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<h1 class="SECT1"><a name="CDEFS" id="CDEFS">CDEFs</a></h1>
<p>CDEFs allow you to apply mathematical functions to graph data to alter output. The concept of a CDEF comes straight from <span class="APPLICATION">RRDTool</span>, and are written in reverse polish notation (RPN). For more information regarding the syntax of CDEFs, check out the <a href="http://people.ee.ethz.ch/~oetiker/webtools/rrdtool/doc/rrdgraph_data.en.html" target="_top">CDEF tutorial</a>.</p>
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<h2 class="SECT2"><a name="ADD_NEW_CDEF" id="ADD_NEW_CDEF">Creating a CDEF</a></h2>
<p>To create a new CDEF in Cacti, select the <span class="GUILABEL">Graph Management</span> option under the <span class="GUILABEL">Management</span> heading, and select <span class="GUILABEL">CDEFs</span>. Once at this screen, click <span class="GUILABEL">Add</span> to the right. You will be prompted for a CDEF name, for which you can type anything used to describe your CDEF. Click the <span class="GUILABEL">Create</span> button so you are redirected back to the edit page, now with an empty <span class="GUILABEL">CDEF Items</span> box. Construct your CDEF by adding an item for each element in the CDEF string, common types such as operators and functions are enumerated for your convenience. Below is a basic description of each CDEF item type.</p>
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<p><b>Table 16-1. CDEF Item Types</b></p>
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<td>Function</td>
<td>You can choose a CDEF function to use as the item. The <a href="http://people.ee.ethz.ch/~oetiker/webtools/rrdtool/doc/rrdgraph.en.html" target="_top">RRDTool graph manual</a> describes the purpose of each CDEF function.</td>
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<td>Operator</td>
<td>Just your standard math operators, including modulo (%).</td>
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<td>Special Data Source</td>
<td>A special data source is basically a flag to tell Cacti to do some special processing when it encounters this CDEF item. The "Current Graph Item Data Source" type basically inserts the name of the data source that is referenced by the graph item that references to this CDEF. Both of the "All Data Sources" types insert a summation of all data sources used on a graph.</td>
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<td>Another CDEF</td>
<td>You can recursively use another CDEF within this CDEF.</td>
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<td>Custom String</td>
<td>Sometimes it's just easier to type out the literal CDEF string manually. When referencing to data sources on the graph, remember that Cacti names them 'a', 'b', 'c', '...', starting with the first data source on the graph.</td>
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