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List: xml-dev
Subject: [xml-dev] A simple and non-brittle way to create XML that evolves over time
From: "Costello, Roger L." <costello () mitre ! org>
Date: 2015-04-29 13:39:39
Message-ID: DM2PR09MB035180AE18F7E248BD576350C8D70 () DM2PR09MB0351 ! namprd09 ! prod ! outlook ! com
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Hi Folks,
Acknowledgement: Some of the below text and idea comes from section 4.2 of =
RFC 7493 (http://www.rfc-editor.org/rfc/rfc7493.txt)
It is frequently the case that changes to XML Schemas are required after th=
ey have been put in production. XML Schemas that allow the introduction of=
new elements in a way that does not disrupt the operation of existing soft=
ware have proven advantageous in practice.
This can be referred to as a "Must-Ignore" policy, meaning that when an imp=
lementation encounters an element that it does not recognize, it should tre=
at the rest of the document as if the new element simply did not appear, an=
d in particular, the implementation must not treat this as an error conditi=
on. The converse "Must-Understand" policy does not tolerate the introducti=
on of new elements, and while this has proven necessary in certain designs,=
in general it has been found to be overly restrictive and brittle.
A good way to support the use of Must-Ignore in XML designs is to require t=
hat the root element must contain a list of key/value pairs where each key =
is unique, and to specify that keys whose names are unrecognized must be ig=
nored.
Example. Here is an XML document where the root element contains a list of =
key/value pairs and each key is unique:
<Book>
<Title>The First and Last Freedom</Title>
<Author>J. Krishnamurti</Author>
<Date>1954</Date>
<ISBN>0-06-064831-7</ISBN>
<Publisher>Harper & Row</Publisher>
</Book>
This example shows simple values for each key. Of course the values could b=
e complex.
Suppose that at a later date the XML Schema is extended to include a NumPag=
es element. Now XML instances contain this:
<Book>
<Title>The First and Last Freedom</Title>
<Author>J. Krishnamurti</Author>
<Date>1954</Date>
<NumPages>289</NumPages>
<ISBN>0-06-064831-7</ISBN>
<Publisher>Harper & Row</Publisher>
</Book>
If a consumer of this XML has not been coded to understand that new key/val=
ue pair, the consumer must ignore it.
Recap: design your XML so that the root element contains a list of key/valu=
e pairs where each key is unique. Notify consumers that over time the set o=
f root element keys will expand and they should ignore any root element key=
s they do not understand.
/Roger
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<p class="MsoNormal">Hi Folks,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Acknowledgement: Some of the below text and idea comes from \
section 4.2 of RFC 7493 (<a \
href="http://www.rfc-editor.org/rfc/rfc7493.txt">http://www.rfc-editor.org/rfc/rfc7493.txt</a>)
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">It is frequently the case that changes to XML Schemas are \
required after they have been put in production. XML Schemas that allow the \
introduction of new elements in a way that does not disrupt the operation of existing \
software have proven advantageous in practice.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This can be referred to as a "Must-Ignore" policy, \
meaning that when an implementation encounters an element that it does not recognize, \
it should treat the rest of the document as if the new element simply did not appear, \
and in particular, the implementation must not treat this as an error \
condition. The converse "Must-Understand" policy does not tolerate \
the introduction of new elements, and while this has proven necessary in certain \
designs, in general it has been found to be overly restrictive and \
brittle.<o:p></o:p></p> <p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">A good way to support the use of Must-Ignore in XML designs is \
to require that the root element must contain a list of key/value pairs where each \
key is unique, and to specify that keys whose names are unrecognized must be \
ignored.<o:p></o:p></p> <p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Example. Here is an XML document where the root element contains \
a list of key/value pairs and each key is unique:<o:p></o:p></p> <p \
class="MsoNormal"><o:p> </o:p></p> <p class="MsoNormal"><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><Book></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><Title></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black">The First and Last Freedom</span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></Title></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><Author></span><span \
style="font-size:12.0pt;font-family:"Times New Roman",serif;color:black">J. \
Krishnamurti</span><span style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></Author></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><Date></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black">1954</span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></Date></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><ISBN></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black">0-06-064831-7</span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></ISBN></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><Publisher></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black">Harper </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#969600">&amp;</span><span \
style="font-size:12.0pt;font-family:"Times New Roman",serif;color:black"> \
Row</span><span style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></Publisher></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></Book></span><o:p></o:p></p> <p \
class="MsoNormal"><o:p> </o:p></p> <p class="MsoNormal">This example shows \
simple values for each key. Of course the values could be complex.<o:p></o:p></p> <p \
class="MsoNormal"><o:p> </o:p></p> <p class="MsoNormal">Suppose that at a later \
date the XML Schema is extended to include a NumPages element. Now XML instances \
contain this:<o:p></o:p></p> <p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><Book></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><Title></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black">The First and Last Freedom</span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></Title></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><Author></span><span \
style="font-size:12.0pt;font-family:"Times New Roman",serif;color:black">J. \
Krishnamurti</span><span style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></Author></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><Date></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black">1954</span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></Date></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><b><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><NumPages></span></b><b><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black">289</span></b><b><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></NumPages></span></b><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><ISBN></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black">0-06-064831-7</span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></ISBN></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"><Publisher></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black">Harper </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#969600">&amp;</span><span \
style="font-size:12.0pt;font-family:"Times New Roman",serif;color:black"> \
Row</span><span style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></Publisher></span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:black"><br> </span><span \
style="font-size:12.0pt;font-family:"Times New \
Roman",serif;color:#000096"></Book></span><o:p></o:p></p> <p \
class="MsoNormal"><o:p> </o:p></p> <p class="MsoNormal">If a consumer of this \
XML has not been coded to understand that new key/value pair, the consumer must \
ignore it.<o:p></o:p></p> <p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Recap: design your XML so that the root element contains a \
list of key/value pairs where each key is unique. Notify consumers that over time the \
set of root element keys will expand and they should ignore any root element keys \
they do not understand.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">/Roger<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
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