institutions and partners interest in joining node runners & feeder whitelist to provide more decentralized price feeds on flow, welcome to contact us.
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