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Depending on the currency pair, and the EVM chain in which its corresponding Price Feed contract is deployed, price updates will be issued according to the following parameters:
The Witnet Foundation is every few seconds attesting for price changes and comparing new off-chain prices with respect the last one stored on-chain. If the price either raises or decreases in a percentage greater than the established deviation threshold, a new price update request will be immediately posted to Witnet oracle in order to ultimately update the on-chain price value that is served via the corresponding Price Router and Price Feed contracts.
To preclude the possibility of not detecting significant price deviations during long periods of time, the heartbeat parameter guarantees a currency pair to be updated at least once every certain amount of minutes.
The cooldown period establishes the minimum time that must elapse since the last on-chain price update until the next one. This parameter alone contents from having a thrashing number of updates during highly volatile situations.
While it is the Witnet Foundation that pays for every single price update according to these triggering conditions, every single supported currency pair involves a selection of well-known and reputed sources (i.e. exchanges), proven to trade significant volumes in a daily basis, and totally unconnected to the Witnet Foundation itself.
Requesting price updates to the Witnet oracle indeed requires the payment of a fee, proportional to the gas price of the request transaction. This fee, however, can be paid with the same native currency (🎉) that is used for paying gas in whatever EVM chain your application is running.