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 Multi-Region Active-Active Disaster Recovery Architecture? — System Design Interview Answer & Trade-offs

Architectural Breakdown & AnalysisFAANG Aligned

Active-Active multi-region deployments run live application stacks in two or more geographic regions simultaneously.

Recovery Metrics

RPO (Recovery Point Objective): Maximum acceptable data loss duration during failover.
RTO (Recovery Time Objective): Maximum acceptable system downtime duration.

Architecture Challenges

Global Traffic Routing: Use Route53 / Cloudflare Latency-Based DNS to route users to the nearest healthy region.
Cross-Region Replication: Replicate database writes asynchronously across regions. Set up Conflict-Free Replicated Data Types (CRDTs) or last-write-wins to resolve write collisions.
Split-Brain Mitigation: Deploy a third tie-breaker quorum region to prevent both regions from declaring master status independently.

WARNING — Active-Active Complexity

Active-Active data replication incurs high operational complexity. Many systems opt for Active-Passive with automated database failover.

Key Architectural Comparison

Disaster Recovery Strategy	RTO (Recovery Time)	RPO (Data Loss)	Cost & Complexity
Backup & Restore	Hours to Days	Hours	Lowest
Active-Passive (Pilot Light)	10–30 minutes	Minutes	Moderate
Active-Passive (Warm Standby)	Under 5 minutes	Sub-minute	High
Active-Active (Multi-Region)	Near Zero (Immediate failover)	Zero to Sub-second	Highest (Requires CRDTs & multi-region consensus)

How do you design a Multi-Region Active-Active Disaster Recovery Architecture?

Recovery Metrics

Architecture Challenges

Key Architectural Comparison

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