Intro to CCIP-Read: Secure offchain data retrieval

CCIP Read is an Ethereum standard, now at Final status, that lets a smart contract fetch external data without storing it all onchain. Under the specification, a contract reverts with an OffchainLookup error containing a gateway URL template, the callData to send, a callback function selector, and extraData. The client queries the gateway, then calls the callback on the contract, which decodes and verifies the response. The standard is authored by Nick Johnson (@arachnid) and was created on 2020-07-19. The text states that in many cases this can be done without trust assumptions beyond those needed for data stored onchain.