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I am fairly new to blockchain development. I want to call a function in my contract when a condition in the API is true. Should I make this API call in my contract, or in my frontend?

  • Can you give more details? Smart contracts do not have direct access to resources outside of the blockchain, you need to use an oracle for such things. – Ismael Dec 26 '17 at 4:38
  • @Ismael I want to complete a transaction when a certain parameter in the RESTful API i want to access is true. Is it better to use an orcale or just use normal methods in javascript? – ACluelessProgramer Dec 26 '17 at 12:31
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If your condition occurs within your smart contract, e.g. you've reached a certain number of crowdfunding subscribers, you can track the activities of your smart contracts using EVENTS and an external monitor (e.g. using a node.js daemon). When the event is triggered within your contract, it will be detected by the daemon which will in turn trigger the execution of your pre-programmed code/script. Ethers.io has a great smart contract library which you can use to build your daemon.

If you need to call a method of your smart contract when an external event occurs (e.g. a user signs in to your website) its as simple as using the Web3.js library's contract methods.


Here is a snippet of how to call a method on an Ethereum contract (from a JS-based API server):

const Web3 = require('web3')
const web3 = new Web3('https://mainnet.infura.io')

function myFunction(myParam) {
    var contractAddress = '<ENTER-YOUR-CONTRACT-ADDRESS-HERE>'
    var contractABI = [<ENTER-YOUR-ABI-HERE>]
    var contract = new web3.eth.Contract(contractABI, contractAddress)
    var callData = contract.methods.<CONTRACT-FUNCTION>(myParam).call()
    callData.then(function(result) {
        console.log(result)
    })
}

myFunction(<MY-PARAM>)
  • See, I want to complete a transaction when a certain parameter in the RESTful API i want to access is true. Should I be using an oracle to access this data from within my smart contract, or just use normal methods in javascript? – ACluelessProgramer Dec 26 '17 at 12:30
  • added an example to my answer. once your api condition becomes true, then you can then call that snippet to execute your contract. – nyusternie Dec 26 '17 at 22:11
  • So, what your saying is, I can just keep track using events and such, and do my API calls in the frontend? Rather than using an oracle. – ACluelessProgramer Dec 29 '17 at 11:49
  • Others may disagree, but I find the oracle approach to be overly complicated atm. I've used the approach described above and it works quite excellent. Regardless, I do plan to keep exploring the oracle method of real-world interactions. – nyusternie Jan 1 '18 at 1:23
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Just call a method on contract to check the condition ( this does not cost gas ) if it's true call the API else don't . If you want to countinuously check the value of variable you can use web sockets .

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