System Design Interview Questions

Clarify functional and non-functional requirements and scale, do rough capacity estimates, define the APIs and data model, sketch a high-level architecture, then deep-dive components and discuss trade-offs and bottlenecks.

Vertical scaling means a bigger machine — simple but has a ceiling and a single point of failure. Horizontal scaling adds more machines — it scales much further but needs load balancing and stateless services.

It distributes incoming traffic across multiple servers for availability and scale, using strategies like round-robin or least-connections, and typically does health checks and can terminate TLS.

Use SQL for structured data, complex queries, and strong transactional consistency. Use NoSQL (document, key-value, wide-column) for flexible schemas, very high write throughput, and easy horizontal scaling. Decide by access patterns.

Caching stores frequent results closer to the user — browser, CDN, in-memory stores like Redis, or a query cache — to cut latency and load. The hard part is invalidation, so discuss TTLs and staleness.

Replication copies the same data to multiple nodes for read scaling and availability; sharding partitions data across nodes by a key for write and storage scaling. Large systems often use both together.

During a network partition you can guarantee only two of Consistency, Availability, and Partition tolerance. In practice you trade consistency against availability, giving CP or AP systems.

Remove single points of failure with redundancy, load balancing, and replication across zones; add health checks, automatic failover, and graceful degradation; and measure against SLAs.

Use an algorithm like token bucket or sliding window, enforced per user or API key at the gateway, backed by a fast store such as Redis, and return HTTP 429 when the limit is exceeded.

A queue like Kafka, RabbitMQ, or SQS decouples producers from consumers, absorbs traffic spikes, and enables asynchronous processing and retries — improving resilience and scalability.