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git-svn-id: https://svn.mcs.anl.gov/repos/bcfg/trunk/bcfg2@867 ce84e21b-d406-0410-9b95-82705330c041

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+<chapter>
+    <title>Generators</title>
+    
+    <para>Generators are modules are are loaded by the Bcfg2 server,
+      based on directives in <filename>/etc/bcfg2.conf</filename>. They
+      provide concrete, fully-specified configuration entries for
+      clients. This chapter documents the function and usage of generators
+      bundled with Bcfg2 releases. It also describes the interface used to
+      communicate with generators; modeles implementing this interface can
+      provide configuration elements for clients based on any
+      representation or requirements that may exist. 
+    </para>
+
+  <section>
+    <title>Bundled Generators</title>
+
+    <para>This section describes the generators that come bundled with
+    Bcfg2. As a general rule, generators requiring more than one
+    configuration file will use a generator specific directory in the
+    configuration repository. 
+    </para>
+
+    <section>
+      <title>Cfg</title>
+      <para>
+	The Cfg generator provides a configuration file repository
+	that uses literal file contents to provide client-tailored
+	configuration file entries. The Cfg generator chooses which
+	data to provide for a given client based on the aspect-based
+	metadata system used for high-level client configuration. 
+      </para>
+      <para>
+	The Cfg repository is structured much like the filesystem
+	hierarchy being configured. Each configuration file being
+	served has a corresponding directory in the configuration
+	repository. These directories have the same relative path as
+	the absolute path of the configuration file on the target
+	system. For example, if Cfg was serving data for the
+	configuration file <filename>/etc/services</filename>, then
+	its directory would be in the relative path
+	<filename>./etc/services</filename> inside of the Cfg
+	repository. 
+      </para>
+      <para>
+	Inside of this file-specific directory, three types of files
+	may exist. Base files are complete instances of configuration
+	file. Deltas are differences between a base file and the
+	target file contents. Base files and deltas are tagged with
+	metadata specifiers, which describe which groups of clients
+	the fragment pertains to. Configuration files are constructed
+	by finding the most specific base file and applying any more
+	specific deltas.
+      </para>
+      <para>
+	Specifiers are embedded in fragment filenames. For example, in
+	the fragment <filename>services.C99_webserver</filename>,
+	"C99_webserver" is the specifier. This specifier applies to
+	the class (C) webserver with a priority of 99. Other metadata
+	categories which can be used include bundle (B), profile (P),
+	hostname (H), attribute (A), and image (I). These are ordered
+	from least to most specific: image, profile, class, bundle,
+	and hostname. Global files are the least specific. Priorities
+	are used as to break ties.
+      </para>
+      <para>
+	Info files, named <filename>:info</filename> are used to
+	specify target configuration file metadata, such as owner,
+	group and permissions. If no <filename>:info</filename> is
+	provided, targets are installed with default
+	information. Default metadata is root ownership, root group
+	memberships, and 0644 file permissions.
+      </para>
+      <example>
+	<title>Cfg generator :info files</title>
+	<programlisting>
+	  owner:root
+	  group:root
+	  perms:0755
+	</programlisting>
+      </example>
+
+      <example>
+	<title>Cfg file repository example</title>
+	<programlisting>
+	  $ ls
+	  :info            passwd	  passwd.C99_chiba-login
+	  passwd.H_bio-debian  passwd.H_cvstest   passwd.H_foxtrot  
+	  passwd.H_reboot  passwd.H_rudy2    passwd.C99_netserv
+	  passwd.B99_tacacs-server.cat  passwd.H_adenine        
+	</programlisting>
+      </example>
+      
+      <para>
+	In the previous example, there exists files with each of the
+	characteristics mentioned above. All files ending in ".cat"
+	are deltas; ones with ".H_" are host specific files. There
+	exists a base file, a <filename>:info</filename> file, two
+	class-specified base files, and a bundle-specified base file.
+      </para>
+    </section>
+
+    <section>
+      <title>Pkgmgr</title>
+      <para/>
+    </section>
+
+  </section>
+
+  <section>
+    <title>The Generator API</title>
+    <para>
+      The Bcfg2 core has a well-formed API used to call
+      generators. This mechanism allows all stock generators to be
+      runtime selected; no stock generators are required. The
+      generator API has two main functions. The first is communication
+      to the Bcfg2 core: the list of entries a particular generator
+      can bind must be communicated to the core so that the proper
+      generator can be called. The second function is the actual
+      production of client-specific configuration element data; this
+      data is then included in client configurations.
+    </para>
+
+    <para>
+      The inventory function is provided by a python dictionary,
+      called __provides__ in each generator object. This dictionary
+      has a key for each type of configuration entry (ConfigFile,
+      Package, Directory, SymLink, Service), whose value is a
+      dictionary indexed by configuration element name. For example,
+      the data path to information about the service "sshd" could be
+      reached at __provides__['Service']['sshd']. The value of each of
+      these keys is a function that can be called to bind
+      client-specific values to a configuration entry. This function
+      is used in the next section. 
+    </para>
+
+    <para>
+      The handler function located by the __provides__ dictionary is
+      called with a static API. The function prototype for each of
+      these handlers is:
+    </para>
+    
+    <example>
+      <title>The Generator handler API</title>
+      <programlisting>
+	def Handler(self, entry, metadata):
+	    generator logic here
+      </programlisting>
+    </example>
+
+    <para>
+      The data supplied upon handler invokation includes two
+      parts. The first is the entry. This is a ElementTree.Element
+      object, which already contains the configuration element type
+      (ie Service) and name. All other data is bound into this object
+      in this function. The range of data bound depends on the data
+      type. The other data provided to handlers is client metadata,
+      information about the current client, including hostname, image,
+      profile, classes and bundles. The metadata is typically used to
+      choose entry contents.
+    </para>
+  </section>
+
+  <section>
+    <title>Writing a Generator</title>
+    <para>
+      Writing a generator is a fairly straightforward task. At a high
+      level, generators are instantiated by the Bcfg2 core, and then
+      used to provide configuration entry contents. This means that
+      the two points where control passes into a generator from Bcfg2
+      are during initial object instantiation, and every time a
+      generator-provided configuration entry is bound.
+    </para>
+
+    <para>
+      Currently, generators must be written in python. They can
+      perform arbitrary operations, hence, a generator could be
+      written that executed logic in another language, but this
+      functionality is currently not implemented. 
+    </para>
+  </section>
+</chapter>
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