Smart contracts

pragma solidity >=0.4.0 <0.6.0;

contract SimpleStorage {

uint storedData;

function set(uint x) public {

storedData = x;

}

function get() public view returns (uint) {

return storedData;

}

Interact with external world

Traditionally (javascript)

const request = require('request');

request('https://min-api.cryptocompare.com/data/price?fsym=ETH&tsyms=USD', function (error, response, body) {

console.error('error:', error); // Print the error if one occurred

console.log('statusCode:', response && response.statusCode); // 200

console.log('body:', body); // {"USD":266.86}

});

Say I make an API call like above and get a result; there’s no way for other nodes to verify my call; they makes the same API call, but may get different results or error

How to interact with smart contracts

Smart contracts can call functions of other smart contracts or even deploy new contracts

Smart contracts can emit events and write those events to logs. You can read the logs to observe the events

You (off-chain nodes) can make transactions/calls to smart contracts to execute contract functions

Solution

Instead of making an API call, smart contracts emit the API request as an event together with a callback function