What is the Reverse Oracle pattern, and how does it differ from a plain oracle?

A reverse oracle flips the direction: an existing off-chain system queries the blockchain (via a transaction or block ID) so its smart contracts can validate data or check conditions — integrating a chain into legacy software without rewriting the software's core.

• Problem. Some domains run on large, mature, or legacy systems that comply with existing standards. You want to add blockchain's guarantees non-intrusively, without changing the core of those systems.
• Solution. A transaction or block ID on the chain is just a small piece of data that slots easily into an existing system's records. Validation logic lives in a smart contract; an off-chain component simply queries the chain using that ID to check a condition or fetch validated data.
• Difference from the oracle. An oracle pushes external data onto the chain for on-chain contracts to use; a reverse oracle has off-chain systems pull from the chain. Same bridge, opposite direction.
• Trade-off. Benefit: connectivity with minimal disruption. Drawback: it's only truly non-intrusive if the existing system is extensible enough — reading from and writing to the chain may still force changes.