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+ <div class="section" id="reclass-concepts">
+<h1>reclass concepts<a class="headerlink" href="#reclass-concepts" title="Permalink to this headline">¶</a></h1>
+<p><strong>reclass</strong> assumes a node-centric perspective into your inventory. This is
+obvious when you query <strong>reclass</strong> for node-specific information, but it might not
+be clear when you ask <strong>reclass</strong> to provide you with a list of groups. In that
+case, <strong>reclass</strong> loops over all nodes it can find in its database, reads all
+information it can find about the nodes, and finally reorders the result to
+provide a list of groups with the nodes they contain.</p>
+<p>Since the term “groups” is somewhat ambiguous, it helps to start off with
+a short glossary of <strong>reclass</strong>-specific terminology:</p>
+<table border="1" class="docutils">
+<colgroup>
+<col width="16%" />
+<col width="84%" />
+</colgroup>
+<thead valign="bottom">
+<tr class="row-odd"><th class="head">Concept</th>
+<th class="head">Description</th>
+</tr>
+</thead>
+<tbody valign="top">
+<tr class="row-even"><td>node</td>
+<td>A node, usually a computer in your infrastructure</td>
+</tr>
+<tr class="row-odd"><td>class</td>
+<td>A category, tag, feature, or role that applies to a node
+Classes may be nested, i.e. there can be a class hierarchy</td>
+</tr>
+<tr class="row-even"><td>application</td>
+<td>A specific set of behaviour to apply</td>
+</tr>
+<tr class="row-odd"><td>parameter</td>
+<td>Node-specific variables, with inheritance throughout the class
+hierarchy.</td>
+</tr>
+</tbody>
+</table>
+<p>A class consists of zero or more parent classes, zero or more applications,
+and any number of parameters.</p>
+<p>A class name must not contain spaces.</p>
+<p>A node is almost equivalent to a class, except that it usually does not (but
+can) specify applications.</p>
+<p>When <strong>reclass</strong> parses a node (or class) definition and encounters a parent
+class, it recurses to this parent class first before reading any data of the
+node (or class). When <strong>reclass</strong> returns from the recursive, depth first walk, it
+then merges all information of the current node (or class) into the
+information it obtained during the recursion.</p>
+<p>Furthermore, a node (or class) may define a list of classes it derives from,
+in which case classes defined further down the list will be able to override
+classes further up the list.</p>
+<p>Information in this context is essentially one of a list of applications or
+a list of parameters.</p>
+<p>The interaction between the depth-first walk and the delayed merging of data
+means that the node (and any class) may override any of the data defined by
+any of the parent classes (ancestors). This is in line with the assumption
+that more specific definitions (“this specific host”) should have a higher
+precedence than more general definitions (“all webservers”, which includes all
+webservers in Munich, which includes “this specific host”, for example).</p>
+<p>Here’s a quick example, showing how parameters accumulate and can get
+replaced.</p>
+<blockquote>
+<div><p>All “unixnodes” (i.e. nodes who have the <tt class="docutils literal"><span class="pre">unixnode</span></tt> class in their
+ancestry) have <tt class="docutils literal"><span class="pre">/etc/motd</span></tt> centrally-managed (through the <tt class="docutils literal"><span class="pre">motd</span></tt>
+application), and the <cite>unixnode</cite> class definition provides a generic
+message-of-the-day to be put into this file.</p>
+<p>All descendants of the class <tt class="docutils literal"><span class="pre">debiannode</span></tt>, a descendant of <tt class="docutils literal"><span class="pre">unixnode</span></tt>,
+should include the Debian codename in this message, so the
+message-of-the-day is overwritten in the <tt class="docutils literal"><span class="pre">debiannodes</span></tt> class.</p>
+<p>The node <tt class="docutils literal"><span class="pre">quantum.example.org</span></tt> (a <cite>debiannode</cite>) will have a scheduled
+downtime this weekend, so until Monday, an appropriate message-of-the-day is
+added to the node definition.</p>
+<p>When the <tt class="docutils literal"><span class="pre">motd</span></tt> application runs, it receives the appropriate
+message-of-the-day (from <tt class="docutils literal"><span class="pre">quantum.example.org</span></tt> when run on that node) and
+writes it into <tt class="docutils literal"><span class="pre">/etc/motd</span></tt>.</p>
+</div></blockquote>
+<p>At this point it should be noted that parameters whose values are lists or
+key-value pairs don’t get overwritten by children classes or node definitions,
+but the information gets merged (recursively) instead.</p>
+<p>Similarly to parameters, applications also accumulate during the recursive
+walk through the class ancestry. It is possible for a node or child class to
+<em>remove</em> an application added by a parent class, by prefixing the application
+with <cite>~</cite>.</p>
+<p>Finally, <strong>reclass</strong> happily lets you use multiple inheritance, and ensures that
+the resolution of parameters is still well-defined. Here’s another example
+building upon the one about <tt class="docutils literal"><span class="pre">/etc/motd</span></tt> above:</p>
+<blockquote>
+<div><p><tt class="docutils literal"><span class="pre">quantum.example.org</span></tt> (which is back up and therefore its node definition
+no longer contains a message-of-the-day) is at a site in Munich. Therefore,
+it is a child of the class <tt class="docutils literal"><span class="pre">hosted@munich</span></tt>. This class is independent of
+the <tt class="docutils literal"><span class="pre">unixnode</span></tt> hierarchy, <tt class="docutils literal"><span class="pre">quantum.example.org</span></tt> derives from both.</p>
+<p>In this example infrastructure, <tt class="docutils literal"><span class="pre">hosted@munich</span></tt> is more specific than
+<tt class="docutils literal"><span class="pre">debiannode</span></tt> because there are plenty of Debian nodes at other sites (and
+some non-Debian nodes in Munich). Therefore, <tt class="docutils literal"><span class="pre">quantum.example.org</span></tt> derives
+from <tt class="docutils literal"><span class="pre">hosted@munich</span></tt> _after_ <tt class="docutils literal"><span class="pre">debiannodes</span></tt>.</p>
+<p>When an electricity outage is expected over the weekend in Munich, the admin
+can change the message-of-the-day in the <tt class="docutils literal"><span class="pre">hosted@munich</span></tt> class, and it
+will apply to all hosts in Munich.</p>
+<p>However, not all hosts in Munich have <tt class="docutils literal"><span class="pre">/etc/motd</span></tt>, because some of them
+are of class <tt class="docutils literal"><span class="pre">windowsnode</span></tt>. Since the <tt class="docutils literal"><span class="pre">windowsnode</span></tt> ancestry does not
+specify the <tt class="docutils literal"><span class="pre">motd</span></tt> application, those hosts have access to the
+message-of-the-day in the node variables, but the message won’t get used…</p>
+<p>… unless, of course, <tt class="docutils literal"><span class="pre">windowsnode</span></tt> specified a Windows-specific
+application to bring such notices to the attention of the user.</p>
+</div></blockquote>
+<p>It’s also trivial to ensure a certain order of class evaluation. Here’s
+another example:</p>
+<blockquote>
+<div><p>The <tt class="docutils literal"><span class="pre">ssh.server</span></tt> class defines the <tt class="docutils literal"><span class="pre">permit_root_login</span></tt> parameter to <tt class="docutils literal"><span class="pre">no</span></tt>.</p>
+<p>The <tt class="docutils literal"><span class="pre">backuppc.client</span></tt> class defines the parameter to <tt class="docutils literal"><span class="pre">without-password</span></tt>,
+because the BackupPC server might need to log in to the host as root.</p>
+<p>Now, what happens if the admin accidentally provides the following two
+classes?</p>
+<ul class="simple">
+<li><tt class="docutils literal"><span class="pre">backuppc.client</span></tt></li>
+<li><tt class="docutils literal"><span class="pre">ssh.server</span></tt></li>
+</ul>
+<p>Theoretically, this would mean <tt class="docutils literal"><span class="pre">permit_root_login</span></tt> gets set to <tt class="docutils literal"><span class="pre">no</span></tt>.</p>
+<p>However, since all <tt class="docutils literal"><span class="pre">backuppc.client</span></tt> nodes need <tt class="docutils literal"><span class="pre">ssh.server</span></tt> (at least
+in most setups), the class <tt class="docutils literal"><span class="pre">backuppc.client</span></tt> itself derives from
+<tt class="docutils literal"><span class="pre">ssh.server</span></tt>, ensuring that it gets parsed before <tt class="docutils literal"><span class="pre">backuppc.client</span></tt>.</p>
+<p>When <strong>reclass</strong> returns to the node and encounters the <tt class="docutils literal"><span class="pre">ssh.server</span></tt> class
+defined there, it simply skips it, as it’s already been processed.</p>
+</div></blockquote>
+<p>Now read about <a class="reference internal" href="operations.html"><em>reclass operations</em></a>!</p>
+</div>
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