Cache Replacement vs Cache Invalidation
💡 Concept Name
Cache Replacement vs Cache Invalidation – Two distinct strategies used in caching to manage data lifecycle and storage space.
Cache Replacement refers to automatically removing older cache entries to make room for new ones. Cache Invalidation refers to explicitly removing or updating cache when the underlying data changes.
📘 Quick Intro
Cache replacement decides which existing item to remove when the cache is full. Cache invalidation explicitly removes or updates outdated or incorrect items in the cache.
🧠 Analogy / Short Story
Think of a fridge:
- Cache Replacement: Tossing out the oldest food to make room for new groceries.
- Cache Invalidation: Throwing away expired or spoiled items to avoid using bad data.
🔧 Technical Explanation
- 🧠 Cache Replacement: Uses algorithms like LRU, LFU, FIFO to remove least useful data when full.
- ♻️ Cache Invalidation: Happens when the source data changes, requiring cached data to be updated or deleted.
- 🧱 Replacement is based on space; invalidation is based on data correctness.
- 🛠️ Both help maintain cache efficiency and accuracy.
🎯 Purpose & Use Case
- ✅ Use cache replacement to manage limited memory effectively.
- ✅ Use cache invalidation to ensure up-to-date data (e.g., after a database update).
- ✅ Often both are used together in real systems.
🔍 Key Differences
- 📦 Replacement happens due to space constraints, often using policies like LRU or FIFO.
- ❌ Invalidation is triggered manually or by logic when cached data is outdated.
- 🎯 Goal of Replacement: Maximize space usage efficiency.
- 🧠 Goal of Invalidation: Maintain data accuracy and freshness.
💻 Code Example
// Invalidation
_cache.Remove("product_42");
// Replacement (automatic via policy like LRU)
// Cache may evict least recently used entries when full
❓ Interview Q&A
Q1: What is cache replacement?
A: Automatic eviction of cached entries based on memory limits and replacement policies.
Q2: What is cache invalidation?
A: Removing cached data when it becomes outdated or inconsistent with the source.
Q3: Is cache invalidation manual or automatic?
A: It can be both — manually triggered or logic-based using events or TTL.
Q4: When does cache replacement happen?
A: When the cache is full and a new item needs to be added.
Q5: Which one ensures data consistency?
A: Cache invalidation focuses on consistency, while replacement optimizes space.
📝 MCQs
Q1. What is the purpose of cache replacement?
- Ensure data freshness
- Manage memory usage
- Avoid TTL
- Track users
Q2. When does cache invalidation happen?
- When cache is full
- When app restarts
- When data changes
- On server boot
Q3. What policy is used in replacement?
- TTL
- LRU
- DNS
- SSL
Q4. Which aims for data freshness?
- Replacement
- Compression
- Invalidation
- Eviction
Q5. Is replacement manual?
- Yes
- No
- Sometimes
- Only in Redis
Q6. Which removes stale data on change?
- Replacement
- Refetch
- Invalidation
- Hashing
Q7. What triggers replacement?
- Data updated
- Key missing
- Cache is full
- User logout
Q8. What is LRU?
- Latest Read Update
- Last Record Unused
- Least Recently Used
- Limited RAM Utility
Q9. Which pattern uses invalidation?
- Factory
- Decorator
- Cache-aside
- Mediator
Q10. Which strategy is automatic?
- Invalidation
- Polling
- Replacement
- Queueing
💡 Bonus Insight
Use cache invalidation when data accuracy is critical, and rely on replacement policies for automatic memory management.
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