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Basic Request Model

Contracts Overview

All source code is open source and available in the Chainlink Github repository.


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.

To build your own client contract using ChainlinkClient, see Introduction to Using Any API, or view the ChainlinkClient API Reference for the ChainlinkClient contract.

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 Contract

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 following items:

  • The oracle address.
  • The job ID, so the oracle knows which tasks to perform.
  • The callback function, which the oracle sends 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.

Consumer UML

Below is a UML diagram describing the contract structure of ATestnetConsumer, a deployed example contract implementing ChainlinkClient.

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