Caching Strategies (APCu, Redis, Memcached)

Also Known As

TL;DR

Explanation

Caching strategies include: in-process caching (APCu, OPcache for bytecode), distributed caching (Redis, Memcached for shared state across servers), HTTP caching (ETags, Cache-Control headers for browser/CDN caching), and database query result caching. Key cache design decisions are TTL (time-to-live), invalidation strategy (TTL expiry vs. explicit eviction), cache stampede prevention (probabilistic early expiry, locking), and what to cache (expensive queries, rendered fragments, API responses). Cache invalidation is famously one of the two hard problems in computer science.

Diagram

flowchart LR
    REQ[Request] --> CHECK{Cache hit?}
    CHECK -->|HIT| RETURN[Return cached<br/>response O 1]
    CHECK -->|MISS| DB[(Database<br/>or API)]
    DB --> STORE[Store in cache<br/>with TTL]
    STORE --> RETURN2[Return response]
    subgraph Cache Layers
        L1[Browser cache]
        L2[CDN edge cache]
        L3[App cache Redis]
        L4[DB query cache]
    end
style RETURN fill:#238636,color:#fff
style L1 fill:#238636,color:#fff
style L4 fill:#1f6feb,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Caching everything without considering cache invalidation — stale data is often worse than no cache.
• Using the same TTL for all data regardless of how often it changes — user sessions and product catalogues have very different staleness tolerances.
• Not caching at the right layer — caching a slow SQL result but not the rendered HTML misses bigger gains.
• Forgetting to handle cache misses gracefully — if the cache server is down, the app should still work.

Code Examples

// No caching — expensive DB query on every request:
function getDashboardStats(): array {
    return $db->query('SELECT COUNT(*), SUM(total) FROM orders WHERE ...')->fetch();
    // Called 1000 times/min — same result every minute
}

// With caching:
function getDashboardStats(): array {
    return apcu_fetch('dashboard_stats', $ok)
        ?: tap(expensiveQuery(), fn($r) => apcu_store('dashboard_stats', $r, 60));
}

// Cache-aside pattern with PSR-6
$item = $cache->getItem('user:' . $id);

if (!$item->isHit()) {
    $user = $this->db->fetchUser($id);          // expensive
    $item->set($user);
    $item->expiresAfter(300);                   // 5 min TTL
    $cache->save($item);
}

return $item->get();

// Cache invalidation on write
public function updateUser(User $user): void {
    $this->db->save($user);
    $this->cache->deleteItem('user:' . $user->id); // invalidate
}

References

• ↗ https://www.php.net/manual/en/book.apcu.php
• ↗ https://redis.io/docs/

Tags

Related Terms