prometheus/service/price.js

/**
 * BCH ↔ USD price lookup, with a 60-second in-memory cache.
 *
 * We pull from CoinGecko by default — public, no API key, generous for the
 * once-a-minute cadence we need. The price feeds the invoice quote; a
 * publisher's quote is locked at issue time and does not re-quote while they
 * decide whether to pay.
 *
 * On fetch failure we return the last good value if it is under 15 minutes
 * old. Beyond that, we throw — handing out invoices with stale prices is
 * worse than refusing to quote until the oracle recovers.
 */

const CACHE_TTL_MS = 60_000;
const STALE_FALLBACK_MS = 15 * 60_000;
const SOURCE = process.env.PROMETHEUS_PRICE_SOURCE
  || 'https://api.coingecko.com/api/v3/simple/price?ids=bitcoin-cash&vs_currencies=usd';

let cached = null; // { price_usd, at }

export async function bchUsd() {
  const now = Date.now();
  if (cached && now - cached.at < CACHE_TTL_MS) return cached.price_usd;

  try {
    const resp = await fetch(SOURCE, {
      headers: { accept: 'application/json' },
      signal: AbortSignal.timeout(5000),
    });
    if (!resp.ok) throw new Error(`HTTP ${resp.status}`);
    const body = await resp.json();
    const p = Number(body?.['bitcoin-cash']?.usd);
    if (!Number.isFinite(p) || p <= 0) throw new Error('bad price in response');
    cached = { price_usd: p, at: now };
    return p;
  } catch (err) {
    if (cached && now - cached.at < STALE_FALLBACK_MS) return cached.price_usd;
    throw new Error(`price oracle: ${err?.message || err}`);
  }
}

/** USD → sats, using the current (or last-good) BCH/USD price. */
export async function usdToSats(usd) {
  const price = await bchUsd();            // USD per BCH
  const bch = Number(usd) / price;         // BCH per `usd` dollars
  const sats = Math.round(bch * 1e8);      // satoshis per BCH = 1e8
  return { sats, rate_sat_per_usd: Math.round(1e8 / price), price_usd: price };
}