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1. Understand the Role of a Checker

A Checker acts as a bridge between conditions and smart contract executions. Its purpose? To check conditions and determine whether a task should be executed by Gelato. Every Checker returns two main things:
  • canExec (Boolean): Indicates if Gelato should execute the task.
  • execData (Bytes): Contains the data that executors will use during execution.
Solidity functions must adhere to the block gas limit for checker calls; exceeding it will cause the call to fail.

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2. Solidity Function Example

Before we delve into complexities, let’s understand the structure of a simple Checker: 
contract CounterChecker{
    ICounter public immutable counter;

    constructor(ICounter _counter) {
        counter = _counter;
    }

    function checker()
        external
        view
        returns (bool canExec, bytes memory execPayload)
    {
        uint256 lastExecuted = counter.lastExecuted();

        canExec = (block.timestamp - lastExecuted) > 180;

        execPayload = abi.encodeCall(ICounter.increaseCount, (1));
    }
}
In the above, the checker checks the state of a counter and prompts Gelato to execute if 3 minutes (180 seconds) have elapsed since its last execution.

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3. Making your Checker Reusable

Avoid hardcoding addresses. Instead, allow the passing of arguments to your checker. This lets you reuse the checker for multiple tasks: 
function checker(address _counter)
    external
    view
    returns (bool canExec, bytes memory execPayload)
{
    uint256 lastExecuted = ICounter(_counter).lastExecuted();

    canExec = (block.timestamp - lastExecuted) > 180;

    execPayload = abi.encodeCall(ICounter.increaseCount, (1));
}

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4. Advanced: Checking Multiple Functions

Suppose you’re automating tasks across different pools. Instead of creating multiple tasks, iterate through your list of pools within a single checker: 
function checker()
    external
    view
    returns (bool canExec, bytes memory execPayload)
{
    uint256 delay = harvester.delay();

    for (uint256 i = 0; i < vaults.length(); i++) {
        IVault vault = IVault(getVault(i));

        canExec = block.timestamp >= vault.lastDistribution().add(delay);

        execPayload = abi.encodeWithSelector(
            IHarvester.harvestVault.selector,
            address(vault)
        );

        if (canExec) return(true, execPayload);
    }

    return(false, bytes("No vaults to harvest"));
}

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5. Incorporating Feedback with Logs

With the Gelato Web3 Functions UI, you can use custom return messages to pinpoint where your checker might be “stuck”: 
function checker()
    external
    view
    returns (bool canExec, bytes memory execPayload)
{
    uint256 lastExecuted = counter.lastExecuted();

    if(block.timestamp - lastExecuted < 180) return(false, bytes("Time not elapsed"));

    execPayload = abi.encodeCall(ICounter.increaseCount, (1));
    return(true, execPayload);
}

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6. Limit the Gas Price of your execution

On networks such as Ethereum, gas will get expensive at certain times. If what you are automating is not time-sensitive and don’t mind having your transaction mined at a later point, you can limit the gas price used in your execution in your checker. 
function checker()
    external
    view
    returns (bool canExec, bytes memory execPayload)
{
    // condition here
    
    if(tx.gasprice > 80 gwei) return (false, bytes("Gas price too high"));
}
This way, Gelato will not execute your transaction if the gas price is higher than 80 GWEI.