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 do you choose Eventual Consistency vs. Strong Consistency (PACELC Theorem)? — System Design Interview Answer & Trade-offs

Architectural Breakdown & AnalysisFAANG Aligned

The CAP Theorem proves that during a network partition (P), a distributed system must choose between Availability (A) and Consistency (C).

The PACELC Theorem Extension

PACELC extends CAP by incorporating normal operating conditions when no network partition exists:

If Partition (P): Choose between Availability (A) vs. Consistency (C).
Else (E): Choose between Latency (L) vs. Consistency (C).

System Classifications

PA/EL (DynamoDB, Cassandra): Prioritizes Availability during partitions and low Latency during normal operations. Reads may return stale data briefly.
PC/EC (Spanner, Relational DBs): Prioritizes Consistency at all times. Will reject writes/reads or wait for global consensus before returning.

NOTE — Financial vs Social Systems

Financial balances require PC/EC (Strong Consistency). Social media likes and view counters leverage PA/EL (Eventual Consistency).

Key Architectural Comparison

PACELC Model	If Partitioned ($P$) Choose:	Else ($E$) Choose:	Concrete Database Example
PC / EC	Consistency ($C$) over Availability.	Consistency ($C$) over Latency.	Google Cloud Spanner, CockroachDB
PC / EL	Consistency ($C$) over Availability.	Latency ($L$) over Consistency.	PostgreSQL / MySQL with Async Read Replicas
PA / EL	Availability ($A$) over Consistency.	Latency ($L$) over Consistency.	Apache Cassandra, Amazon DynamoDB, Couchbase
PA / EC	Availability ($A$) over Consistency.	Consistency ($C$) over Latency.	VoltDB, BigTable (Tunable configurations)

When do you choose Eventual Consistency vs. Strong Consistency (PACELC Theorem)?

The PACELC Theorem Extension

System Classifications

Key Architectural Comparison

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