Cache Debugging — Hit Ratio, Status, and Performance

What is Cache Hit Ratio?

Hit Ratio = Cache Hits / (Cache Hits + Cache Misses) * 100

Target: > 90% for production

Measuring Hit Ratio

// Redis cache hit ratio
$redis = new Redis();
$redis->connect('127.0.0.1', 6379);

$info = $redis->info();
$hits = $info['keyspace_hits'];
$misses = $info['keyspace_misses'];
$ratio = $hits / ($hits + $misses) * 100;

// Output: Hit ratio: 95.5%

Monitoring Tools

# Redis stats
redis-cli info stats | grep -E 'keyspace_hits|keyspace_misses'

# Check specific database
redis-cli -n 0 info keyspace
redis-cli -n 1 info keyspace  # FPC

Cache Performance Metrics

-- Monitor cache operations
SELECT
    cache_type,
    COUNT(*) as operations,
    AVG(execution_time) as avg_time
FROM cache_log
WHERE created_at > DATE_SUB(NOW(), INTERVAL 1 HOUR)
GROUP BY cache_type;

Cache Status Command

php bin/magento cache:status

+------------------+----------+
| Cache Type       | Status   |
+------------------+----------+
| config           | Enabled  |
| layout           | Enabled  |
| block_html       | Enabled  |
| collections      | Enabled  |
| fpc              | Enabled  |
+------------------+----------+

Redis Cache Monitoring

# Total keys per database
redis-cli -n 0 dbsize  # Default cache
redis-cli -n 1 dbsize  # FPC
redis-cli -n 2 dbsize  # Sessions

# Memory usage
redis-cli info memory

# Cache size breakdown
redis-cli --bigkeys

Cache Status API

$cacheManager = $objectManager->get(\Magento\Framework\App\Cache\Manager::class);
$types = $cacheManager->getAvailableTypes();

foreach ($types as $type => $status) {
    echo "$type: " . ($status ? 'Enabled' : 'Disabled') . PHP_EOL;
}

Debug Mode

// Enable cache debugging in env.php
return [
    'cache' => [
        'frontend' => [
            'default' => [
                'backend_options' => [
                    'log_level' => 3  // Verbose logging
                ]
            ]
        ]
    ]
];

Cache Header Debug

// Add cache debug headers
public function render($resultPage)
{
    $response = $this->getResponse();
    $response->setHeader('X-Cache-Debug', '1');
    $response->setHeader('X-Magento-Cache-Debug', 'HIT');
    return $resultPage;
}

Varnish Debug

# Enable Varnish debug
varnishadm param.set debug +esi

# Check cache status
curl -I http://magento-url/
# Look for: X-Varnish: 200 (hit) or 500 (miss)

Debug Commands

# Check cache configuration
php bin/magento config:show | grep cache

# Show cache types
php bin/magento cache:status

# Clear specific cache
php bin/magento cache:clean block_html

# Monitor cache in real-time
watch -n 1 'redis-cli -n 0 info keyspace'

Page Load Time with Cache

// Measure page load time
$start = microtime(true);
// ... page render ...
$elapsed = microtime(true) - $start;

// With FPC: 50-100ms
// Without FPC: 500-2000ms

Cache Impact by Type

Debugging Slow Pages

// Profile cache operations
$start = microtime(true);
$data = $cache->load($key);
$loadTime = microtime(true) - $start;

if ($loadTime > 0.01) { // > 10ms
    $logger->warning('Slow cache load: ' . $key . ' took ' . $loadTime . 's');
}

Cache Optimization Checklist

1. Verify all cache types enabled
2. Check Redis memory isn't swapping
3. Monitor cache hit ratio > 90%
4. Check Varnish hit rate
5. Review cache TTL settings
6. Optimize block cache keys
7. Clean stale cache entries