Lets walk through how to make your contract keeper-compatible. While this will get you up and running quickly and showcase how easy it is to use, we highly recommend taking the time to fully understand and become a proficient user of Chainlink Keepers. We want you to take full advantage of the automation infrastructure we’ve built.

1. The Interface
2. Example Contract

KeeperCompatibleInterface

Functions

checkUpkeep

The Keeper node runs this method as an eth_call in order to determine if your contract requires some work to be done. If the off-chain simulation of your checkUpkeep confirms your predefined conditions are met, the Keeper will broadcast a transaction to the blockchain executing the performUpkeep method described below.

  function checkUpkeep(
    bytes calldata checkData
  )
    external
    returns (
        bool upkeepNeeded,
        bytes memory performData
    );

Parameters

• checkData: Data passed to the contract when checking for Upkeep. Specified in the Upkeep registration so it is always the same for a registered Upkeep.

Return Values

• upkeepNeeded: Indicates whether the Keeper should call performUpkeep or not.
• performData: Bytes that the Keeper should call performUpkeep with, if Upkeep is needed. If you would like to encode data to decode later, try abi.encode.

performUpkeep

When your checkUpkeep returns upkeepNeeded == true, the Keeper node broadcasts a transaction to the blockchain to execute your contract code with performData as an input.

  function performUpkeep(
    bytes calldata performData
  ) external;

Parameters

• performData: Data which was passed back from the checkData simulation. If it is encoded, it can easily be decoded into other types by calling abi.decode. This data should always be validated against the contract's current state.

Example Contract

The example below represents a simple counter contract. Each time performUpkeep is called, it increments its counter by one.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

import "@chainlink/contracts/src/v0.8/interfaces/KeeperCompatibleInterface.sol";

contract Counter is KeeperCompatibleInterface {
    /**
    * Public counter variable
    */
    uint public counter;

    /**
    * Use an interval in seconds and a timestamp to slow execution of Upkeep
    */
    uint public immutable interval;
    uint public lastTimeStamp;
    
    constructor(uint updateInterval) {
      interval = updateInterval;
      lastTimeStamp = block.timestamp;

      counter = 0;
    }

    function checkUpkeep(bytes calldata /* checkData */) external override returns (bool upkeepNeeded, bytes memory /* performData */) {
        upkeepNeeded = (block.timestamp - lastTimeStamp) > interval;
        // We don't use the checkData in this example. The checkData is defined when the Upkeep was registered.
    }

    function performUpkeep(bytes calldata /* performData */) external override {
        lastTimeStamp = block.timestamp;
        counter = counter + 1;
        // We don't use the performData in this example. The performData is generated by the Keeper's call to your checkUpkeep function
    }   
}

Once deployed, your contract doesn't automatically start to receive Upkeep after deployment, it must be registered. See Register for Upkeep for more details.