Databases

Database fundamentals: transactions, consistency guarantees, and the concepts that hold up regardless of which specific database you're using.

Articles

ACID — Atomicity, Consistency, Isolation, and Durability, plus isolation levels and the phenomena they prevent.
Idempotency — why retries need it, and the standard techniques (idempotency keys, upserts, compare-and-swap) for getting it.
Redis as a Caching Layer — key design, TTLs and eviction policies, cache-aside/write-through/write-behind, cache stampedes, and invalidation.
SQL vs. NoSQL: PostgreSQL, MySQL, and MongoDB — schema-on-write vs. schema-on-read, transaction support, relationship modeling, and a focused Postgres-vs-MySQL comparison.
Locking, Deadlocks, and Concurrency — pessimistic vs. optimistic locking, row vs. table locks, and how databases detect and break deadlocks.
Query Performance Bottlenecks — reading EXPLAIN output, when indexes help, and common slow-query anti-patterns.
SQLAlchemy 2.0 and Alembic Migrations — the 2.0 declarative style, autogeneration's real limitations, and managing migrations safely.
Zero-Downtime Migrations — the expand/contract pattern for schema changes that survive a rolling deploy.

Learned something about databases worth keeping — a concept, a gotcha, a tuning tip? Add it here as its own file. See Contributing for the how-to.