--- name: caching model: standard description: Caching strategies, invalidation, eviction policies, HTTP caching, distributed caching, and anti-patterns. Use when designing cache layers, choosing eviction policies, debugging stale data, or optimizing read-heavy workloads. ---

Caching Patterns

> A well-placed cache is the cheapest way to buy speed. A misplaced cache is the most expensive way to buy bugs.

Cache Strategies

| Strategy | How It Works | When to Use | |----------|-------------|-------------| | Cache-Aside (Lazy) | App checks cache → miss → reads DB → writes to cache | Default choice — general purpose | | Read-Through | Cache fetches from DB on miss automatically | ORM-integrated caching, CDN origin fetch | | Write-Through | Writes go to cache AND DB synchronously | Read-heavy with strong consistency | | Write-Behind | Writes go to cache, async flush to DB | High write throughput, eventual consistency OK | | Refresh-Ahead | Cache proactively refreshes before expiry | Predictable access patterns, low-latency critical |

Cache-Aside Flow:

  App ──► Cache ──► HIT? ──► Return data
              │
              ▼ MISS
          Read DB ──► Store in Cache ──► Return data

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Cache Invalidation

| Method | Consistency | When to Use | |--------|-------------|-------------| | TTL-based | Eventual (up to TTL) | Simple data, acceptable staleness | | Event-based | Strong (near real-time) | Inventory, profile updates | | Version-based | Strong | Static assets, API responses, config | | Tag-based | Strong | CMS content, category-based purging |

TTL Guidelines

| Data Type | TTL | Rationale | |-----------|-----|-----------| | Static assets (CSS/JS/images) | 1 year + cache-busting hash | Immutable by filename | | API config / feature flags | 30–60 seconds | Fast propagation needed | | User profile data | 5–15 minutes | Tolerable staleness | | Product catalog | 1–5 minutes | Balance freshness vs load | | Session data | Match session timeout | Security requirement |

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HTTP Caching

Cache-Control Directives

| Directive | Meaning | |-----------|---------| | max-age=N | Cache for N seconds | | s-maxage=N | CDN/shared cache max age (overrides max-age) | | no-cache | Must revalidate before using cached copy | | no-store | Never cache anywhere | | must-revalidate | Once stale, must revalidate | | private | Only browser can cache, not CDN | | public | Any cache can store | | immutable | Content will never change (within max-age) | | stale-while-revalidate=N | Serve stale for N seconds while fetching fresh |

Common Recipes

# Immutable static assets (hashed filenames)
Cache-Control: public, max-age=31536000, immutable

# API response, CDN-cached, background refresh
Cache-Control: public, s-maxage=60, stale-while-revalidate=300

# Personalized data, browser-only
Cache-Control: private, max-age=0, must-revalidate
ETag: "abc123"

# Never cache (auth tokens, sensitive data)
Cache-Control: no-store

Conditional Requests

| Mechanism | Request Header | Response Header | How It Works | |-----------|---------------|-----------------|-------------| | ETag | If-None-Match: "abc" | ETag: "abc" | Hash-based — 304 if match | | Last-Modified | If-Modified-Since: | Last-Modified: | Date-based — 304 if unchanged |

Prefer ETag over Last-Modified — ETags detect content changes regardless of timestamp granularity.

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Application Caching

| Solution | Speed | Shared Across Processes | When to Use | |----------|-------|------------------------|-------------| | In-memory LRU | Fastest | No | Single-process, bounded memory, hot data | | Redis | Sub-ms (network) | Yes | Production default — TTL, pub/sub, persistence | | Memcached | Sub-ms (network) | Yes | Simple key-value at extreme scale | | SQLite | Fast (disk) | No | Embedded apps, edge caching |

Redis vs Memcached

| Feature | Redis | Memcached | |---------|-------|-----------| | Data structures | Strings, hashes, lists, sets, sorted sets | Strings only | | Persistence | AOF, RDB snapshots | None | | Pub/Sub | Yes | No | | Max value size | 512 MB | 1 MB | | Verdict | Default choice | Pure cache at extreme scale |

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Distributed Caching

| Concern | Solution | |---------|----------| | Partitioning | Consistent hashing — minimal reshuffling on node changes | | Replication | Primary-replica — writes to primary, reads from replicas | | Failover | Redis Sentinel or Cluster auto-failover |

Rule of thumb: 3 primaries + 3 replicas minimum for production Redis Cluster.

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Cache Eviction Policies

| Policy | How It Works | When to Use | |--------|-------------|-------------| | LRU | Evicts least recently accessed | Default — general purpose | | LFU | Evicts least frequently accessed | Skewed popularity distributions | | FIFO | Evicts oldest entry | Simple, time-ordered data | | TTL | Evicts after fixed duration | Data with known freshness window |

> Redis default is noeviction. Set maxmemory-policy to allkeys-lru or volatile-lru for production.

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Caching Layers

Browser Cache → CDN → Load Balancer → App Cache → DB Cache → Database

| Layer | What to Cache | Invalidation | |-------|--------------|--------------| | Browser | Static assets, API responses | Versioned URLs, Cache-Control | | CDN | Static files, public API responses | Purge API, surrogate keys | | Application | Computed results, DB queries, external API | Event-driven, TTL | | Database | Query plans, buffer pool, materialized views | ANALYZE, manual refresh |

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Cache Stampede Prevention

When a hot key expires, hundreds of requests simultaneously hit the database.

| Technique | How It Works | |-----------|-------------| | Mutex / Lock | First request locks, fetches, populates; others wait | | Probabilistic early expiration | Random chance of refreshing before TTL | | Request coalescing | Deduplicate in-flight requests for same key | | Stale-while-revalidate | Serve stale, refresh asynchronously |

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Cache Warming

| Strategy | When to Use | |----------|-------------| | On-deploy warm-up | Predictable key set, latency-sensitive | | Background job | Reports, dashboards, catalog data | | Shadow traffic | Cache migration, new infrastructure | | Priority-based | Limited warm-up time budget |

> Cold start impact: A full cache flush can increase DB load 10–100x. Always warm gradually or use stale-while-revalidate.

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Monitoring

| Metric | Healthy Range | Action if Unhealthy | |--------|--------------|---------------------| | Hit rate | > 90% | Low → cache too small, wrong TTL, bad key design | | Eviction rate | Near 0 steady state | High → increase memory or tune policy | | Latency (p99) | < 1ms (Redis) | High → network issue, large values, hot key | | Memory usage | < 80% of max | Approaching max → scale up or tune eviction |

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NEVER Do

1. NEVER cache without a TTL or invalidation plan — data rots; every entry needs an expiry path
2. NEVER treat cache as durable storage — caches evict, crash, and restart; always fall back to source of truth
3. NEVER cache sensitive data (tokens, PII) without encryption — cache breaches expose everything in plaintext
4. NEVER ignore cache stampede on hot keys — one expired popular key can take down your database
5. NEVER use unbounded in-memory caches in production — memory grows until OOM-killed
6. NEVER cache mutable data with immutable Cache-Control — browsers will never re-fetch
7. NEVER skip monitoring hit/miss rates — you won't know if your cache is helping or hurting