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.

What are the trade-offs of Layer 4 vs Layer 7 Load Balancing (HAProxy vs Envoy)? — System Design Interview Answer & Trade-offs

Architectural Breakdown & AnalysisFAANG Aligned

Load balancers operate at different OSI layers, balancing packet processing speed against application routing intelligence.

Layer 4 Load Balancing (Transport Layer)

Routes TCP/UDP packets without reading application payloads.
Pros: Blazing fast throughput, minimal CPU usage, handles high connection counts.
Cons: Cannot inspect HTTP headers, cookies, or URL paths. No smart routing.

Layer 7 Load Balancing (Application Layer)

Terminates TLS, parses HTTP headers, cookies, and JSON URIs.
Pros: Intelligent path routing (/api/v1/orders → Order Pods), header rewrite, gRPC balancing.
Cons: Higher CPU overhead due to packet decryption and HTTP parsing.

NOTE — Production Hierarchy

Place a Layer 4 Load Balancer (AWS NLB / HAProxy) at the edge for TCP scale, pointing to Layer 7 Proxies (Envoy / Nginx) for intelligent application routing.

Key Architectural Comparison

Load Balancer Tier	Operating OSI Layer	Routing Decisions Based On	Throughput / Overhead	SSL / TLS Termination
Layer 4 (L4 / NLB)	Transport Layer (TCP / UDP).	IP address & Port number.	Maximum (Zero packet inspection).	Passthrough without decryption.
Layer 7 (L7 / ALB / Envoy)	Application Layer (HTTP / gRPC).	URL Path, Headers, Cookies, JSON body.	Moderate (Requires full HTTP/2 framing).	Full TLS offload & header inspection.

What are the trade-offs of Layer 4 vs Layer 7 Load Balancing (HAProxy vs Envoy)?

Layer 4 Load Balancing (Transport Layer)

Layer 7 Load Balancing (Application Layer)

Key Architectural Comparison

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