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From Oliver Heger <oliver.he...@oliver-heger.de>
Subject Re: [configuration] Combined Configuration examples
Date Sun, 13 Nov 2016 18:51:46 GMT
Hi Greg,

Am 13.11.2016 um 18:41 schrieb Greg Torrance:
> Hi Oliver,
> 
> Thanks for the reply. That is helpful!
> 
>> Hm, it looks as if the examples are partly incorrect. I assume that
> the channel with id="4" should not be part of testfile1.xml, but only be
> contained in the second file. It should appear in the results of union
> and  merge combiner. Does that make sense?
> 
> I'm not clear why -- if the channel with id="4" is not contained in
> testfile1.xml -- why it will not end up in the results from all three
> combiners. Why just union and merge and not override, too?
On a higher level, OverrideCombiner detects that both files have a list
of channels. Therefore, it copies the first list and ignores the second
one. So the channels element in the first file overrides the one in the
second file.

> 
> As for the attributes, do I understand correctly that the attribute
> values for elements are used, essentially, as "key" values to
> distinguish the various elements for combining purposes? And if the
> values of any attributes of two elements differ, then the elements would
> be considered different elements for combining purposes? (Said another
> way, if two elements have a set of attributes in common, if the values
> for those common attributes are the same, the elements will be
> considered to match.) Does that make sense? Is that correct?
This is true only for MergeCombiner. The other combiners do not try to
match nodes in both configurations based on their attribute values; they
take two source nodes at the same position in the hierarchy and produce
a result node that has all the attributes of the two source nodes.

Oliver

> 
> Thanks again for the reply.
> 
> Greg
> 
> On 11/13/2016 12:05 PM, Oliver Heger wrote:
>> Hi Greg,
>>
>> Am 13.11.2016 um 15:07 schrieb Greg Torrance:
>>> Hi all,
>>>
>>> I am trying to work out the precise differences between
>>> OverrideCombiner, UnionCombiner, and MergeCombiner. However, the
>>> examples on this page
>>> (https://commons.apache.org/proper/commons-configuration/userguide/howto_combinedconfiguration.html)
>>>
>>> are confusing to me.
>>>
>>> testfile1.xml and testfile2.xml both contain Channels with id="4",
>>> however none of the Combiner Results shows any Channels with id="4".
>>> And the Notes on the right-hand side of the page contain no mention of
>>> Channel 4.
>>>
>>> Am I missing something? Are the examples correct?
>> Hm, it looks as if the examples are partly incorrect. I assume that the
>> channel with id="4" should not be part of testfile1.xml, but only be
>> contained in the second file. It should appear in the results of union
>> and merge combiner. Does that make sense?
>>
>>> What I'm really trying to understand is the impact of attributes in the
>>> way the various combiners choose to combine elements. (Any pointers to
>>> further documentation on this would be appreciated.)
>> Some more information can be found in the Javadocs of the combiner
>> classes:
>> - UnionCombiner states: "Another limitation is the handling of
>> attributes: Attributes can only have a single value. So if two nodes are
>> to be combined which both have an attribute with the same name, it is
>> not possible to construct a proper union attribute. In this case, the
>> attribute value from the first node is used."
>>
>> - OverrideCombiner has an addAttributes() method with the following
>> documentation: "Handles the attributes during a combination process.
>> First all attributes of the first node are added to the result. Then all
>> attributes of the second node, which are not contained in the first
>> node, are also added."
>>
>> - The merging behavior of MergeCombiner is also described in its class
>> documentation.
>>
>> Does this help?
>> Oliver
>>
>>> Thoughts?
>>>
>>> Thanks,
>>> Greg
>>>
>>>
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> 
> 
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