Preventing Cache Stampedes in PHP

Use short-lived locks or stale-while-revalidate to avoid many workers recomputing one missing key.

What It Does and When to Use It

A local flock prevents duplicate refreshes on one host; distributed apps need a shared lock. APCu stores local application data, Redis is external and can be shared among hosts, and OPcache stores compiled code.

Requirements, Inputs and Expected Behavior

Confirm the runtime, extension, environment variables, and permissions before running the examples in production. Lock TTL must cover work duration and the fallback must remain safe if lock acquisition fails.

Example 1: Basic Operation

A local flock prevents duplicate refreshes on one host; distributed apps need a shared lock.

<?php
$lockName='cache-refresh:popular-products';
$handle=fopen(sys_get_temp_dir().'/cache-refresh.lock','c');
if (!$handle) throw new RuntimeException('Lock unavailable');
try { if (flock($handle,LOCK_EX|LOCK_NB)) { echo 'Only one local process refreshes'; flock($handle,LOCK_UN); } }
finally { fclose($handle); }

Example 2: Behavior and Validation

Spread cache expirations so hot entries do not all refresh simultaneously.

<?php
$ttl=300+random_int(-30,30);
echo 'Jittered TTL: ',$ttl,' seconds';

Practical Example: Application Use

Stale-while-revalidate may smooth spikes; bound the stale window for correctness.

<?php
$stale=['value'=>['count'=>9],'expires_at'=>time()-5];
if ($stale['expires_at']<time()) echo 'Serve stale briefly while one worker refreshes';

Important Behavior and Edge Cases

The second example highlights: Spread cache expirations so hot entries do not all refresh simultaneously. The practical example adds: Stale-while-revalidate may smooth spikes; bound the stale window for correctness.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• Use short-lived locks or stale-while-revalidate to avoid many workers recomputing one missing key.
• Stale-while-revalidate may smooth spikes; bound the stale window for correctness.
• Define TTL, invalidation, and concurrency behavior separately from the cache storage API.