Caching Strategies Knowledge Base

Quick reference for caching patterns, invalidation strategies, and Redis implementation guidelines. Focuses on caching theory and Redis patterns — for Cache-Aside code generation, see create-cache-aside.

Caching Strategies

Strategy	How It Works	Consistency	Performance	Use Case
Cache-Aside	App reads cache first, fetches from DB on miss, writes to cache	Eventual	Read-heavy	General purpose, most common
Read-Through	Cache itself fetches from DB on miss	Eventual	Read-heavy	Transparent caching layer
Write-Through	App writes to cache and DB synchronously	Strong	Write-heavy (slower writes)	Consistency-critical data
Write-Behind	App writes to cache, cache writes to DB asynchronously	Eventual	Write-heavy (fast writes)	High write throughput
Write-Around	App writes directly to DB, cache populated on read	Eventual	Infrequent-read data	Write-once, read-later

Strategy Flow Diagrams

Cache-Aside (Lazy Loading):
  Read:  App → Cache (hit?) → yes → return
                             → no  → DB → write to Cache → return
  Write: App → DB → invalidate Cache

Read-Through:
  Read:  App → Cache (hit?) → yes → return
                             → no  → Cache fetches from DB → return
  Write: App → DB → invalidate Cache

Write-Through:
  Read:  App → Cache (hit?) → yes → return
                             → no  → DB → return
  Write: App → Cache → Cache writes to DB (sync)

Write-Behind (Write-Back):
  Read:  App → Cache (hit?) → yes → return
                             → no  → DB → return
  Write: App → Cache → Cache writes to DB (async, batched)

Cache Invalidation Approaches

Approach	Description	Consistency	Complexity
TTL (Time-To-Live)	Cache expires after fixed duration	Eventual (stale window)	Low
Event-Driven	Invalidate on domain event	Near real-time	Medium
Versioned Keys	Include version in cache key	Immediate (new key)	Medium
Tag-Based	Group related keys by tag, purge by tag	Immediate	High
Write-Through	Update cache on write	Immediate	Medium
Manual	Explicit invalidation in code	Depends on discipline	Low

TTL Selection Guide

Data Type	TTL	Reasoning
Static config	1-24 hours	Rarely changes
User profile	5-15 minutes	Moderate change frequency
Session data	30 minutes	Linked to session timeout
Product catalog	1-5 minutes	Moderate updates
Search results	30-60 seconds	Frequent updates
Real-time data	5-15 seconds	High change frequency
Counters/stats	No TTL (event-driven)	Update on write

Multi-Level Caching

┌─────────────────────────────────────────────────┐
│                 MULTI-LEVEL CACHE                 │
│                                                   │
│   Request → L1 (In-Process)  hit → return         │
│                              miss ↓               │
│            L2 (Redis/Memcached) hit → populate L1  │
│                              miss ↓               │
│            L3 (CDN/HTTP Cache) hit → populate L2   │
│                              miss ↓               │
│            Database → populate L2 → populate L1    │
└─────────────────────────────────────────────────┘

Level	Storage	Latency	Capacity	Scope
L1	In-process (APCu, static)	< 1μs	Small (MB)	Per-process
L2	Distributed (Redis)	1-5ms	Large (GB)	Shared
L3	CDN / HTTP Cache	5-50ms	Very large	Global
Origin	Database	10-100ms	Unlimited	Source of truth

Redis Data Structures for Caching

Structure	When to Use	Example
String	Simple key-value, serialized objects	User session, JSON blob
Hash	Object with fields, partial reads	User profile (name, email, role)
Sorted Set	Ranked data, leaderboards, time-series	Top products, recent activity
List	Queues, recent items, feeds	Recent notifications
Set	Unique collections, tags	User permissions, online users
HyperLogLog	Cardinality estimation	Unique visitors count

Strategy Selection by Workload

Workload	Strategy	Why
Read-heavy, tolerance for stale	Cache-Aside + TTL	Simple, effective
Read-heavy, consistency needed	Cache-Aside + event invalidation	Fresh data
Write-heavy, read-after-write	Write-Through	Immediate consistency
Write-heavy, async OK	Write-Behind	Best write performance
Mixed, complex invalidation	Tag-based + event-driven	Granular control
API responses	HTTP Cache (CDN) + L2	Reduce server load

Detection Patterns

# Cache usage
Grep: "Cache|Redis|Memcached|APCu|apc_" --glob "**/*.php"
Grep: "CacheInterface|CacheItemPoolInterface|SimpleCacheInterface" --glob "**/*.php"

# Cache-Aside pattern
Grep: "->get\(.*\).*->set\(" --glob "**/*.php"
Grep: "cache->has|cache->get|cache->set" --glob "**/*.php"

# TTL configuration
Grep: "ttl|expire|setex|SETEX|TTL" --glob "**/*.php"
Grep: "CACHE_TTL|CACHE_LIFETIME" --glob "**/.env*"

# Cache invalidation
Grep: "->delete\(|->invalidate\(|->clear\(|->flush\(" --glob "**/*.php"
Grep: "invalidateTag|invalidateTags|clearByTag" --glob "**/*.php"

# Redis patterns
Grep: "Predis|PhpRedis|Redis::|new Redis" --glob "**/*.php"
Grep: "REDIS_HOST|REDIS_URL" --glob "**/.env*"

# Multi-level caching
Grep: "ChainCache|StackedCache|MultiLevelCache" --glob "**/*.php"
Grep: "apcu_fetch|apcu_store" --glob "**/*.php"

Advanced Patterns

Cache Stampede Prevention

Method	How It Works	Complexity	Best For
Locking (Mutex)	One process recomputes, others wait	Medium	Most cases
Probabilistic Early Expiry (XFetch)	Recompute before TTL with probability	Medium	High concurrency
Stale-While-Revalidate	Serve stale, refresh async	Medium	Latency-critical
External refresh	Cron/worker refreshes before expiry	Low	Predictable access

Distributed Cache Coherence

Strategy	Consistency	Latency	Complexity
TTL only	Eventual (stale window)	None	Low
Pub/Sub invalidation	Near-real-time	~1-5ms	Medium
Write-through all nodes	Strong	High	High
Version-based (ETag)	Strong (on read)	Per-read check	Medium

Write-Back vs Write-Through

Aspect	Write-Through	Write-Back
Write latency	Higher (sync)	Lower (cache only)
Data safety	Safe	Risk of loss
Consistency	Strong	Eventual
DB load	Per-write	Batched
Use case	Financial, orders	Analytics, counters

References

For detailed information, load these reference files:

• references/strategies.md — Detailed strategy analysis, cache warming, stampede prevention, distributed consistency
• references/redis-patterns.md — Eviction policies, data structure guide, cluster/sentinel, Lua scripting, PHP patterns
• references/advanced-patterns.md — Cache stampede prevention (locking, XFetch, stale-while-revalidate), cache warming strategies, write-back vs write-through comparison, distributed cache coherence, key design patterns