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diff --git a/doc/merging.txt b/doc/merging.txt new file mode 100644 index 00000000..f719aec8 --- /dev/null +++ b/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). |