Payment Gateway Expert / Ahli Payment Gateway

English | Bahasa Indonesia

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English

Description

Expert guide for integrating major payment gateways (Stripe, PayPal, Xendit, Midtrans, DOKU SNAP BI) into modern SaaS platforms. Covers checkout flows, secure raw-body webhook handling, atomic idempotency to prevent race conditions, subscription state machines, and local database synchronization.

Core Principles

• Security First: Always validate webhook signatures using the raw, unparsed request body before processing any payment event. Never trust client-side data for prices or payment status. For Indonesian gateways (DOKU, Midtrans, Xendit), accurately apply the required signature protocol: DOKU Checkout uses HMAC-SHA256 with body Digest, while SNAP BI Direct APIs enforce HMAC-SHA512 with ISO8601 timestamps.
• Idempotency & Race Condition Prevention: Implement idempotency keys for all payment creation requests. For webhook ingestion, DO NOT rely solely on findUnique checks, as concurrent webhook retries cause race conditions. Always use Atomic Updates (UPDATE ... WHERE status = 'PENDING') or database pessimistic locks (SELECT ... FOR UPDATE).
• Raw Body Ingestion: Webhook signature verification fails if JSON is parsed or re-serialized with modified key ordering. Extract the exact raw string buffer (req.text() in Fetch API or express.raw({ type: 'application/json' }) in Express).
• Early 200 OK Acknowledgment: If a webhook event is verified but already processed, immediately return 200 OK so the payment gateway halts retries.
• State Synchronization: Ensure the local database (e.g., PostgreSQL, Supabase via Drizzle/Prisma) is updated transactionally upon receiving successful webhook events.
• Subscription Management: Map provider subscription statuses (trialing, active, past_due, canceled) accurately to internal SaaS state machines.

Implementation Checklist

• Create dedicated raw Webhook endpoint (e.g., /api/webhooks/stripe, /api/webhooks/doku/checkout).
• Use raw request body string for signature verification (never parsed JSON).
• Verify signature using proper algorithm (HMAC-SHA256 + Digest for DOKU Checkout, Stripe, PayPal; HMAC-SHA512 for SNAP BI Direct API).
• Implement Atomic Update idempotency (WHERE status = 'PENDING') to eliminate race conditions.
• Acknowledge duplicate webhooks immediately with HTTP 200 OK.
• Offload heavy post-payment operations (invoicing, emails, webhooks) to background job queues (BullMQ/Temporal).

Example: SNAP BI Webhook & Atomic Idempotency (Next.js App Router)

import { headers } from 'next/headers';
import crypto from 'crypto';
import { db } from '@/lib/db';

export async function POST(req: Request) {
  const rawBody = await req.text(); // Raw body is mandatory
  const headerList = await headers();
  
  const timestamp = headerList.get('x-timestamp') || '';
  const receivedSig = headerList.get('x-signature') || '';
  const authHeader = headerList.get('authorization') || '';
  const token = authHeader.replace(/^Bearer\s+/i, '');
  const path = new URL(req.url).pathname;

  // 1. SNAP BI Body Hash & Signature Verification (HMAC-SHA512)
  const bodyHash = crypto.createHash('sha256').update(rawBody, 'utf8').digest('hex').toLowerCase();
  const stringToSign = `POST:${path}:${token}:${bodyHash}:${timestamp}`;
  const calculatedSig = crypto
    .createHmac('sha512', process.env.DOKU_SECRET_KEY!)
    .update(stringToSign, 'utf8')
    .digest('base64');

  const isValid = crypto.timingSafeEqual(Buffer.from(receivedSig), Buffer.from(calculatedSig));
  if (!isValid) {
    return new Response('Invalid Signature', { status: 401 });
  }

  const payload = JSON.parse(rawBody);
  const invoiceNumber = payload.order?.invoice_number;

  // 2. Atomic Update: Eliminates Race Conditions by locking on 'PENDING' status
  const updated = await db.transaction.updateMany({
    where: {
      invoiceNumber: invoiceNumber,
      status: 'PENDING', // Key race condition prevention gate
    },
    data: {
      status: 'PAID',
      paidAt: new Date(),
      externalReference: payload.transaction?.original_reference_no,
    },
  });

  // 3. Duplicate Webhook Handling (Idempotency)
  if (updated.count === 0) {
    // Already processed or invalid invoice — return 200 to halt gateway retry spam
    return new Response(JSON.stringify({ status: 'ALREADY_PROCESSED' }), { status: 200 });
  }

  // 4. Fulfillment (Execute only once)
  await grantUserSubscription(invoiceNumber);

  return new Response(JSON.stringify({ status: 'SUCCESS' }), { status: 200 });
}

async function grantUserSubscription(invoiceNumber: string) {
  // Safe business logic execution
}

Orchestration & Integration

• Integrates with: saas-billing, doku-payment-gateway, doku-mcp-server, async-queue-temporal-expert, database-orm-expert.

Trigger Conditions

Active whenever the user is working on billing integration, payment checkout, webhook handling, or integrating platforms like PayPal, Stripe, Xendit, Midtrans, or DOKU SNAP BI.

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Bahasa Indonesia

Deskripsi

Panduan ahli untuk mengintegrasikan payment gateway utama (Stripe, PayPal, Xendit, Midtrans, DOKU SNAP BI) ke platform SaaS modern. Mencakup alur checkout, penanganan webhook raw-body aman, pencegahan race condition melalui atomic update, state machine langganan, dan sinkronisasi database lokal.

Prinsip Utama

• Keamanan Utama: Selalu validasi signature webhook menggunakan raw request body murni sebelum memproses event pembayaran. Jangan pernah mempercayai data dari sisi klien untuk harga atau status pembayaran. Untuk gateway Indonesia (DOKU, Midtrans, Xendit), gunakan algoritma yang tepat: DOKU Checkout menggunakan HMAC-SHA256 dengan Digest body, sedangkan Direct API SNAP BI menggunakan HMAC-SHA512 dengan timestamp ISO8601.
• Idempotensi & Pencegahan Race Condition: Implementasikan kunci idempotensi untuk semua pembuatan pembayaran. Pada penerimaan webhook, JANGAN hanya mengandalkan pengecekan findUnique, karena panggilan webhook paralel dari gateway dapat memicu race condition. Selalu gunakan Atomic Update (UPDATE ... WHERE status = 'PENDING') atau database lock (SELECT ... FOR UPDATE).
• Raw Body Webhook: Verifikasi tanda tangan akan gagal jika JSON di-parse atau di-serialize ulang karena perubahan urutan key atau spasi. Ambil buffer teks mentah (req.text() pada Fetch API atau express.raw({ type: 'application/json' }) di Express).
• Balasan Cepat 200 OK: Jika signature valid namun transaksi sudah berstatus lunas (webhook duplikat/retry), segera kembalikan HTTP 200 OK agar payment gateway berhenti mengirimkan retry.
• Sinkronisasi State: Pastikan database lokal (PostgreSQL/Supabase via Drizzle/Prisma) diperbarui secara transaksional saat menerima webhook sukses.
• Manajemen Langganan: Petakan status langganan dari provider (trialing, active, past_due, canceled) secara akurat ke state machine internal SaaS.

Checklist Implementasi

• Buat endpoint Webhook raw khusus (misal: /api/webhooks/stripe, /api/webhooks/doku/checkout).
• Gunakan raw request body string untuk verifikasi signature (jangan parse JSON sebelum verifikasi).
• Verifikasi signature dengan algoritma yang tepat (HMAC-SHA256 + Digest untuk DOKU Checkout, Stripe, PayPal; HMAC-SHA512 untuk SNAP BI Direct API).
• Terapkan Atomic Update (WHERE status = 'PENDING') untuk mematikan peluang race condition.
• Balas webhook duplikat secara instan dengan HTTP 200 OK.
• Lemparkan proses berat pasca-bayar (pembuatan invoice PDF, email, push notification) ke antrean latar belakang (BullMQ/Temporal).

Integrasi Orkestrasi

• Terintegrasi dengan: saas-billing, doku-payment-gateway, doku-mcp-server, async-queue-temporal-expert, database-orm-expert.

Kondisi Pemicu

Aktif setiap kali pengguna sedang mengerjakan integrasi billing, checkout pembayaran, penanganan webhook, atau mengintegrasikan platform seperti PayPal, Stripe, Xendit, Midtrans, atau DOKU SNAP BI.