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+==================
+reclass operations
+==================
+
+Data merging
+------------
+While |reclass| has been built to support different storage backends through
+plugins, currently only the ``yaml_fs`` storage backend exists. This is a very
+simple, yet powerful, YAML-based backend, using flat files on the filesystem
+(as suggested by the ``_fs`` postfix).
+
+``yaml_fs`` works with two directories, one for node definitions, and another
+for class definitions. It is possible to use a single directory for both, but
+that could get messy and is therefore not recommended.
+
+Files in those directories are YAML-files, specifying key-value pairs. The
+following three keys are read by |reclass|:
+
+============ ================================================================
+Key          Description
+============ ================================================================
+classes      a list of parent classes
+appliations  a list of applications to append to the applications defined by
+             ancestors. If an application name starts with ``~``, it would
+             remove this application from the list, if it had already been
+             added — but it does not prevent a future addition.
+             E.g. ``~firewalled``
+parameters   key-value pairs to set defaults in class definitions, override
+             existing data, or provide node-specific information in node
+             specifications.
+             \
+             By convention, parameters corresponding to an application
+             should be provided as subkey-value pairs, keyed by the name of
+             the application, e.g.::
+
+                applications:
+                - ssh.server
+                parameters:
+                  ssh.server:
+                    permit_root_login: no
+============ ================================================================
+
+|reclass| starts out reading a node definition file, obtains the list of
+classes, then reads the files corresponding to these classes, recursively
+reading parent classes, and finally merges the applications list and the
+parameters.
+
+Merging of parameters is done recursively, meaning that lists and dictionaries
+are extended (recursively), rather than replaced. However, a scalar value
+*does* overwrite a dictionary or list value. While the scalar could be
+appended to an existing list, there is sane default assumption in the context
+of a dictionary, so this behaviour seems the most logical.
+
+Parameter interpolation
+------------------------
+Parameters may reference each other, including deep references, e.g.::
+
+  parameters:
+    location: Munich, Germany
+    motd:
+      header: This node sits in ${location}
+    for_demonstration: ${motd:header}
+    dict_reference: ${motd}
+
+After merging and interpolation, which happens automatically inside the
+storage modules, the ``for_demonstration`` parameter will have a value of
+"This node sits in Munich, Germany".
+
+Types are preserved if the value contains nothing but a reference. Hence, the
+value of ``dict_reference`` will actually be a dictionary.
+
+You should now be ready to :doc:`use reclass <usage>`!
+
+.. include:: substs.inc