ACID Transactions
ACID is the vocabulary for what a database promises about a transaction: atomicity, consistency, isolation, durability. Covers tunable isolation levels too.
SQL vs NoSQL
SQL enforces a fixed schema and relational integrity; NoSQL trades that away for flexibility and easier horizontal scale. Neither one is just 'better.'
Database Indexes
A database index turns an O(n) full table scan into an O(log n) lookup, at the cost of slower writes. Covers B-trees, selectivity, and composite indexes.
Database Sharding
Sharding splits one database across many machines by a shard key. Covers range vs hash-based sharding and what cross-shard queries actually cost.
Data Replication
Replication keeps copies of the same data on multiple machines for read scale and failover. Covers leader-based, synchronous vs async replication.
Database Scaling
Scaling a database follows a real order: indexes first, then vertical scaling, read replicas, caching, and only then sharding. Skipping ahead is a tell.
Database Types
Relational, document, key-value, graph, time-series, search — each database type is fast at exactly the access pattern it was built for, and slow elsewhere.
Bloom Filters
A Bloom filter answers 'definitely not in the set' cheaply, with occasional false positives but never a false negative — great for skipping expensive lookups.
Database Architectures
A database architecture is the full topology — how many nodes, who accepts writes, how they sync. Covers single-leader, multi-leader, and leaderless.