Cache Stampede / Thundering Herd

Also Known As

TL;DR

Explanation

A cache stampede occurs when a popular cached item expires and many concurrent requests all find a cache miss simultaneously — each generates the same expensive query, flooding the database. Solutions include: mutex/lock-based regeneration (only one process rebuilds, others wait), probabilistic early expiry (start rebuilding before TTL expires with a small probability), stale-while-revalidate (serve stale data while regenerating in the background), and cache warming on deploy. In PHP with Redis, implement a lock with SET key value NX EX seconds — the first process acquires it and rebuilds, others serve stale or wait.

Diagram

sequenceDiagram
    participant W1 as Worker 1
    participant W2 as Worker 2
    participant W3 as Worker 3
    participant C as Cache
    participant DB as Database
    C-->>W1: Cache MISS - expired
    C-->>W2: Cache MISS - expired
    C-->>W3: Cache MISS - expired
    W1->>DB: expensive query
    W2->>DB: expensive query duplicate
    W3->>DB: expensive query duplicate
    Note over DB: DB overwhelmed - stampede!
    Note over C: Fix: mutex lock or probabilistic early expiry

Common Misconception

Why It Matters

Common Mistakes

• No mutex or locking around cache miss regeneration — all concurrent requests hit the backend.
• Setting all cache entries to expire at the same time — use TTL jitter to spread expiry.
• Not using probabilistic early expiry — start regenerating before expiry to avoid a cliff-edge miss.
• Short TTLs on expensive-to-compute values — frequent expiry increases stampede probability.

Code Examples

// On cache miss, 100 simultaneous requests all hit the DB
$value = $cache->get('popular_data');
if ($value === null) {
    $value = $this->db->expensiveQuery(); // thundering herd
    $cache->set('popular_data', $value, 300);
}

// Option 1 — Mutex/lock (only one request recomputes)
$lock = $this->locks->acquire('popular_data', ttl: 10);
if ($lock) {
    try {
        $value = $this->db->expensiveQuery();
        $cache->set('popular_data', $value, 300);
    } finally { $lock->release(); }
} else {
    // Other requests wait briefly then read from cache
    sleep(1);
    $value = $cache->get('popular_data');
}

// Option 2 — Probabilistic early recomputation (no lock needed)
// Recompute before expiry with increasing probability as TTL decreases

References

• ↗ https://en.wikipedia.org/wiki/Cache_stampede

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