doc: correct grammar and improve clarity in merging documentation.

Move merging.rst into doc/rtd/topics with small fixes.

1 files changed, 199 insertions, 1 deletions

diff --git a/doc/rtd/topics/merging.rst b/doc/rtd/topics/merging.rst
index eca118f5..2f927a47 100644
--- a/doc/rtd/topics/merging.rst
+++ b/doc/rtd/topics/merging.rst
@@ -2,5 +2,203 @@
 Merging User-Data Sections
 **************************
 
-.. include:: ../../merging.rst
+Overview
+========
+
+This was implemented because it has been a common feature request that there be
+a way to specify how cloud-config yaml "dictionaries" provided as user-data are
+merged together when there are multiple yamls to merge together (say when
+performing an #include).
+
+Since previously the merging algorithm was very simple and would only overwrite
+and not append lists, or strings, and so on it was decided to create a new and
+improved way to merge dictionaries (and their contained objects) together in a
+way that is customizable, thus allowing for users who provide cloud-config
+user-data to determine exactly how their objects will be merged.
+
+For example.
+
+.. code-block:: yaml
+
+   #cloud-config (1)
+   run_cmd:
+     - bash1
+     - bash2
+
+   #cloud-config (2)
+   run_cmd:
+     - bash3
+     - bash4
+
+The previous way of merging the two objects above would result in a final
+cloud-config object that contains the following.
+
+.. code-block:: yaml
+
+   #cloud-config (merged)
+   run_cmd:
+     - bash3
+     - bash4
+
+Typically this is not what users want; instead they would likely prefer:
+
+.. code-block:: yaml
+
+   #cloud-config (merged)
+   run_cmd:
+     - bash1
+     - bash2
+     - bash3
+     - bash4
+
+This way makes it easier to combine the various cloud-config objects you have
+into a more useful list, thus reducing duplication necessary to accomplish the
+same result with the previous method.
+
+Customizability
+===============
+
+Because the above merging algorithm may not always be desired (just as the
+previous merging algorithm was not always the preferred one), the concept of
+customized merging was introduced through 'merge classes'.
+
+A merge class is a class definition which provides functions that can be used
+to merge a given type with another given type.
+
+An example of one of these merging classes is the following:
+
+.. code-block:: python
+
+   class Merger(object):
+       def __init__(self, merger, opts):
+           self._merger = merger
+           self._overwrite = 'overwrite' in opts
+
+       # This merging algorithm will attempt to merge with
+       # another dictionary, on encountering any other type of object
+       # it will not merge with said object, but will instead return
+       # the original value
+       #
+       # On encountering a dictionary, it will create a new dictionary
+       # composed of the original and the one to merge with, if 'overwrite'
+       # is enabled then keys that exist in the original will be overwritten
+       # by keys in the one to merge with (and associated values). Otherwise
+       # if not in overwrite mode the 2 conflicting keys themselves will
+       # be merged.
+       def _on_dict(self, value, merge_with):
+           if not isinstance(merge_with, (dict)):
+               return value
+           merged = dict(value)
+           for (k, v) in merge_with.items():
+               if k in merged:
+                   if not self._overwrite:
+                       merged[k] = self._merger.merge(merged[k], v)
+                   else:
+                       merged[k] = v
+               else:
+                   merged[k] = v
+           return merged
+
+As you can see there is a '_on_dict' method here that will be given a source
+value and a value to merge with. The result will be the merged object. This
+code itself is called by another merging class which 'directs' the merging to
+happen by analyzing the types of the objects to merge and attempting to find a
+know object that will merge that type. I will avoid pasting that here, but it
+can be found in the `mergers/__init__.py` file (see `LookupMerger` and
+`UnknownMerger`).
+
+So following the typical cloud-init way of allowing source code to be
+downloaded and used dynamically, it is possible for users to inject there own
+merging files to handle specific types of merging as they choose (the basic
+ones included will handle lists, dicts, and strings). Note how each merge can
+have options associated with it which affect how the merging is performed, for
+example a dictionary merger can be told to overwrite instead of attempt to
+merge, or a string merger can be told to append strings instead of discarding
+other strings to merge with.
+
+How to activate
+===============
+
+There are a few ways to activate the merging algorithms, and to customize them
+for your own usage.
+
+1. The first way involves the usage of MIME messages in cloud-init to specify
+   multipart documents (this is one way in which multiple cloud-config is
+   joined together into a single cloud-config). Two new headers are looked
+   for, both of which can define the way merging is done (the first header to
+   exist wins).  These new headers (in lookup order) are 'Merge-Type' and
+   'X-Merge-Type'. The value should be a string which will satisfy the new
+   merging format defintion (see below for this format).
+
+2. The second way is actually specifying the merge-type in the body of the
+   cloud-config dictionary. There are 2 ways to specify this, either as a
+   string or as a dictionary (see format below). The keys that are looked up
+   for this definition are the following (in order), 'merge_how',
+   'merge_type'.
+
+String format
+-------------
+
+The string format that is expected is the following.
+
+::
+
+   classname1(option1,option2)+classname2(option3,option4)....
+
+The class name there will be connected to class names used when looking for the
+class that can be used to merge and options provided will be given to the class
+on construction of that class.
+
+For example, the default string that is used when none is provided is the
+following:
+
+::
+
+   list()+dict()+str()
+
+Dictionary format
+-----------------
+
+A dictionary can be used when it specifies the same information as the
+string format (i.e. the second option above), for example:
+
+.. code-block:: python
+
+   {'merge_how': [{'name': 'list', 'settings': ['extend']},
+                  {'name': 'dict', 'settings': []},
+                  {'name': 'str', 'settings': ['append']}]}
+
+This would be the equivalent format for default string format but in dictionary
+form instead of string form.
+
+Specifying multiple types and its effect
+========================================
+
+Now you may be asking yourself, if I specify a merge-type header or dictionary
+for every cloud-config that I provide, what exactly happens?
+
+The answer is that when merging, a stack of 'merging classes' is kept, the
+first one on that stack is the default merging classes, this set of mergers
+will be used when the first cloud-config is merged with the initial empty
+cloud-config dictionary. If the cloud-config that was just merged provided a
+set of merging classes (via the above formats) then those merging classes will
+be pushed onto the stack. Now if there is a second cloud-config to be merged
+then the merging classes from the cloud-config before the first will be used
+(not the default) and so on. This way a cloud-config can decide how it will
+merge with a cloud-config dictionary coming after it.
+
+Other uses
+==========
+
+In addition to being used for merging user-data sections, the default merging
+algorithm for merging 'conf.d' yaml files (which form an initial yaml config
+for cloud-init) was also changed to use this mechanism so its full
+benefits (and customization) can also be used there as well. Other places that
+used the previous merging are also, similarly, now extensible (metadata
+merging, for example).
+
+Note, however, that merge algorithms are not used *across* types of
+configuration.  As was the case before merging was implemented,
+user-data will overwrite conf.d configuration without merging.
+
 .. vi: textwidth=78