Chapter 7 of 9

The program that brings the price every morning

The contract does not wake up by itself: someone must bring it the number every morning. That someone is a small program (the backend) running on an always-on computer.

The program takes five steps: 1) query the Ministry, 2) compute the median, 3) read the POL price, 4) save everything in a small local archive, 5) call setPrices on the contract, signing with the updater key, the only address with the role from chapter 4. Each day is recorded once, so running it again creates no duplicates.

import { Contract, JsonRpcProvider, Wallet } from "ethers";

function median(values) {
  const s = [...values].sort((a, b) => a - b);
  const m = s.length >> 1;
  return s.length % 2 ? s[m] : (s[m - 1] + s[m]) / 2;
}

const abi = ["function setPrices(uint256 fuelEurPerLitre, uint256 polEur, uint64 sourceDate)"];
const provider = new JsonRpcProvider(process.env.RPC_URL);
const wallet = new Wallet(process.env.UPDATER_PRIVATE_KEY, provider);
const oracle = new Contract(process.env.ORACLE_ADDRESS, abi, wallet);

const fuelEur = median(pricesOfStations);   // e.g. 1.989
const polEur = 0.10;                        // from a price service
const scale = (x) => BigInt(Math.round(x * 1e6));
const today = Math.floor(Date.parse(new Date().toISOString().slice(0, 10)) / 1000);

const tx = await oracle.setPrices(scale(fuelEur), scale(polEur), today);
await tx.wait();

• median is the function from chapter 2.
• abi is the list of functions the program wants to call, written as text.
• provider is the doorway to the network (the RPC address); wallet is the updater, with its private key taken from an environment variable, never written in the code.
• oracle combines the contract address, the function list and the signing wallet.
• scale turns 1.989 into 1989000, the six-decimal format.
• today is today's date at midnight UTC in seconds, the data's sourceDate.
• setPrices sends the transaction; tx.wait() waits for it to enter a block.

In the project the program starts by itself at 8:30 (Rome time) with a timer. If the request fails, it retries three times, ten minutes apart.

If the source is down

If the Ministry is unreachable or changes format, the program writes nothing. The blockchain keeps yesterday's price, with its date. After 3 days isFresh (chapter 4) answers "no" and buy() and sell() stop with an "old price" error. This is deliberate: better to stop than to operate on a number that no longer reflects reality.