Tutorials System Design Tutorial
Cache Invalidation in Real Systems — Complete Guide
Cache Invalidation in Real Systems — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of System Design Tutorial on Toolliyo Academy.
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System Design Tutorial · Lesson 34 of 100
Cache Invalidation in Real Systems
Basics → Scale → Interview
Basics · 1 — Building blocks · ~6 min · Module 4: Caching and Storage
What is this?
Invalidation decides when cached data dies — TTL, explicit delete, version bumps, or event-driven busts.
Why should you care?
ShopNest cannot show yesterday’s stock as “in stock” forever after a sell-out.
See it live — copy this example
Sketch the architecture on paper. These lessons focus on concepts and trade-offs.
On stock change event:
DEL product:42
DEL product:42:card
TTL backup: 30–120s even if events miss
Versioned keys: product:42:v17 (bump v on write)Run Example »
This lesson uses terminal or setup steps. Run commands on your computer — the live editor appears on coding lessons.
What happened?
- Event-driven deletes are precise; TTL is the safety net.
- Versioned keys avoid racing old writers.
- Invalidation is famously hard — keep the strategy simple.
Practice next
- Publish stock_changed for ShopNest SKUs.
- Delete related cache keys in the consumer.
- Keep a short TTL anyway.
- Invalidate on price and stock events only.
- Measure stale complaint rate from support.
Remember
Events + TTL together. Version keys when useful. Keep key layouts boring and documented.
Stock event busts cache
Inventory service emits stock_changed; cache listener deletes keys.
Outcome: Sold-out items stop appearing in stock quickly.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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