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| <h1>Database Caching Class</h1> | |
| <p>The Database Caching Class permits you to cache your queries as text files for reduced database load.</p> | |
| <p class="important"><strong>Important:</strong> This class is initialized automatically by the database driver | |
| when caching is enabled. Do NOT load this class manually.<br /><br /> | |
| <strong>Also note:</strong> Not all query result functions are available when you use caching. Please read this page carefully.</p> | |
| <h2>Enabling Caching</h2> | |
| <p>Caching is enabled in three steps:</p> | |
| <ul> | |
| <li>Create a writable directory on your server where the cache files can be stored.</li> | |
| <li>Set the path to your cache folder in your <dfn>application/config/database.php</dfn> file.</li> | |
| <li>Enable the caching feature, either globally by setting the preference in your <dfn>application/config/database.php</dfn> file, or manually as described below.</li> | |
| </ul> | |
| <p>Once enabled, caching will happen automatically whenever a page is loaded that contains database queries.</p> | |
| <h2>How Does Caching Work?</h2> | |
| <p>CodeIgniter's query caching system happens dynamically when your pages are viewed. | |
| When caching is enabled, the first time a web page is loaded, the query result object will | |
| be serialized and stored in a text file on your server. The next time the page is loaded the cache file will be used instead of | |
| accessing your database. Your database usage can effectively be reduced to zero for any pages that have been cached.</p> | |
| <p>Only <dfn>read-type</dfn> (SELECT) queries can be cached, since these are the only type of queries that produce a result. | |
| <dfn>Write-type</dfn> (INSERT, UPDATE, etc.) queries, since they don't generate a result, will not be cached by the system.</p> | |
| <p>Cache files DO NOT expire. Any queries that have been cached will remain cached until you delete them. The caching system | |
| permits you clear caches associated with individual pages, or you can delete the entire collection of cache files. | |
| Typically you'll to use the housekeeping functions described below to delete cache files after certain | |
| events take place, like when you've added new information to your database.</p> | |
| <h2>Will Caching Improve Your Site's Performance?</h2> | |
| <p>Getting a performance gain as a result of caching depends on many factors. | |
| If you have a highly optimized database under very little load, you probably won't see a performance boost. | |
| If your database is under heavy use you probably will see an improved response, assuming your file-system is not | |
| overly taxed. Remember that caching simply changes how your information is retrieved, shifting it from being a database | |
| operation to a file-system one.</p> | |
| <p>In some clustered server environments, for example, caching may be detrimental since file-system operations are so intense. | |
| On single servers in shared environments, caching will probably be beneficial. Unfortunately there is no | |
| single answer to the question of whether you should cache your database. It really depends on your situation.</p> | |
| <h2>How are Cache Files Stored?</h2> | |
| <p>CodeIgniter places the result of EACH query into its own cache file. Sets of cache files are further organized into | |
| sub-folders corresponding to your controller functions. To be precise, the sub-folders are named identically to the | |
| first two segments of your URI (the controller class name and function name).</p> | |
| <p>For example, let's say you have a controller called <dfn>blog</dfn> with a function called <dfn>comments</dfn> that | |
| contains three queries. The caching system will create a cache folder | |
| called <kbd>blog+comments</kbd>, into which it will write three cache files.</p> | |
| <p>If you use dynamic queries that change based on information in your URI (when using pagination, for example), each instance of | |
| the query will produce its own cache file. It's possible, therefore, to end up with many times more cache files than you have | |
| queries.</p> | |
| <h2>Managing your Cache Files</h2> | |
| <p>Since cache files do not expire, you'll need to build deletion routines into your application. For example, let's say you have a blog | |
| that allows user commenting. Whenever a new comment is submitted you'll want to delete the cache files associated with the | |
| controller function that serves up your comments. You'll find two delete functions described below that help you | |
| clear data.</p> | |
| <h2>Not All Database Functions Work with Caching</h2> | |
| <p>Lastly, we need to point out that the result object that is cached is a simplified version of the full result object. For that reason, | |
| some of the query result functions are not available for use.</p> | |
| <p>The following functions <kbd>ARE NOT</kbd> available when using a cached result object:</p> | |
| <ul> | |
| <li>num_fields()</li> | |
| <li>field_names()</li> | |
| <li>field_data()</li> | |
| <li>free_result()</li> | |
| </ul> | |
| <p>Also, the two database resources (result_id and conn_id) are not available when caching, since result resources only | |
| pertain to run-time operations.</p> | |
| <br /> | |
| <h1>Function Reference</h1> | |
| <h2>$this->db->cache_on() / $this->db->cache_off()</h2> | |
| <p>Manually enables/disables caching. This can be useful if you want to | |
| keep certain queries from being cached. Example:</p> | |
| <code> | |
| // Turn caching on<br /> | |
| $this->db->cache_on();<br /> | |
| $query = $this->db->query("SELECT * FROM mytable");<br /> | |
| <br /> | |
| // Turn caching off for this one query<br /> | |
| $this->db->cache_off();<br /> | |
| $query = $this->db->query("SELECT * FROM members WHERE member_id = '$current_user'");<br /> | |
| <br /> | |
| // Turn caching back on<br /> | |
| $this->db->cache_on();<br /> | |
| $query = $this->db->query("SELECT * FROM another_table"); | |
| </code> | |
| <h2>$this->db->cache_delete()</h2> | |
| <p>Deletes the cache files associated with a particular page. This is useful if you need to clear caching after you update your database.</p> | |
| <p>The caching system saves your cache files to folders that correspond to the URI of the page you are viewing. For example, if you are viewing | |
| a page at <dfn>www.your-site.com/index.php/blog/comments</dfn>, the caching system will put all cache files associated with it in a folder | |
| called <dfn>blog+comments</dfn>. To delete those particular cache files you will use:</p> | |
| <code>$this->db->cache_delete('blog', 'comments');</code> | |
| <p>If you do not use any parameters the current URI will be used when determining what should be cleared.</p> | |
| <h2>$this->db->cache_delete_all()</h2> | |
| <p>Clears all existing cache files. Example:</p> | |
| <code>$this->db->cache_delete_all();</code> | |
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