220 lines
9.4 KiB
HTML
220 lines
9.4 KiB
HTML
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<table cellpadding="0" cellspacing="0" border="0" style="width:100%">
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<tr>
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<td><h1>CodeIgniter User Guide Version 1.7.3</h1></td>
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<td id="breadcrumb_right"><a href="../toc.html">Table of Contents Page</a></td>
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<a href="http://codeigniter.com/">CodeIgniter Home</a> ›
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<a href="../index.html">User Guide Home</a> ›
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<a href="index.html">Database Library</a> ›
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Database Caching Class
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<td id="searchbox"><form method="get" action="http://www.google.com/search"><input type="hidden" name="as_sitesearch" id="as_sitesearch" value="codeigniter.com/user_guide/" />Search User Guide <input type="text" class="input" style="width:200px;" name="q" id="q" size="31" maxlength="255" value="" /> <input type="submit" class="submit" name="sa" value="Go" /></form></td>
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<div id="content">
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<h1>Database Caching Class</h1>
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<p>The Database Caching Class permits you to cache your queries as text files for reduced database load.</p>
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<p class="important"><strong>Important:</strong> This class is initialized automatically by the database driver
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when caching is enabled. Do NOT load this class manually.<br /><br />
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<strong>Also note:</strong> Not all query result functions are available when you use caching. Please read this page carefully.</p>
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<h2>Enabling Caching</h2>
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<p>Caching is enabled in three steps:</p>
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<ul>
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<li>Create a writable directory on your server where the cache files can be stored.</li>
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<li>Set the path to your cache folder in your <dfn>application/config/database.php</dfn> file.</li>
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<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>
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</ul>
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<p>Once enabled, caching will happen automatically whenever a page is loaded that contains database queries.</p>
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<h2>How Does Caching Work?</h2>
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<p>CodeIgniter's query caching system happens dynamically when your pages are viewed.
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When caching is enabled, the first time a web page is loaded, the query result object will
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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
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accessing your database. Your database usage can effectively be reduced to zero for any pages that have been cached.</p>
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<p>Only <dfn>read-type</dfn> (SELECT) queries can be cached, since these are the only type of queries that produce a result.
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<dfn>Write-type</dfn> (INSERT, UPDATE, etc.) queries, since they don't generate a result, will not be cached by the system.</p>
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<p>Cache files DO NOT expire. Any queries that have been cached will remain cached until you delete them. The caching system
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permits you clear caches associated with individual pages, or you can delete the entire collection of cache files.
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Typically you'll want to use the housekeeping functions described below to delete cache files after certain
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events take place, like when you've added new information to your database.</p>
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<h2>Will Caching Improve Your Site's Performance?</h2>
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<p>Getting a performance gain as a result of caching depends on many factors.
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If you have a highly optimized database under very little load, you probably won't see a performance boost.
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If your database is under heavy use you probably will see an improved response, assuming your file-system is not
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overly taxed. Remember that caching simply changes how your information is retrieved, shifting it from being a database
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operation to a file-system one.</p>
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<p>In some clustered server environments, for example, caching may be detrimental since file-system operations are so intense.
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On single servers in shared environments, caching will probably be beneficial. Unfortunately there is no
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single answer to the question of whether you should cache your database. It really depends on your situation.</p>
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<h2>How are Cache Files Stored?</h2>
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<p>CodeIgniter places the result of EACH query into its own cache file. Sets of cache files are further organized into
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sub-folders corresponding to your controller functions. To be precise, the sub-folders are named identically to the
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first two segments of your URI (the controller class name and function name).</p>
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<p>For example, let's say you have a controller called <dfn>blog</dfn> with a function called <dfn>comments</dfn> that
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contains three queries. The caching system will create a cache folder
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called <kbd>blog+comments</kbd>, into which it will write three cache files.</p>
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<p>If you use dynamic queries that change based on information in your URI (when using pagination, for example), each instance of
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the query will produce its own cache file. It's possible, therefore, to end up with many times more cache files than you have
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queries.</p>
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<h2>Managing your Cache Files</h2>
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<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
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that allows user commenting. Whenever a new comment is submitted you'll want to delete the cache files associated with the
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controller function that serves up your comments. You'll find two delete functions described below that help you
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clear data.</p>
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<h2>Not All Database Functions Work with Caching</h2>
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<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,
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some of the query result functions are not available for use.</p>
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<p>The following functions <kbd>ARE NOT</kbd> available when using a cached result object:</p>
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<ul>
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<li>num_fields()</li>
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<li>field_names()</li>
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<li>field_data()</li>
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<li>free_result()</li>
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</ul>
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<p>Also, the two database resources (result_id and conn_id) are not available when caching, since result resources only
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pertain to run-time operations.</p>
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<br />
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<h1>Function Reference</h1>
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<h2>$this->db->cache_on() / $this->db->cache_off()</h2>
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<p>Manually enables/disables caching. This can be useful if you want to
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keep certain queries from being cached. Example:</p>
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<code>
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// Turn caching on<br />
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$this->db->cache_on();<br />
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$query = $this->db->query("SELECT * FROM mytable");<br />
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<br />
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// Turn caching off for this one query<br />
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$this->db->cache_off();<br />
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$query = $this->db->query("SELECT * FROM members WHERE member_id = '$current_user'");<br />
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<br />
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// Turn caching back on<br />
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$this->db->cache_on();<br />
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$query = $this->db->query("SELECT * FROM another_table");
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</code>
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<h2>$this->db->cache_delete()</h2>
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<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>
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<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
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a page at <dfn>example.com/index.php/blog/comments</dfn>, the caching system will put all cache files associated with it in a folder
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called <dfn>blog+comments</dfn>. To delete those particular cache files you will use:</p>
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<code>$this->db->cache_delete('blog', 'comments');</code>
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<p>If you do not use any parameters the current URI will be used when determining what should be cleared.</p>
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<h2>$this->db->cache_delete_all()</h2>
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<p>Clears all existing cache files. Example:</p>
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<code>$this->db->cache_delete_all();</code>
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</div>
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<p>
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Previous Topic: <a href="call_function.html">Custom Function Calls</a>
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<a href="#top">Top of Page</a> ·
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<a href="../index.html">User Guide Home</a> ·
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