Relational vs non-relational, caching strategies, sharding, and the trade-offs behind every data layer decision.
SQL vs NoSQL, ACID properties, replication, and indexes — the foundations of database design.
Cache layers, strategies, eviction policies, invalidation approaches, and Redis vs Memcached.
Horizontal partitioning strategies, the challenges of cross-shard queries, and how consistent hashing minimises data movement when the cluster changes.