PolyglotDB

Demonstrates real-world backend architecture using multiple databases in a single system

A single Express.js API powered by multiple databases, each chosen deliberately based on its strengths.

Built to understand database design decisions from first principles, not just syntax.

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Core Idea

Different problems require different data models.

Instead of forcing one database to do everything, this system uses polyglot persistence —
choosing the right database for each use case.

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Tech Stack & Responsibilities

Database	Responsibility	Why
PostgreSQL	Users, Orders, Payments	ACID transactions, strong consistency
MongoDB	Product Catalog	Flexible schema
Redis	Sessions, Caching	TTL-based expiry, ultra-fast
Cassandra	Activity Logs	High write throughput
Neo4j	Recommendations	Graph traversal

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Key Insight

"What are your access patterns?"

• Transactions → PostgreSQL
• Flexible data → MongoDB
• Speed → Redis
• High writes → Cassandra
• Relationships → Neo4j

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API Endpoints

Endpoint	Database	Description
POST /auth/login	Redis	Session with TTL
GET /users/:id	PostgreSQL	Indexed read
GET /products	MongoDB	Flexible schema
POST /orders	Multi-DB	Transaction + logs + graph
POST /activity	Cassandra	Event logging
GET /recommend/:userId	Neo4j	Graph traversal

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Example: Multi-Database Flow

When placing an order:

• PostgreSQL → transaction
• Cassandra → logging
• Neo4j → relationship update

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API Screenshots

Login (Redis)

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User (PostgreSQL)

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Products (MongoDB)

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Activity (Cassandra)

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Full API Documentation

APIs

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Full Documentation

Explore complete API flows, screenshots, and explanations:

🔗 Documentation

Deep Dive Blog Series

This project was built in phases — each database explored in depth:

1. 🔗 https://satyam-dev-7.vercel.app/blog/why-i-built-one-app-with-six-databases
2. 🔗 https://satyam-dev-7.vercel.app/blog/postgresql-indexes-deep-dive-polyglotdb-phase-1
3. 🔗 https://satyam-dev-7.vercel.app/blog/acid-transactions-postgresql-polyglotdb-phase-1
4. 🔗 https://satyam-dev-7.vercel.app/blog/redis-is-not-a-database-its-a-decision
5. 🔗 https://satyam-dev-7.vercel.app/blog/why-mongodb-exists-and-when-its-the-wrong-choice
6. 🔗 https://satyam-dev-7.vercel.app/blog/cassandra-thinking-in-queries-not-tables-polyglotdb

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Running Locally

docker compose up -d
node src/index.js