How do you choose between SQL and NoSQL databases?

Choose SQL (PostgreSQL, MySQL) for structured data requiring ACID transactions, complex JOINs, and strict schema enforcement. Choose NoSQL (MongoDB, DynamoDB, Cassandra) for flexible schemas, massive horizontal scale, high write throughput, and eventual consistency workloads.

When should you choose Vertical Scaling vs Horizontal Scaling?

Start with vertical scaling (adding CPU/RAM to one server) for simplicity. Transition to horizontal scaling (adding more servers) when hardware limits are reached, cost curves become prohibitive, or high availability is required. Horizontal scaling introduces distributed system complexity.

What is the difference between Sharding, Partitioning, and Indexing?

Indexing speeds up read queries within a single table using B-Tree or Hash structures. Partitioning splits a single table into logical physical chunks within one database instance. Sharding distributes rows across multiple distinct database servers using a shard key, scaling write throughput and storage.

How do you scale WebSockets to millions of concurrent connections?

Increase Linux file descriptor limits, use Layer 4 load balancing with sticky sessions, and implement a Redis Pub/Sub backplane so any WebSocket server can relay messages to clients connected to other servers. Pattern: Client → HAProxy → WS Servers ↔ Redis Pub/Sub.

What is the difference between LSI and GSI in DynamoDB?

A Local Secondary Index (LSI) shares the same partition key as the base table but uses a different sort key. It is limited to 10GB per item collection and supports strong consistency. A Global Secondary Index (GSI) uses a completely different partition key, spans all partitions, supports only eventual consistency, and has its own dedicated throughput capacity.

Which database should I use for transactions, user profiles, logs, and analytics?

Use PostgreSQL for financial transactions (ACID compliance). Use MongoDB for user profiles (flexible JSON). Use Cassandra for logs and high-throughput writes (LSM tree). Use Apache Druid for real-time analytics (columnar storage). Use Elasticsearch for search and catalog (inverted index). Use Redis for sessions and caching (in-memory).

How does Consistent Hashing work?

Consistent hashing maps both servers and keys onto a circular ring. A key routes clockwise to the nearest server. When a node is added or removed, only 1/N keys need to migrate (vs nearly 100% in modulo hashing). Virtual nodes (100-200 per server) ensure even distribution and prevent hotspots.

How do you design a distributed rate limiter?

Use a Token Bucket algorithm for burst-friendly limiting. For distributed enforcement across multiple API gateway instances, use Redis with Lua scripts to atomically evaluate and increment counters, preventing race conditions (dirty reads) where concurrent requests could bypass the limit.

When should you choose Long Polling vs Server-Sent Events (SSE) vs WebSockets? — System Design Interview Answer & Trade-offs

Architectural Breakdown & AnalysisFAANG Aligned

Choosing the right real-time mechanism depends on directionality, network environments, and connection lifecycle requirements.

1. Long Polling

Client requests data, server holds connection open until update arrives or timeout occurs.
Best For: Fallback environments with strict corporate HTTP proxies blocking WebSockets.

2. Server-Sent Events (SSE)

Single persistent HTTP/2 connection. Server streams text/event data to client continuously.
Pros: Built-in auto-reconnect, native HTTP/2 support, zero custom proxy configuration.
Best For: AI chat response streaming (ChatGPT), live stock charts, notification feeds.

3. WebSockets

Full-duplex persistent TCP connection for instant bi-directional messaging.
Best For: Real-time multiplayer games, collaborative canvas editors, live chat rooms.

TIP — Modern Rule

Default to SSE for AI text generation streaming and notification updates. Reserve WebSockets when client-to-server real-time push is required.

Key Architectural Comparison

Real-Time Communication	Directionality	Connection State	Server Resource Utilization	Mobile Battery Efficiency
Short Polling	Unidirectional	Stateless	High (Repeated TCP handshakes)	Poor
Long Polling	Unidirectional	Stateful HTTP hold	Moderate	Fair
Server-Sent Events (SSE)	Unidirectional (Server -> Client)	Persistent HTTP/2 stream	Low (Multiplexed)	High
WebSockets	Full-Duplex Bidirectional	Persistent TCP socket	Moderate	Good

When should you choose Long Polling vs Server-Sent Events (SSE) vs WebSockets?

1. Long Polling

2. Server-Sent Events (SSE)

3. WebSockets

Key Architectural Comparison

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