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
- Don't use maxOccurs="unbounded"
- Don't use recursive constructions
- Set maxOccurs to a number no larger than the amount of resources you have
available
