Idempotency

import express from "express";
const app = express();
app.use(express.json());

// Mock Database table for idempotent requests
// Schema: { payment_id: string, status: "PROCESSING" | "COMPLETED", result: any }
const idempotencyStore = new Map();

app.post("/api/pay", async (req, res) => {
  const { amount, from_user, to_user } = req.body;
  const payment_id = req.headers["idempotency-key"];

  if (!payment_id) {
    return res.status(400).json({ error: "Idempotency-Key header is required" });
  }

  // 1. Check if the payment_id has been seen before (Generate ID / Check Status)
  if (idempotencyStore.has(payment_id)) {
    const record = idempotencyStore.get(payment_id);

    if (record.status === "COMPLETED") {
      // Return exactly what was returned the first time (Already Completed)
      return res.status(200).json({
        message: "Already completed.",
        data: record.result
      });
    }

    if (record.status === "PROCESSING") {
      // Concurrent request issue (e.g. user double clicked quickly)
      return res.status(409).json({ error: "Request is already being processed." });
    }
  }

  // 2. Mark as processing to handle concurrent identical requests
  idempotencyStore.set(payment_id, { status: "PROCESSING", result: null });

  try {
    // 3. Process the actual payment logic (A -> B, $20,000)
    const result = await processPayment(from_user, to_user, amount);

    // 4. Update the record status to COMPLETED
    idempotencyStore.set(payment_id, { status: "COMPLETED", result: result });

    return res.status(200).json({ message: "Transfer done", data: result });
  } catch (error) {
    // If it fails, remove the key so it can be safely retried
    idempotencyStore.delete(payment_id);
    return res.status(500).json({ error: "Internal processing error." });
  }
});