Never Charge the Same Customer Twice (Idempotency)

// Standard pattern: client sends an idempotency-key header per intent
// Server checks if this key has been seen; if so, return cached response.

import { Request, Response } from "express";

const KEY_TTL_MS = 24 * 60 * 60 * 1000; // 24 hours

interface IdempotencyEntry {
  status: number;
  body: object;
  createdAt: number;
}

const cache = new Map<string, IdempotencyEntry>(); // → Redis in prod

async function chargeWithIdempotency(req: Request, res: Response) {
  const key = req.header("Idempotency-Key");
  if (!key) return res.status(400).json({ error: "Idempotency-Key required" });

  // Check cache
  const cached = cache.get(key);
  if (cached && Date.now() - cached.createdAt < KEY_TTL_MS) {
    return res.status(cached.status).json(cached.body);
  }

  // Reserve the slot (mark as "in progress" to prevent concurrent dupes)
  cache.set(key, { status: 202, body: { status: "pending" }, createdAt: Date.now() });

  try {
    // The actual money operation
    const charge = await stripe.paymentIntents.create({
      amount: req.body.amount,
      currency: req.body.currency,
      payment_method: req.body.payment_method,
    }, { idempotencyKey: key }); // Pass it down to Stripe too

    cache.set(key, { status: 200, body: charge, createdAt: Date.now() });
    return res.status(200).json(charge);
  } catch (err) {
    // Don't cache 5xx errors — let the client retry safely
    cache.delete(key);
    throw err;
  }
}

// Key rules:
//   - Client generates key (UUID v4 or similar); never server-generated
//   - Key valid for at least 24 hours
//   - Same key + same request body → same response
//   - Same key + DIFFERENT request body → 409 Conflict
//   - In Redis or DB, NOT in-memory (survives restarts)