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  <
div class="section" id="database-caching-class">
<
h1>Database Caching Class<a class="headerlink" href="#database-caching-class" title="Permalink to this headline"></a></h1>
<
p>The Database Caching Class permits you to cache your queries as text
files 
for reduced database load.</p>
<
div class="admonition important">
<
p class="first admonition-title">Important</p>
<
p class="last">This class is initialized automatically by the database
driver when caching is enabled
. Do NOT load this class manually.</p>
</
div>
<
div class="admonition important">
<
p class="first admonition-title">Important</p>
<
p class="last">Not all query result functions are available when you
use caching. Please read this page carefully.</p>
</
div>
<
div class="section" id="enabling-caching">
<
h2>Enabling Caching<a class="headerlink" href="#enabling-caching" title="Permalink to this headline"></a></h2>
<
p>Caching is enabled in three steps:</p>
<
ul class="simple">
<
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
application
/config/database.php file.</li>
<
li>Enable the caching feature, either globally by setting the preference
in your application
/config/database.php 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>
</
div>
<
div class="section" id="how-does-caching-work">
<
h2>How Does Caching Work?<a class="headerlink" href="#how-does-caching-work" title="Permalink to this headline"></a></h2>
<
p>CodeIgniter&#8217;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 read-type (SELECT) queries can be cached, since these are the only
type of queries that produce a result
. Write-type (INSERT, UPDATE, etc.)
queries, since they don&#8217;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&#8217;ll want to use the housekeeping
functions described below to delete cache files after certain events
take place
, like when you&#8217;ve added new information to your database.</p>
</div>
<
div class="section" id="will-caching-improve-your-site-s-performance">
<
h2>Will Caching Improve Your Site&#8217;s Performance?<a class="headerlink" href="#will-caching-improve-your-site-s-performance" title="Permalink to this headline">¶</a></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&#8217;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>
</
div>
<
div class="section" id="how-are-cache-files-stored">
<
h2>How are Cache Files Stored?<a class="headerlink" href="#how-are-cache-files-stored" title="Permalink to this headline"></a></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&#8217;s say you have a controller called blog with a function
called comments that contains three queries. The caching system will
create a cache folder called blog
+comments, 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&#8217;s possible, therefore, to end up with
many times more cache files than you have queries.</p>
</
div>
<
div class="section" id="managing-your-cache-files">
<
h2>Managing your Cache Files<a class="headerlink" href="#managing-your-cache-files" title="Permalink to this headline"></a></h2>
<
p>Since cache files do not expire, you&#8217;ll need to build deletion routines
into your application. For example, let&#8217;s say you have a blog that
allows user commenting. Whenever a new comment is submitted you&#8217;ll want
to delete the cache files associated with the controller function that
serves up your comments
. You&#8217;ll find two delete functions described
below that help you clear data.</p>
</
div>
<
div class="section" id="not-all-database-functions-work-with-caching">
<
h2>Not All Database Functions Work with Caching<a class="headerlink" href="#not-all-database-functions-work-with-caching" title="Permalink to this headline"></a></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 ARE NOT available when using a cached result
object
:</p>
<
ul class="simple">
<
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>
<
div class="section" id="function-reference">
<
h3>Function Reference<a class="headerlink" href="#function-reference" title="Permalink to this headline"></a></h3>
</
div>
</
div>
<
div class="section" id="this-db-cache-on-this-db-cache-off">
<
h2>$this-&gt;db-&gt;cache_on() / $this-&gt;db-&gt;cache_off()<a class="headerlink" href="#this-db-cache-on-this-db-cache-off" title="Permalink to this headline"></a></h2>
<
p>Manually enables/disables caching. This can be useful if you want to
keep certain queries from being cached
. Example:</p>
<
div class="highlight-ci"><div class="highlight"><pre><span class="c1">// Turn caching on</span>
<span class="nv">$this</span><span class="o">-&gt;</span><span class="na">db</span><span class="o">-&gt;</span><span class="na">cache_on</span><span class="p">();</span>
<
span class="nv">$query</span> <span class="o">=</span> <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">db</span><span class="o">-&gt;</span><span class="na">query</span><span class="p">(</span><span class="s2">&quot;SELECT * FROM mytable&quot;</span><span class="p">);</span>

<
span class="c1">// Turn caching off for this one query</span>
<span class="nv">$this</span><span class="o">-&gt;</span><span class="na">db</span><span class="o">-&gt;</span><span class="na">cache_off</span><span class="p">();</span>
<
span class="nv">$query</span> <span class="o">=</span> <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">db</span><span class="o">-&gt;</span><span class="na">query</span><span class="p">(</span><span class="s2">&quot;SELECT * FROM members WHERE member_id = &#39;</span><span class="si">$current_user</span><span class="s2">&#39;&quot;</span><span class="p">);</span>

<span class="c1">// Turn caching back on</span>
<span class="nv">$this</span><span class="o">-&gt;</span><span class="na">db</span><span class="o">-&gt;</span><span class="na">cache_on</span><span class="p">();</span>
<
span class="nv">$query</span> <span class="o">=</span> <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">db</span><span class="o">-&gt;</span><span class="na">query</span><span class="p">(</span><span class="s2">&quot;SELECT * FROM another_table&quot;</span><span class="p">);</span>
</
pre></div>
</
div>
</
div>
<
div class="section" id="this-db-cache-delete">
<
h2>$this-&gt;db-&gt;cache_delete()<a class="headerlink" href="#this-db-cache-delete" title="Permalink to this headline"></a></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 example
.com/index.php/blog/comments, the caching system will put
all cache files associated with it in a folder called blog
+comments. To
delete those particular cache files you will 
use:</p>
<
div class="highlight-ci"><div class="highlight"><pre><span class="nv">$this</span><span class="o">-&gt;</span><span class="na">db</span><span class="o">-&gt;</span><span class="na">cache_delete</span><span class="p">(</span><span class="s1">&#39;blog&#39;</span><span class="p">,</span> <span class="s1">&#39;comments&#39;</span><span class="p">);</span>
</pre></div>
</
div>
<
p>If you do not use any parameters the current URI will be used when
determining what should be cleared
.</p>
</
div>
<
div class="section" id="this-db-cache-delete-all">
<
h2>$this-&gt;db-&gt;cache_delete_all()<a class="headerlink" href="#this-db-cache-delete-all" title="Permalink to this headline"></a></h2>
<
p>Clears all existing cache files. Example:</p>
<
div class="highlight-ci"><div class="highlight"><pre><span class="nv">$this</span><span class="o">-&gt;</span><span class="na">db</span><span class="o">-&gt;</span><span class="na">cache_delete_all</span><span class="p">();</span>
</
pre></div>
</
div>
</
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