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Phil Sturgeon9c63d0b2011-10-27 01:55:44 +0100[diff] [blame]31<td><h1>CodeIgniter User Guide Version 2.1.0</h1></td>
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]32<td id="breadcrumb_right"><a href="../toc.html">Table of Contents Page</a></td>
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59
60<h1>Database Caching Class</h1>
61
62<p>The Database Caching Class permits you to cache your queries as text files for reduced database load.</p>
63
64<p class="important"><strong>Important:</strong>&nbsp; This class is initialized automatically by the database driver
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]65when caching is enabled. Do NOT load this class manually.<br /><br />
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]66
67<strong>Also note:</strong>&nbsp; Not all query result functions are available when you use caching. Please read this page carefully.</p>
68
69<h2>Enabling Caching</h2>
70
71<p>Caching is enabled in three steps:</p>
72
73<ul>
74<li>Create a writable directory on your server where the cache files can be stored.</li>
75<li>Set the path to your cache folder in your <dfn>application/config/database.php</dfn> file.</li>
76<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>
77</ul>
78
79<p>Once enabled, caching will happen automatically whenever a page is loaded that contains database queries.</p>
80
81
82<h2>How Does Caching Work?</h2>
83
84<p>CodeIgniter's query caching system happens dynamically when your pages are viewed.
85When caching is enabled, the first time a web page is loaded, the query result object will
86be 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
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]87accessing your database. Your database usage can effectively be reduced to zero for any pages that have been cached.</p>
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]88
89<p>Only <dfn>read-type</dfn> (SELECT) queries can be cached, since these are the only type of queries that produce a result.
90<dfn>Write-type</dfn> (INSERT, UPDATE, etc.) queries, since they don't generate a result, will not be cached by the system.</p>
91
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]92<p>Cache files DO NOT expire. Any queries that have been cached will remain cached until you delete them. The caching system
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]93permits you clear caches associated with individual pages, or you can delete the entire collection of cache files.
94Typically you'll want to use the housekeeping functions described below to delete cache files after certain
95events take place, like when you've added new information to your database.</p>
96
97<h2>Will Caching Improve Your Site's Performance?</h2>
98
99<p>Getting a performance gain as a result of caching depends on many factors.
100If you have a highly optimized database under very little load, you probably won't see a performance boost.
101If your database is under heavy use you probably will see an improved response, assuming your file-system is not
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]102overly taxed. Remember that caching simply changes how your information is retrieved, shifting it from being a database
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]103operation to a file-system one.</p>
104
105<p>In some clustered server environments, for example, caching may be detrimental since file-system operations are so intense.
106On single servers in shared environments, caching will probably be beneficial. Unfortunately there is no
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]107single answer to the question of whether you should cache your database. It really depends on your situation.</p>
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]108
109<h2>How are Cache Files Stored?</h2>
110
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]111<p>CodeIgniter places the result of EACH query into its own cache file. Sets of cache files are further organized into
112sub-folders corresponding to your controller functions. To be precise, the sub-folders are named identically to the
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]113first two segments of your URI (the controller class name and function name).</p>
114
115<p>For example, let's say you have a controller called <dfn>blog</dfn> with a function called <dfn>comments</dfn> that
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]116contains three queries. The caching system will create a cache folder
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]117called <kbd>blog+comments</kbd>, into which it will write three cache files.</p>
118
119<p>If you use dynamic queries that change based on information in your URI (when using pagination, for example), each instance of
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]120the query will produce its own cache file. It's possible, therefore, to end up with many times more cache files than you have
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]121queries.</p>
122
123
124<h2>Managing your Cache Files</h2>
125
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]126<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
127that allows user commenting. Whenever a new comment is submitted you'll want to delete the cache files associated with the
128controller function that serves up your comments. You'll find two delete functions described below that help you
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]129clear data.</p>
130
131
132<h2>Not All Database Functions Work with Caching</h2>
133
134<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,
135some of the query result functions are not available for use.</p>
136
137<p>The following functions <kbd>ARE NOT</kbd> available when using a cached result object:</p>
138
139<ul>
140<li>num_fields()</li>
141<li>field_names()</li>
142<li>field_data()</li>
143<li>free_result()</li>
144</ul>
145
146<p>Also, the two database resources (result_id and conn_id) are not available when caching, since result resources only
147pertain to run-time operations.</p>
148
149
150<br />
151
152<h1>Function Reference</h1>
153
154
155
156<h2>$this->db->cache_on()&nbsp; / &nbsp; $this->db->cache_off()</h2>
157
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]158<p>Manually enables/disables caching. This can be useful if you want to
159keep certain queries from being cached. Example:</p>
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]160
161<code>
162// Turn caching on<br />
163$this->db->cache_on();<br />
164$query = $this->db->query("SELECT * FROM mytable");<br />
165<br />
166// Turn caching off for this one query<br />
167$this->db->cache_off();<br />
168$query = $this->db->query("SELECT * FROM members WHERE member_id = '$current_user'");<br />
169<br />
170// Turn caching back on<br />
171$this->db->cache_on();<br />
172$query = $this->db->query("SELECT * FROM another_table");
173</code>
174
175
176<h2>$this->db->cache_delete()</h2>
177
178<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>
179
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]180<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
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]181a page at <dfn>example.com/index.php/blog/comments</dfn>, the caching system will put all cache files associated with it in a folder
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]182called <dfn>blog+comments</dfn>. To delete those particular cache files you will use:</p>
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]183
184<code>$this->db->cache_delete('blog', 'comments');</code>
185
186<p>If you do not use any parameters the current URI will be used when determining what should be cleared.</p>
187
188
189<h2>$this->db->cache_delete_all()</h2>
190
Derek Jones4b9c6292011-07-01 17:40:48 -0500[diff] [blame]191<p>Clears all existing cache files. Example:</p>
Derek Allard2067d1a2008-11-13 22:59:24 +0000[diff] [blame]192
193<code>$this->db->cache_delete_all();</code>
194
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