Why a single database eventually stops being enough, how sharding splits data across multiple nodes, and why consistent hashing is the elegant solution to the painful problem of adding or removing shards.

What CQRS is and why it separates reads from writes, what Event Sourcing means when you store events instead of state, how they fit together, and the real tradeoffs before you reach for either pattern.

Why Kafka partitions become unequal, what happens when one partition hogs all the work, and the concrete strategies teams use to fix hot keys, rebalance load, and prevent consumer starvation.

What Kafka solves when many things happen at once, how streams, topics, partitions, producers, and consumer groups fit together, and why it stays fast, ordered, and durable.

Monolith and microservice fundamentals with pros and cons, how they differ, the role of an API gateway, OAuth vs OpenID Connect, and a concise introduction to event-driven architecture.

Why basic TTL isn't enough, how expiry causes traffic spikes, and practical strategies: jitter, early expiration, mutex locking, SWR, and cache warming, with clear tradeoffs and when to use which.

What the thundering herd problem is, where it occurs, its impact on systems, and practical techniques to prevent or reduce it, with a focus on understanding system behavior and interview relevance.