Implementing Cache-Aside in PHP

Cache-aside checks the cache, loads from the system of record on a miss and stores a bounded copy.

What It Does and When to Use It

Load through the repository only on cache miss; a real backend must also enforce TTL. 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. Distinguish a cached null from a miss and invalidate entries after writes.

Example 1: Basic Operation

Load through the repository only on cache miss; a real backend must also enforce TTL.

<?php
$cache=[];
function readArticle(int $id,array &$cache,callable $load): array {
  $key='article:v1:'.$id;
  if (!array_key_exists($key,$cache)) $cache[$key]=$load($id);
  return $cache[$key];
}
$load=static fn(int $id)=>['id'=>$id,'title'=>'Guide'];
print_r(readArticle(9,$cache,$load));

Example 2: Behavior and Validation

Invalidate a changed item after successful database update.

<?php
$cache=['article:v1:3'=>['title'=>'Old']];
$id=3; unset($cache['article:v1:'.$id]);
echo isset($cache['article:v1:3'])?'Stale':'Invalidated';

Practical Example: Application Use

Distinguish a cached miss (null) from a key that is absent.

<?php
$cache=['article:v1:2'=>null];
$key='article:v1:2';
var_dump(array_key_exists($key,$cache));

Important Behavior and Edge Cases

The second example highlights: Invalidate a changed item after successful database update. The practical example adds: Distinguish a cached miss (null) from a key that is absent.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• Cache-aside checks the cache, loads from the system of record on a miss and stores a bounded copy.
• Distinguish a cached miss (null) from a key that is absent.
• Define TTL, invalidation, and concurrency behavior separately from the cache storage API.