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X-Fetch & Stampede Protection

One of the most difficult problems in caching is the Cache Stampede (also known as the dog-pile effect). This happens when a frequently accessed cache item expires, and multiple requests try to regenerate it simultaneously, crashing the backend.

How X-Fetch Works

X-Fetch (Probabilistic Early Recomputation) is an algorithm that randomly refreshes a cache item before it actually expires.

Instead of waiting for the hard TTL (Time-To-Live), the system calculates a probabilistic value based on: 1. The time remaining until expiration. 2. The time it took to generate the value previously (generation_time). 3. A configured beta coefficient (x_fetch_beta).

If the check passes, one lucky request triggers the refresh early, while others continue to use the current value. This spreads the recomputation load and prevents the "cliff edge" of expiration.

Configuration

You can enable X-Fetch by setting the x_fetch_beta option in CacheOptions.

use Fyennyi\AsyncCache\CacheOptions;

$options = new CacheOptions(
    ttl: 600,
    x_fetch_beta: 1.0 // Standard value. Set to 0 to disable.
);

Tuning Beta

  • 1.0: Standard setting. Effectively mimics the optimal recomputation point.
  • Higher values (> 1.0): Aggressive early recomputation. Good for very expensive queries.
  • Lower values (< 1.0): Lazy recomputation. Closer to standard expiration.
  • 0: Disables X-Fetch logic.

Why Use It?

  • Zero Downtime: The cache is refreshed in the background before it disappears.
  • No Locking Required: Unlike traditional stampede protection (which uses locks), X-Fetch is non-blocking and statistically efficient.