Share Simple File Based Caching in PHP
Friday, July 30th, 2010
Depending on the access pattern for the your data, file based caches are often more economical, despite being slower, than an in memory cache like Memcached or APC. Disk space is cheap and is therefore a good option for caching data that’s not frequently accessed, but is expensive to generate. That’s exactly where this class comes in. On a small EC2 instance, I was able to cache API calls, time was reduced from 0.4 seconds to 0.001 seconds per call. The reduced the overall latency of the application and freed up system resources to work on other tasks.
Because API result sets can be very large, consider a friend list from Facebook which may be 10-50K per person, caching them in memory can often push out more valuable data reducing the effectiveness of the cache. The file based cache is of course persistent and can also be distributed across multiple servers using rsync (or if you’re feeling adventurous, you alter this code to store it in an S3 bucket).
One drawback of this approach is the cache is not limited in size, leaving the files on disk until they’re deleted or replaced with new data. For that reason, I recommend you store data on a different drive where possible. File base caches, also have a potential security risk if the data is stored in a web accessible directory. This risk is not unique to this implementation. The potential exists for a clever hacker to discover your cache and gain access to sensitive data or worse compromise your server by storing executable code and executing it. You can reduce this risk by encrypting your data before storing it in the cache and by moving the data to a directory outside of the web root.
To use this cache, save this source to disk, I recommend a filename like FileCache.php and include it in your existing code files where you will be using it. Then, start caching your data! See the usage example in the comments for how to get started. Because the files are not encrypted, you can easily look in them to see what your data looks like.
<?php /** * * @package FileCache - A simple file based cache * @author Erik Giberti * @copyright 2010 Erik Giberti, all rights reserved * @license http://opensource.org/licenses/gpl-license.php GNU Public License * * Class to implement a file based cache. This is useful for caching large objects such as * API/Curl responses or HTML results that aren't well suited to storing in small memory caches * or are infrequently accessed but are still expensive to generate. * * For security reasons, it's *strongly* recommended you set your cache directory to be outside * of your web root and on a drive independent of your operating system. * * Uses JSON to serialize the data object. * * Sample usage: * * $cache = new FileCache('/var/www/cache/'); * $data = $cache->get('sampledata'); * if(!$data){ * $data = array('a'=>1,'b'=>2,'c'=>3); * $cache->set('sampledata', $data, 3600); * } * print $data['a']; * */ // Requires the native JSON library if (!function_exists('json_decode') || !function_exists('json_encode')) { throw new Exception('Cache needs the JSON PHP extensions.'); } class FileCache { /** * Value is pre-pended to the cache, should be the full path to the directory */ protected $root = null; /** * For holding any error messages that may have been raised */ protected $error = null; /** * @param string $root The root of the file cache. */ function __construct($root = '/tmp/'){ $this->root = $root; } /** * Saves data to the cache. Anything that evaluates to false, null, '', boolean false, 0 will * not be saved. * @param string $key An identifier for the data * @param mixed $data The data to save * @param int $ttl Seconds to store the data * @returns boolean True if the save was successful, false if it failed */ public function set($key, $data = false, $ttl = 3600){ if(!$key) { $this->error = "Invalid key"; return false; } if(!$data){ $this->error = "Invalid data"; return false; } $key = $this->_make_file_key($key); $store = array( 'data' => $data, 'ttl' => time() + $ttl, ); $status = false; try { $fh = fopen($key, "w+"); if(flock($fh, LOCK_EX)){ ftruncate($fh, 0); fwrite($fh, json_encode($store)); flock($fh, LOCK_UN); $status = true; } fclose($fh); } catch (Exception $e) { $this->error = "Exception caught: " . $e->getMessage(); return false; } return $status; } /** * Reads the data from the cache * @param string $key An identifier for the data * @returns mixed Data that was stored */ public function get($key){ if(!$key) { $this->error = "Invalid key"; return false; } $key = $this->_make_file_key($key); $file_content = null; // Get the data from the file try { $fh = fopen($key, "r"); if(flock($fh, LOCK_SH)){ $file_content = fread($fh, filesize($key)); } fclose($fh); } catch (Exception $e) { $this->error = "Exception caught: " . $e->getMessage(); return false; } // Assuming we got something back... if($file_content){ $store = json_decode($file_content, true); if($store['ttl'] < time()){ unlink($key); // remove the file $this->error = "Data expired"; return false; } } return $store['data']; } /** * Remove a key, regardless of it's expire time * @param string $key An identifier for the data */ public function delete($key){ if(!$key) { $this->error = "Invalid key"; return false; } $key = $this->_make_file_key($key); try { unlink($key); // remove the file } catch (Exception $e) { $this->error = "Exception caught: " . $e->getMessage(); return false; } return true; } /** * Reads and clears the internal error * @returns string Text of the error raised by the last process */ public function get_error(){ $message = $this->error; $this->error = null; return $message; } /** * Can be used to inspect internal error * @returns boolean True if we have an error, false if we don't */ public function have_error(){ return ($this->error !== null) ? true : false; } /** * Create a key for the cache * @todo Beef up the cleansing of the file. * @param string $key The key to create * @returns string The full path and filename to access */ private function _make_file_key($key){ $safe_key = str_replace(array('.','/',':','\''), array('_','-','-','-'), trim($key)); return $this->root . $safe_key; } } |
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