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1. Without having an infinite amount of data, you cannot create a
legitimate business document with an unbounded number of occurrences in
some structure governed by maxOccurs. Of course, a broken program
generating data (e.g., one in an infinite loop) could do this, but
setting a bound on the number of occurrences doesn't help much, if at
all. (Even with a broken program you cannot create it in finite time.)
2. Without using entity expansion (the famous xml denial of service
attack or something similar) or a broken program, you cannot create an
infinitely recursive document . The entity expansion problem would
likely never occur in a serious business context.
This suggests that the analogy to programming language iterations or
recursions is weak. You can on one line write a program that loops
infinitely and run it, producing some form of resource overflow or
denial of service.
One possible resource overflow could be via an infinitely recursive or
repeating document structure, but hoping that the problem will be fixed
or even gracefully handled by some constraint in the validation of the
document seems unlikely to be producitive.
By avoiding recursive types you might produce documents that are less
understandable than the recursive ones. By setting an upper bound on
occurrences, you give yourself a maintenance proiblem when an unusual
circumstance reveals that your bound was improperly chosen. Neither of
these choices provide benefit commensurate with their disadvantages, IMO.
Jeff
Roger L. Costello wrote:
>Hi Folks,
>
>Below I have jotted down a few thoughts regarding XML Schemas which permit
>an unbounded number of occurrences. Namely, I recommend against using
>maxOccurs="unbounded" in an XML Schema. I am interested in hearing your
>thoughts on this. /Roger
>
>
>Constrain the Number of Occurrences of Elements in your XML Schema
>
>by Roger L. Costello
>August 5, 2005
>
>Constrain your Data!
>
>
>In this message I will argue that you should never create XML Schemas that
>permit an unbounded number of occurrences.
>
>There are two ways in XML Schemas to permit an unbounded number of
>occurrences. The first way is to explicitly state that you are permitting an
>unbounded number of occurrences. For example, this declaration says that
>Bookstore can contain an unbounded number of Book elements:
>
><element name="Bookstore">
>
> <complexType>
>
> <sequence>
>
> <element name="Book" type="..." maxOccurs="unbounded"/>
>
> </sequence>
>
> </complexType>
>
></element>
>
>
>
>The second way of permitting an unbounded number of occurrences is less
>obvious. Unboundedness occurs implicitly when you create a recursive
>structure. In this example there is no limit to the depth of the Section
>elements. That is, a Section can contain a Section which contains a Section
>which contains a Section ...
>
><element name="Section" type="SectionType"/>
>
>
>
><complexType name="SectionType">
>
> <sequence>
>
> <element name="Title" type="..."/>
>
> <element name="Section" type="SectionType"/>
>
> </sequence>
>
></complexType>
>
>
>
>Both of the above forms permit an unbounded number of occurrences. I
>recommend that you never use either form. That is, never declare an element
>with maxOccurs="unbounded", and never declare a recursive structure. Below I
>explain why.
>
>
>Writing a Journal Article? Your Word Count is Limited!
>
>
>The situation with specifying the number of occurrences of an element in an
>XML Schema is analogous to the situation with specifying the number of words
>authors can use in an article.
>
>Suppose that you want to write an article for a journal. How many words can
>you use in your article? All journals have an upper limit on the number of
>words that you can use. Why don't the journals set the word limit to
>unbounded? Answer: there are editors that have to check the articles for
>correctness, readability, etc. The editors have limited resources (i.e.,
>time). Thus, it is necessary to limit the length of the article. Perhaps at
>a later date the journal will increase the word limit (perhaps they hire
>some full-time editors). But they always have a definite upper limit. They
>never allow articles of unbounded length. The reason is because of limited
>resources.
>
>
>Error! Infinite Loop!
>
>
>The situation with specifying the number of occurrences of an element in an
>XML Schema is analogous to an infinite loop in programming languages. Why
>are infinite loops deemed "bad" in programming languages, yet unbounded
>occurrences are embraced in data?
>
>Let's see why infinite loops are bad in programming languages. Suppose that
>a program has a loop, and a computer begins to process the loop. It requires
>a certain amount of resources (memory, cpu cycles) for the computer to
>perform one iteration. Two iterations will require a bit more resources.
>Three iterations require still more. ... Infinite iterations require
>infinite resources. Thus, infinite loops are bad because they require
>infinite resources.
>
>The situation is analogous with data. Consider the Bookstore declaration
>above. It declares that an unbounded number of Book elements are permitted
>within Bookstore. A program that must process XML instances conforming to
>the declaration must have the necessary resources (memory, cpu cycles). To
>process one Book element will require a certain amount of resources. To
>process a second Book element will require a bit more resources. A third
>book will require still more resources. ... Infinite Books require infinite
>resources. Even though XML instance documents are always finite, the schema
>indicates that there is a "potential" for an infinite number of Book
>elements. A program that is designed to process "any" XML instance document
>that conforms to the schema must therefore have an infinite amount of
>resources.
>
>
>Okay, then what Value should I use for maxOccurs?
>
>
>"Suppose that I anticipate that Bookstore will never have more than 30,000
>Books, so I set maxOccurs='30000'. After some time the requirements change
>and BookStore now needs to be able to hold 35,000 Books. Won't I have to
>change the Schema every time my needs change? Wouldn't it be easier if I
>simply declared maxOccurs='unbounded'?"
>
>Answer: yes, you will need to change the Schema whenever your requirements
>change. Yes, it is easier to simply declare maxOccurs='unbounded'. But don't
>do it! The number that you use for maxOccurs should be as big as your
>programs are willing and able to cope with, and no more. If at some point
>the number of actual books exceeds that number then they must either (1)
>extend your program's resources to handle the expanded number, or (2) refuse
>to allow more books.
>
>
>Recap
>
>
>1. Don't use maxOccurs="unbounded"
>
>2. Don't use recursive constructions
>
>3. Set maxOccurs to a number no larger than the amount of resources you
>have available
>
>
>
>
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