Data & Storage

Where state lives, how it stays correct, and why the query got slow.

14 concepts · 580 minutes

• SQL BasicsSELECT, WHERE, GROUP BY — declaring what you want and letting the planner find it.
• Validation & InjectionNever trust the client — schema validation at the boundary, parameterised queries below it.
• ORMs & Query BuildersConvenience with a leak — you still have to know the SQL it writes for you.
• Joins & RelationshipsInner, left, and the one that silently duplicates all your rows.
• Caching with RedisAn in-memory store used as cache, session bag, lock, counter, and queue.
• SQL vs NoSQLAccess patterns decide the store — not fashion, and not scale you don't have.
• IdempotencyMaking 'do it again' safe — the single idea that separates reliable distributed code from hope.
• Schema MigrationsChanging a schema under live traffic without taking the site down.
• Indexes & Query PlansWhy one query is 2ms and the identical-looking one is 8 seconds — read the plan.
• The N+1 ProblemOne query becomes 501 — the most common performance bug in application code.
• Data ModellingNormalise until it hurts, denormalise until it works — and know which one you're doing.
• Cache InvalidationTTLs, tags, stale-while-revalidate — the genuinely hard half of caching.
• Transactions & ACIDAll or nothing — plus the isolation levels that decide which anomalies you're allowing.
• Race ConditionsTwo things interleaving in an order you never tested — the bug class that only shows up in production.