Databases

Tables, schemas and ACID on a single primary — when B+ trees and strong guarantees beat distributed stores

How databases physically store and retrieve data — storage engine internals

Read committed, repeatable read, serializable — and dirty reads, phantom reads

Single-leader, multi-leader, and leaderless replication for high availability

Functional partitioning by domain — splitting users, products, orders into separate databases

Range, hash, and directory-based sharding for multi-tenant large-scale systems

Shared schema, separate schemas, and row-level isolation for multi-tenant SaaS

Copying data to avoid joins, read amplification, write herding and when a cache is better than duplication

Explain plans, covering indexes, sargability and how the wrong index hurts write-heavy tables