How would you design a notification system?

// Notification pipeline: idempotency, preferences, quiet hours, per-user caps, per-channel retries and a DLQ.
console.log("== capacity estimate ==");
const users = 50000000, perUserPerDay = 10, campaignUsers = 50000000, campaignMinutes = 10, bytesPerRow = 1000, retentionDays = 30;
const perDay = users * perUserPerDay;
console.log("notifications/day: " + perDay + ", avg " + Math.ceil(perDay / 86400) + "/s, campaign burst " + Math.ceil(campaignUsers / (campaignMinutes * 60)) + "/s");
console.log("delivery log: " + (perDay * bytesPerRow) / 1000000000 + " GB/day, " + (perDay * bytesPerRow * retentionDays) / 1000000000000 + " TB for " + retentionDays + " days");

const prefs = {
  u1: { channels: ["push", "email"], marketing: true, quiet: [22, 7] },
  u2: { channels: ["email"], marketing: false, quiet: null },
  u3: { channels: ["sms", "push"], marketing: true, quiet: null },
};
const MARKETING_CAP = 2, MAX_ATTEMPTS = 4, BASE_BACKOFF = 30;
// Fake providers. push fails twice for u3 then works; sms always times out for u3.
const calls = new Map();
function provider(channel, user) {
  const key = channel + ":" + user;
  const n = (calls.get(key) || 0) + 1;
  calls.set(key, n);
  if (channel === "push" && user === "u3" && n <= 2) return "503";
  if (channel === "sms" && user === "u3") return "timeout";
  return "ok";
}
const fmt = (t) => "day" + (Math.floor(t / 86400) + 1) + " " + [Math.floor(t / 3600) % 24, Math.floor(t / 60) % 60, t % 60].map((x) => String(x).padStart(2, "0")).join(":");
const inQuiet = (q, t) => { if (!q) return false; const h = Math.floor(t / 3600) % 24; return q[0] > q[1] ? h >= q[0] || h < q[1] : h >= q[0] && h < q[1]; };

const tasks = []; let seq = 0;
const schedule = (t, task) => { tasks.push({ t, seq: seq++, ...task }); tasks.sort((a, b) => a.t - b.t || a.seq - b.seq); };
const T = (h, m) => h * 3600 + m * 60;
const events = [
  { t: T(21, 0), key: "ship-881", user: "u1", kind: "transactional", text: "order shipped" },
  { t: T(21, 0), key: "ship-881", user: "u1", kind: "transactional", text: "order shipped" }, // producer retry
  { t: T(21, 5), key: "promo-1", user: "u2", kind: "marketing", text: "sale" },
  { t: T(21, 10), key: "promo-2", user: "u3", kind: "marketing", text: "sale" },
  { t: T(23, 30), key: "otp-55", user: "u1", kind: "transactional", text: "login code" },
  { t: T(23, 40), key: "promo-3", user: "u1", kind: "marketing", text: "sale" },
  { t: T(23, 41), key: "promo-4", user: "u1", kind: "marketing", text: "sale" },
  { t: T(23, 42), key: "promo-5", user: "u1", kind: "marketing", text: "sale" },
];
for (const e of events) schedule(e.t, { type: "event", e });

const seen = new Set(), marketingSent = new Map();
const stats = { deduped: 0, optedOut: 0, deferred: 0, capped: 0, sent: 0, retries: 0, dlq: 0 };
while (tasks.length) {
  const task = tasks.shift(), now = task.t;
  if (task.type === "event") {
    const e = task.e, p = prefs[e.user];
    if (seen.has(e.key)) { stats.deduped++; console.log(fmt(now) + " " + e.key + " duplicate idempotency key, dropped"); continue; }
    seen.add(e.key);
    if (e.kind === "marketing" && !p.marketing) { stats.optedOut++; console.log(fmt(now) + " " + e.key + " " + e.user + " opted out of marketing"); continue; }
    if (e.kind === "marketing" && inQuiet(p.quiet, now)) {
      let wake = Math.floor(now / 86400) * 86400 + p.quiet[1] * 3600;
      if (wake <= now) wake += 86400;
      stats.deferred++; seen.delete(e.key);
      console.log(fmt(now) + " " + e.key + " quiet hours for " + e.user + ", deferred to " + fmt(wake));
      schedule(wake, { type: "event", e });
      continue;
    }
    if (e.kind === "marketing") {
      const n = marketingSent.get(e.user) || 0;
      if (n >= MARKETING_CAP) { stats.capped++; console.log(fmt(now) + " " + e.key + " " + e.user + " hit the marketing cap of " + MARKETING_CAP); continue; }
      marketingSent.set(e.user, n + 1);
    }
    for (const ch of p.channels) schedule(now, { type: "send", ch, e, attempt: 1 });
  } else {
    const { ch, e, attempt } = task;
    const result = provider(ch, e.user);
    if (result === "ok") { stats.sent++; console.log(fmt(now) + " " + ch + " -> " + e.user + " " + e.key + " " + e.text + " (attempt " + attempt + ")"); continue; }
    if (attempt >= MAX_ATTEMPTS) { stats.dlq++; console.log(fmt(now) + " " + ch + " -> " + e.user + " " + e.key + " failed " + result + " " + attempt + " times, sent to DLQ"); continue; }
    const wait = BASE_BACKOFF * 2 ** (attempt - 1);
    stats.retries++;
    console.log(fmt(now) + " " + ch + " -> " + e.user + " " + e.key + " failed " + result + ", retry in " + wait + "s");
    schedule(now + wait, { type: "send", ch, e, attempt: attempt + 1 });
  }
}
console.log("== summary ==");
console.log(Object.entries(stats).map(([k, v]) => k + " " + v).join(", "));