Don't Panic: Blockchain Oracles and Data Feeds

Smart contracts are deterministic on purpose. Every honest node must compute the same result. That is why your contract cannot casually phone a live price API mid-transaction: different nodes would see different answers, and consensus would fall over.

So the industry invented oracles. Offchain components observe the world, then write an attested value onto the chain. After that write, reading the value is ordinary state access. ethereum.org calls the combination a hybrid smart contract: onchain logic plus offchain infrastructure.

A data feed is the shape you meet most often in DeFi. Many contracts share one onchain answer, usually a price, behind a stable proxy address. Push feeds update on a heartbeat or deviation rule. Pull feeds keep high-frequency reports offchain until a transaction needs them, and the caller brings a fresh signed report along.

The joke that is also the lesson: a successful latestRoundData() call is not a safety certificate. Check the network and proxy. Scale with decimals. Reject stale updatedAt values against a budget you chose. On rollups with a sequencer uptime feed, confirm the sequencer is up and that your grace period has passed before liquidations or other one-way market actions.

The oracle problem is the three-part checklist people skip when demos look green: correctness, availability, and incentives. Oracles do not remove trust. They relocate it into reporters, update policy, and the checks your contract refuses to skip.

If you want the map, read the Intro and Cheatsheet. If you want to practice the read path, use the Practice Reference and Exercise. If you want judgment about what teams underprice, open Field Notes. Landscape and Awesome Links show who ships which delivery model.