Oracles & Off-Chain Data — Hands-on lab

Exercise the edge cases Happy paths are trivial; the real work is rejecting stale/invalid data and handling VRF fulfillment securely. Treat circuit breakers as product features—users should understand why an action is paused.

A distributed system is one in which the failure of a computer you didn’t even know existed can render your own computer unusable.

— Leslie Lamport

Make it observable Emit events when guards trip: stale feed, deviation exceeded, VRF fulfillment errors. This helps dashboards and support teams respond.

Fail closed, practice recovery Simulate outages and show how to unpause or switch feeds. Write down the runbook for ops to follow during incidents.

function readPrice() public view returns (uint256) {
    (
        uint80 roundId,
        int256 answer,
        ,
        uint256 updatedAt,
        uint80 answeredInRound
    ) = feed.latestRoundData();
    require(answer > 0, "bad answer");
    require(updatedAt >= block.timestamp - 1 hours, "stale");
    require(answeredInRound >= roundId, "incomplete");

    uint256 price = uint256(answer) / 10**feed.decimals();
    uint256 prev = lastGoodPrice;
    uint256 deviation = price > prev ? (price - prev) * 1e18 / prev : (prev - price) * 1e18 / prev;
    require(deviation <= 5e16, "deviation too high"); // 5%
    return price;
}