How to debug the apcu_entry cache that does not take effect

gitbox 2025-05-29

In PHP, apcu_entry is a powerful function provided by the APCu extension to implement automatic cache initialization: when the specified cache key does not exist, the callback function will be automatically called to generate cache values and store them. It greatly simplifies cache logic and improves code efficiency.

However, sometimes when using apcu_entry , you may encounter situations where the cache is not effective. This article will combine specific code examples to help you troubleshoot and debug common problems that the apcu_entry cache is not effective.

1. Examples of basic usage of apcu_entry

 <?php
$key = 'user_123';
$value = apcu_entry($key, function() {
    // Simulate database query
    return 'user_data_from_db';
});
echo $value;

In the above code, if there is no user_123 in the cache, the callback function will be called to generate and cache the data. The cache will be read directly the next call.

2. Common reasons for not taking effect of cache and debugging methods

1. The APCu extension is not installed or not enabled

Symptoms : An error is reported by calling apcu_entry or a cache behavior is abnormal.

debug :

 if (!extension_loaded('apcu')) {
    die('APCu The extension is not installed or not enabled，Please install and enable the extension。');
}

Make sure apc.enabled=1 is in the PHP configuration and restart the PHP service.

2. The cache is not available in CLI mode

By default, APCu cache is invalid in CLI mode. apc.enable_cli needs to be set to 1 .

debug :

 var_dump(ini_get('apc.enable_cli')); // If 0，butCLIMode without cache

The solution is to modify php.ini :

 apc.enable_cli=1

Then restart the CLI.

3. Insufficient cache space or cache is cleaned up frequently

When the cache space is insufficient, old data may be recycled, resulting in cache failure.

debug :

View APCu cache statistics:

 print_r(apcu_cache_info());

Observe cache space usage and fragmentation.

Adjust the apc.shm_size configuration to increase cache space.

4. Key names conflict or inconsistent

Make sure that the incoming cache key name is a stable and unique string. Dynamically generated key names may cause the cache to fail to hit.

5. The callback function execution exception causes the cache to not be written.

If an exception is thrown inside the callback function or returns null , apcu_entry may not be written to the cache.

debug :

 $value = apcu_entry('key', function() {
    try {
        // Possible errors
        return some_function();
    } catch (Exception $e) {
        error_log($e->getMessage());
        return false; // Or other reasonable default values
    }
});

6. Code example: Comprehensive debug version

 <?php
$key = 'sample_key';

if (!extension_loaded('apcu')) {
    die('Please install and enable APCu Extended');
}

if (PHP_SAPI === 'cli' && ini_get('apc.enable_cli') != '1') {
    die('CLI In mode APCu Cache not enabled，Please set apc.enable_cli=1');
}

$value = apcu_entry($key, function() {
    // Simulate exceptions
    if (rand(0,1) === 0) {
        throw new Exception('Simulate exceptions，Cache not written');
    }
    return 'Cache data';
});

var_dump($value);

print_r(apcu_cache_info());

3. Summary

Confirm that the APCu extension is installed and enabled.
CLI mode requires apc.enable_cli to be enabled.
Check if the cache space is sufficient.
Ensure that the cache key name is unique and stable.
The callback function should be safe and stable to avoid exceptions.

Through the above steps, the cache is basically solved when using apcu_entry .

For more APCu information, please refer to: https://gitbox.net/manual/apcu.html

apcu_entry