Basic Request Model
ChainlinkClient is a parent contract that enables smart contracts to consume data from oracles. It’s available in the Chainlink smart contract library which can be installed using the latest package managers.
The client constructs and makes a request to a known Chainlink oracle through the transferAndCall function, implemented by the LINK token. This request contains encoded information that is required for the cycle to succeed. In the ChainlinkClient contract, this call is initiated with a call to sendChainlinkRequestTo.
LINK is an ERC-677 compliant token which implements transferAndCall, a function that allows tokens to be transferred whilst also triggering logic in the receiving contract within a single transaction.
Learn more about ERC-677 and the LINK token.
Oracle contracts are owned by oracle node operators, which run alongside off-chain oracle nodes.
The client contract that initiates this cycle must create a request with:
- the oracle address
- the job ID, so the oracle know what tasks to perform
- the callback function, which the oracle will send the response to.
To learn about how to find oracles to suit your needs, see Find Existing Jobs.
Oracle contracts are responsible for handling on-chain requests made through the LINK token, by implementing onTokenTransfer as a LinkTokenReceiver. Upon execution of this function, the oracle contract emits an OracleRequest event containing information about the request. This event is crucial, as it is monitored by the off-chain oracle node which acts upon it.
For fulfillment, the oracle contract has a fulfillOracleRequest function which is used by the node to fulfill a request once it has the result of the job. This function returns the result to the ChainlinkClient using the callback function defined in the original request.
Off-Chain Oracle Node
The off-chain oracle node is responsible for listening for events emitted by its corresponding on-chain smart contract. Once it detects an OracleRequest event, it uses the data emitted to perform a job.
The most common job type for a Node is to make a GET request to an API, retrieve some data from it, parse the response, convert the result into blockchain compatible data, then submit it in a transaction back to the oracle contract, using the fulfillOracleRequest function.
For more information on how to become a node operator, learn how to run a Chainlink node.
Below is a UML diagram describing the contract structure of ATestnetConsumer, a deployed example contract implementing ChainlinkClient.
Originally published @ https://docs.chain.link/docs/architecture-request-model/