10. Cache

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1️⃣ What is Caching?

• Store frequently accessed data in a faster storage layer.
• Reduces:
  • Database load
  • Latency
  • Expensive recomputation
• Usually uses:
  • RAM (Redis, Memcached)
  • CDN
  • Local memory

📌 Memory access is ~10,000x faster than disk.

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2️⃣ Why Use Cache?

Use caching when:

• System is read-heavy
• Queries are expensive
• Low latency is required
• Database CPU/load is high

Example:

• Newsfeed generation
• User profiles
• Trending posts
• API responses

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3️⃣ Types of Caching

Type	Description	Use Case
External Cache	Separate cache server (Redis)	Most common
In-Process Cache	Inside app memory	Ultra low latency
CDN Cache	Edge servers near users	Images/videos
Client Cache	Browser/mobile cache	Offline support

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4️⃣ Cache Architectures

A. Cache Aside (Most Important ⭐)

Flow:

1. Check cache
2. Cache miss → DB
3. Store in cache
4. Return response

✅ Most common in interviews.

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B. Write Through

• Write to cache + DB together.

✅ Fresh reads
❌ Slower writes

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C. Write Behind / Write Back

• Write to cache first
• DB updated asynchronously later

✅ Very fast writes
❌ Possible data loss

Used in:

• Analytics
• Logging systems

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D. Read Through

• Cache itself fetches from DB on miss.

Mostly used in:

• CDN
• Edge caching

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5️⃣ Cache Eviction Policies

Policy	Meaning
LRU	Remove least recently used
LFU	Remove least frequently used
FIFO	Remove oldest item
TTL	Expire after fixed time

📌 Most common:

• LRU
• TTL

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6️⃣ Important Cache Problems

A. Cache Stampede / Thundering Herd

Many requests hit DB after cache expiry.

✅ Solutions:

• Request coalescing / single flight
• Cache warming
• Randomized TTL

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B. Cache Consistency

Cache and DB contain different values.

✅ Solutions:

• Invalidate cache on write
• Short TTL
• Eventual consistency

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C. Hot Keys

One cache key receives huge traffic.

Example:

• Celebrity profile
• Trending post

✅ Solutions:

• Replicate hot keys
• Local in-memory cache
• Load balancing

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7️⃣ Common Tools

Tool	Purpose
Redis	Distributed cache
Memcached	Simple cache
Cloudflare	CDN
Amazon CloudFront	CDN

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8️⃣ What to Mention in Interviews

Always Explain:

1. Why cache is needed
2. What data is cached
3. Cache key
4. TTL / eviction policy
5. Read/write flow
6. Consistency handling
7. Stampede handling

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9️⃣ Interview Default Recommendation

Default Stack

• Cache Type → External cache
• Tool → Redis
• Pattern → Cache Aside
• Eviction → LRU + TTL

This works for most interviews. 🚀

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🔟 Golden Rule

“Don’t add cache blindly.”

Always justify:

• What bottleneck exists?
• How cache solves it?
• What tradeoffs are introduced?

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Quick Revision

Concept	Default Choice
Cache Server	Redis
Pattern	Cache Aside
Eviction	LRU + TTL
Main Goal	Faster reads
Biggest Problems	Stale data, stampede, hot keys

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