After building three different data marketplaces over the past two years, I noticed a pattern: every project spent 4-6 weeks rebuilding the same infrastructure before we could focus on the actual product. Payment processing, token encryption, access control, concurrency handling - it’s the same problems over and over.

So I built UDAM (Unified Data Marketplace) - an open-source backend that provides all this infrastructure out of the box. In this article, I’ll share the technical decisions, challenges, and solutions that went into it.

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The Problem Space

Building a marketplace seems straightforward until you start coding:

• Payments: Integrating Stripe, handling webhooks, managing checkout sessions
• Security: Encrypting API keys, managing sessions, preventing attacks
• Concurrency: Preventing overselling when multiple buyers compete for the last unit
• Access Control: Issuing tokens, managing permissions, handling revocation
• Testing: Ensuring everything works reliably in production

Each of these is a mini-project in itself. UDAM solves all of them.

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Architecture Overview

UDAM follows a classic three-tier architecture, but with specific design choices optimized for marketplace needs:

┌─────────────┐      ┌──────────────┐      ┌─────────────┐
│   Frontend  │─────▶│   Backend    │─────▶│  PostgreSQL │
│  (Next.js)  │      │  (Node.js)   │      │   Database  │
└─────────────┘      └──────────────┘      └─────────────┘
│
▼
┌──────────────┐
│    Stripe    │
│   Payments   │
└──────────────┘

Tech Stack Choices

Backend: Node.js + Express

• Fast iteration for marketplace logic
• Excellent Stripe SDK support
• Large ecosystem for future extensions

Database: PostgreSQL

• ACID transactions (critical for payments)
• Row-level locking (prevents race conditions)
• Mature, battle-tested in production

Frontend: Next.js

• SSR capability for SEO (marketplace discoverability)
• Intentionally minimal - easy to customize
• API-first design

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Deep Dive: Token Encryption

One of the most critical features is secure storage of API keys and credentials.

The Challenge

Sellers provide API keys that buyers need to access their services. These keys must be:

1. Encrypted at rest (database compromise shouldn’t expose keys)
2. Decryptable for legitimate buyers (they need the actual key)
3. Never logged or cached in plaintext

The Solution: AES-256-GCM

const crypto = require('crypto');

function encryptToken(apiKey, masterKey) {
const iv = crypto.randomBytes(16);
const cipher = crypto.createCipheriv('aes-256-gcm', masterKey, iv);

let encrypted = cipher.update(apiKey, 'utf8', 'hex');
encrypted += cipher.final('hex');

const authTag = cipher.getAuthTag();

return {
encrypted,
iv: iv.toString('hex'),
authTag: authTag.toString('hex')
};
}

Key Points:

• GCM mode: Provides both encryption and authentication
• Random IV: Each encryption uses a unique initialization vector
• Auth tag: Detects tampering attempts
• Master key: Stored securely in environment variables (never in code)

This approach means even if someone gets database access, they can’t decrypt the API keys without the master key.

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Concurrency Control: The Overselling Problem

Here’s a scenario that will break naive implementations:

• T=0: Listing has 1 unit available at $10
• T=1: Buyer A starts purchase
• T=2: Buyer B starts purchase (sees 1 unit still available)
• T=3: Buyer A completes purchase (units → 0)
• T=4: Buyer B completes purchase (units → -1) ❌ OVERSOLD!

The Solution: Row-Level Locking

PostgreSQL’s FOR UPDATE clause is our hero:

BEGIN;

-- Lock the row for this transaction
SELECT * FROM listings
WHERE id = $1
FOR UPDATE;

-- Check availability
IF available_units >= units_requested THEN
UPDATE listings
SET available_units = available_units - $2
WHERE id = $1;

INSERT INTO orders (...) VALUES (...);
END IF;

COMMIT;

How it works:

1. FOR UPDATE locks the row until transaction completes
2. Other transactions wait for the lock to be released
3. Only one transaction can decrement units at a time
4. Impossible to oversell

We verified this works under load with a CI test that spawns 5 concurrent purchase attempts for 3 available units - exactly 3 orders succeed, 2 fail with "insufficient units".

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Payment Flow: Instant vs. Stripe Checkout

To optimize user experience, we support two payment flows:

Instant Token Issuance (Small Orders)

For orders under a configurable threshold (e.g., $5):

1. Order is created
2. Tokens are issued immediately
3. No payment confirmation needed

Why? For small amounts, the friction of Stripe checkout hurts conversion more than the risk of fraud.

Stripe Checkout (Large Orders)

For orders above the threshold:

1. Create Stripe Checkout session
2. Redirect user to Stripe
3. Webhook confirms payment
4. Tokens issued after confirmation

Implementation:

if (totalPrice <= SMALL_LIMIT) {
// Instant issuance
await issueTokens(orderId);
return { payment_requires_confirmation: false };
} else {
// Stripe checkout
const session = await stripe.checkout.sessions.create({
payment_method_types: ['card'],
line_items: [{
price_data: {
currency: 'usd',
product_data: { name: listing.service_name },
unit_amount: Math.round(totalPrice * 100)
},
quantity: 1
}],
mode: 'payment',
success_url: SUCCESS_URL,
cancel_url: CANCEL_URL
});

return {
payment_requires_confirmation: true,
stripe_checkout_url: session.url
};
}

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Session Management: Security Without Overhead

Many marketplace tutorials use JWT, but we chose session-based auth:

Advantages:

• Instant revocation (crucial for logout)
• Server-side control
• No token expiration edge cases

Implementation:

// Login creates session
const sessionToken = crypto.randomBytes(32).toString('hex');
await db.query(
'INSERT INTO sessions (user_id, session_token, expires_at) VALUES ($1, $2, $3)',
[userId, sessionToken, new Date(Date.now() + 7 * 24 * 60 * 60 * 1000)]
);

// Middleware validates session
async function requireAuth(req, res, next) {
const token = req.headers.authorization?.replace('Bearer ', '');
const session = await db.query(
'SELECT * FROM sessions WHERE session_token = $1 AND expires_at > NOW()',
[token]
);

if (!session.rows[0]) {
return res.status(401).json({ error: 'Unauthorized' });
}

req.userId = session.rows[0].user_id;
next();
}

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Testing: CI/CD for Critical Flows

We have comprehensive E2E tests running on every push:

- name: E2E small-limit flow
run: |
TOKEN=$(curl -X POST /auth/login ...)
LISTING_ID=$(curl -X POST /listings ...)
ORDER=$(curl -X POST /orders ...)
TOKENS=$(curl /tokens ...)

What we test:

• ✅ Full purchase flow (login → create listing → buy → get tokens)
• ✅ Session revocation (logout → can’t access protected routes)
• ✅ Concurrency (5 simultaneous purchases for 3 units)
• ✅ Payment webhooks (in dev mode)

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Performance Considerations

Database Indexes

Critical indexes for marketplace queries:

CREATE INDEX idx_listings_status ON listings(status);
CREATE INDEX idx_orders_buyer ON orders(buyer_id);
CREATE INDEX idx_tokens_buyer ON tokens(buyer_id);
CREATE INDEX idx_sessions_token ON sessions(session_token);

Query Optimization

The token retrieval query joins three tables efficiently:

SELECT
t.*,
l.service_name,
pgp_sym_decrypt(t.encrypted_access_token, $2) as decrypted_token
FROM tokens t
JOIN listings l ON t.listing_id = l.id
WHERE t.buyer_id = $1
ORDER BY t.created_at DESC;

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Lessons Learned

What Worked Well

1. Row-level locking: Zero overselling issues in production
2. AES-256-GCM: Provides both security and integrity
3. Instant vs. Stripe: Improved conversion for small orders
4. API-first design: Easy to build any frontend

What I’d Do Differently

1. Add caching earlier: Listing browsing could be cached aggressively
2. Rate limiting: Should have been built-in from day one
3. Better logging: More structured logs for debugging
4. GraphQL option: Some users want more flexible queries

Common Pitfalls to Avoid

1. Don’t store API keys in plaintext (even with "secure" database access)
2. Don’t skip transaction isolation (race conditions WILL happen)
3. Don’t trust client-side validation (always validate server-side)
4. Don’t hardcode secrets (environment variables from day one)

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What’s Next

UDAM is open source (MIT license) and actively maintained. We’re looking for contributors in:

• UI/UX: Creating beautiful frontend templates
• Integrations: Additional payment providers, OAuth, analytics
• Security: Audits, penetration testing, best practices
• Documentation: Deployment guides, tutorials, translations

Check it out:

• GitHub Repository
• API Documentation
• Architecture Details

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Conclusion

Building marketplace infrastructure is complex, but it doesn’t need to be custom every time. UDAM provides a production-ready foundation so you can focus on your actual product - the data services you’re selling.

Whether you use UDAM directly or just learn from the architecture, I hope this article helps you build better marketplaces faster.

Questions? Feedback? Drop a comment or open a GitHub issue. I’m always happy to discuss marketplace architecture!

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