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.

How do you design a High-Scale URL Shortener (TinyURL)? — System Design Interview Answer & Trade-offs

Architectural Breakdown & AnalysisFAANG Aligned

Designing a URL shortener requires handling high read-to-write ratios (100:1) with sub-10ms redirection latencies.

1. Short Key Generation: Base62 Encoding

Convert an auto-incrementing 64-bit integer ID into a 7-character string using Base62 ([a-z, A-Z, 0-9]): $62^7 \approx 3.52 \text{ trillion unique URLs}$

2. Preventing Counter Bottlenecks

Instead of hitting a central DB counter per request, pre-allocate ranges of IDs (e.g. Node 1 gets IDs 1–1,000,000, Node 2 gets 1,000,001–2,000,000) to key generator instances.

3. Redirection Flow & Caching

HTTP 301 (Permanent Redirect): Browser caches redirection locally. Reduces server load, but hides click analytics.
HTTP 302 (Found / Temporary): Every click hits the URL shortener service, updating analytics counters while reading destination URLs from Redis.

TIP — Scaling Cache

Cache the top 20% hottest destination URLs in Redis to achieve 99% read cache hit ratios.

Key Architectural Comparison

Redirection Strategy	Browser Caching Behavior	Analytics Capture Accuracy	Server Load Impact
HTTP 301 Moved Permanently	Browser caches destination URL permanently.	Poor (Cached clicks bypass URL shortener).	Lowest (Reduces server traffic by 80%).
HTTP 302 Found (Temporary)	Browser queries URL shortener on every click.	100% Exact click count and telemetry.	Moderate (Every click hits shortener backend).

How do you design a High-Scale URL Shortener (TinyURL)?

1. Short Key Generation: Base62 Encoding

2. Preventing Counter Bottlenecks

3. Redirection Flow & Caching

Key Architectural Comparison

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