Add some nice docs on what this is.

cloudinit/mergers/__init__.py

@@ -85,7 +85,7 @@ class LookupMerger(UnknownMerger):
 
 def dict_extract_mergers(config):
     parsed_mergers = []
-    raw_mergers = config.get('merger_how')
+    raw_mergers = config.get('merge_how')
     if raw_mergers is None:
         raw_mergers = config.get('merge_type')
     if raw_mergers is None:

doc/merging.txt

@@ -0,0 +1,179 @@
+Arriving in 0.7.2 is a new way to handle dictionary merging in cloud-init.
+---
+
+Overview
+--------
+
+This was done because it has been a common feature request that there be a
+way to specify how cloud-config yaml "dictionaries" 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 there contained objects) together in a
+way that is customizable, thus allowing for users who provide cloud-config data
+to determine exactly how there objects will be merged.
+
+For example.
+
+#cloud-config (1)
+run_cmd:
+  - bash1
+  - bash2
+
+#cloud-config (2)
+run_cmd:
+  - bash3
+  - bash4
+
+The previous way of merging the following 2 objects would result in a final 
+cloud-config object that contains the following.
+
+#cloud-config (merged)
+run_cmd:
+  - bash3
+  - bash4
+
+Typically this is not what users want, instead they would likely prefer:
+
+#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 that would have had to
+occur in the previous method to accomplish the same result.
+
+Customizability
+---------------
+
+Since the above merging algorithm may not always be the desired merging
+algorithm (like how the merging algorithm in < 0.7.2 was not always the preferred
+one) the concept of customizing how merging can be done was introduced through
+a new concept call 'merge classes'. 
+
+A merge class is a class defintion 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:
+
+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.
+
+"classname(option1,option2)+classname2(option3,option4)" (and so on)
+
+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(extend)+dict()+str(append)"
+
+*Dictionary format*
+
+In cases where a dictionary can be used to specify the same information as the
+string format (ie option #2 of above) it can be used, for example.
+
+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
+----------
+
+The default merging algorithm for merging conf.d yaml files (which form a 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 similar now extensible (metadata merging for
+example).