Oracle Callback Service

Written in Serpent and deployed on October 12, 2015. It bridges on-chain code to data that lives off-chain, which EVM contracts cannot access directly because every node has to re-execute a transaction and reach the same result.

The contract exposes two functions. call(fetcher, url, fetchId) takes the address of an external fetcher contract, a URL string, and a caller-supplied request id. It invokes the fetcher's get(string) method (ABI selector 693ec85e), forwarding any attached ether, and stores cbids[returnedId] = fetchId so the later response can be matched to the original request. callback(response, responseId) is invoked by the fetcher once it has the data. It reads cbids[responseId] and, if an entry exists, emits LogResponse(response, fetchId) carrying the fetched string. The caller and any off-chain listener read the result from that event.

State is a single storage array, cbids, mapping a fetcher's request id back to the caller's fetchId. The only event is LogResponse(response:string, fetchId:uint256). Before dispatching a request, call also emits LogResponse with the literal string "cow" and a fetchId of 0, a placeholder carried over from the tutorial source the contract derives from.

The request and the response run as two separate transactions: one to submit the request, and a later one from the fetcher to deliver the answer. At deployment in October 2015, Chainlink did not exist and Oraclize (later Provable) was only beginning to appear.