Are CDATA sections really unnecessary?

问题:

This question is prompted by the rather militant refusal of developer Michael Rys to include the parsing of CDATA sections into FOR XML PATH because "There is no semantic difference in the data that you store."

I have stored nuggets of HTML in CDATA nodes and other content that requires the use of special or awkward characters. However I don't feel qualified to challenge Rys's controversial assertion because, I suppose, technically he is correct in the scenarios where I've employed CDATA for convenience.

What's really baking my noodle is that, as developers take to the internet begging for advice on how to render CDATA segments using FOR XML PATH, respondents continually direct them to use FOR XML EXPLICIT instead, the XML rendering method Rys cited as being the "query from hell".

If we can really do without CDATA in every use case that anyone can suggest I guess we should stop moaning and reject CDATA usage henceforth. But if there are clearly defined cases where CDATA is essential Rys already undertook that he would bake it into FOR XML PATH going forward in the topmost link in this question.

So which is it to be? Are CDATA sections really relics of the past? Or should Rys pull his finger out and allow for CDATA parsing in FOR XML PATH? And while we're at it, in the meanwhile, are there any hacks for getting FOR XML PATH to return CDATA sections?

回答1:

CDATA sections are useful if you don't care about the semantics of the data in them (i.e. you do not need to parse it - it is simply a run of characters), and you don't wish to escape any of the XML within them.

The definition, according to w3:

CDATA sections may occur anywhere character data may occur; they are used to escape blocks of text containing characters which would otherwise be recognized as markup.

From wikipedia:

New authors of XML documents often misunderstand the purpose of a CDATA section, mistakenly believing that its purpose is to "protect" data from being treated as ordinary character data during processing. Some APIs for working with XML documents do offer options for independent access to CDATA sections, but such options exist above and beyond the normal requirements of XML processing systems, and still do not change the implicit meaning of the data. Character data is character data, regardless of whether it is expressed via a CDATA section or ordinary markup.

CDATA sections are useful for writing XML code as text data within an XML document. For example, if one wishes to typeset a book with XSL explaining the use of an XML application, the XML markup to appear in the book itself will be written in the source file in a CDATA section. However, a CDATA section cannot contain the string "]]>" and therefore it is not possible for a CDATA section to contain nested CDATA sections. The preferred approach to using CDATA sections for encoding text that contains the triad "]]>" is to use multiple CDATA sections by splitting each occurrence of the triad just before the ">". For example, to encode "]]>" one would write:

回答2:

CDATA sections are unnecessary. They're not a "relic of the past" because they've always been unnecessary.

This does not mean they aren't useful. Look at just about any programming language or library and you can find a large number of things you could do without because they are semantically equivalent to something else, but which are useful if there's a human being sitting there having to write the stuff.

For that matter, even with programmatic production it's also handy that one could take the opposite approach and use CDATA sections for every single piece of c-data (bloaty, but it could have efficiency gains elsewhere).

FOR XML PATH does not involve a human being sitting there having to write the stuff. It's a means of producing valid XML from a the results of an SQL query. (It's also not a matter of parsing CDATA sections, but of producing them - a different matter).

And you can't really complain about FOR XML EXPLICIT being the alternative when you want really fine control - the reason FOR XML EXPLICIT is so nasty to use sometimes is precisely because it gives you really fine control. Indeed, consider if they first added support for CDATA sections and then added support for every other tweak and configuration option that seemed just as vital to someone else out there. How long would it take before FOR XML EXPLICIT was the automatic choice due to it being more straightforward than FOR XML PATH‽

There are four cases where CDATA are useful:

1. You're sitting at a keyboard typing this stuff in yourself.
2. You are dealing with a mixing different technologies with different standards designed at different times and which will be interpreted by different parsers in different ways (e.g javascript embedded into XHTML - though it's not 100% necessary here it's a nightmare to do otherwise).
3. You're trying to parse the XML with something that doesn't understand XML.
4. You're trying to use something built on a parser that allows low-level access that distinguishes between CDATA sections and other character data and using that low-level access inappropriately.

Funnily enough, these four cases are also the four cases where a ban on accepting CDATA sections can make sense.

Case 1 doesn't apply here, it isn't human-generated code. Case 2 could apply here if you are doing something really crazy. Frankly, the lack of CDATA sections is the least of your worries here; switch to producing simpler XML in the query and transforming it elsewhere. Case 3 could apply here, but it's not fair to complain to the SQL people if it does, when you should complain to the broken XML parser that doesn't treat <example> the same as <![CDATA[<example>]]>. Case 4 could apply here, but again complain to the person who wrote the buggy code, not the SQL people.

回答3:

You are absolutely right, CDATA are essential in many scenarios, they're part of XML standard and should be supported by every XML manipulation tool/method. But thing is that MS usually dosn't care .. you know, "640kB should be enough for everyone" kind of approach.

Edit: About FOR XML EXPLICIT - this is THE best method for generating precisely formatted XML data. Yes, syntax is kinda painful to look at and confusing, but once you use it feww times, you'll admire its beauty and power.

回答4:

It is interesting to see how someone can just throw a very valuable piece of the Standard with such whimsical approach. Not everyone is using XML for a few hundred characters of HTML or a list of items for a drop down.

Some of us are actually using XML to exchange data, very complex data like a CCD, CDA CDR, these are all standard document formats in the healthcare arena and are becoming more and more prominent with ObamaCare. Part of these documents structure contain attachments things like DiCOM Images, PDF's and other Binary Data that should not be read by the parser the reason the CDATA definition exists.

Why should I pay the overhead of the parser reading a 3 megabyte DiCom image embedded in a CCD document? Why should I be forced to separate the document when it came in the original data and is part of the XML Standard. And I want the be able to locate and recover the document and is contents with XML.

This bewilders me why you all would support the parsing of data that is intended to not be parsed by the engine. If the engine sees CDATA ignore it, it is very simple. And the continued argument that some do not need it is irrelevant. It is part of the standard and the standard should be maintained. If they would like to add a "Feature" as it has been called then support the default behavior with an option.

Please stop parsing CDATA and ignore it.