XSD Tutorial Parts
- Elements and Attributes
- Conventions and Recommendations
- Extending Existing Types
- Other Useful bits...
This section covers conventions and recommendations when designing your schemas.
When to use Elements or Attributes
There is often some confusion over when to use an
element or an
attribute. Some people say that
elements describe data and
attributes describe the meta data. Another way to look at it is that
attributes are used for small pieces of data such as order id's, but really, it is personal taste that dictates when to use an
attribute. Generally it is best to use a child
element if the information feels like data. Some of the problems with using
Attributes cannot contain multiple values (child
Attributes are not easily expandable (to incorporate future changes to the schema)
Attributes cannot describe structures (child
lf you use
attributes as containers for data, you end up with documents that are difficult to read and maintain. Try to use
elements to describe data. What I am trying to say here is that metadata (data about data) should be stored as
attributes, and that data itself should be stored as
Mixed Element Content
Mixed content is something you should try to avoid as much as possible. It is used heavily on the web in the form of XHTML, but that has many limitations. It is difficult to parse and it can lead to unforeseen complexity in the resulting data. XML Data Binding has limitations associated with it making it difficult to manipulate such documents.
Attributes should use UCC camel case, eg (
PostalAddress), avoid hyphens, spaces or other syntax.
- Readability is more important than tag length. There is always a line to draw between document size and readability, wherever possible favor readability.
- Try to avoid abbreviations and acronyms for
type names. Exceptions should be well known within your business area eg
ID (Identifier), and
POS (Point of Sale).
- Postfix new types with the name '
- Enumerations should use names not numbers, and the values should be UCC camel case.
- Names should not include the name of the containing structure, eg
CustomerName, should be
Name within the sub
- Only produce
types that are likely to be re-used. If the structure only exists in one place, define it inline with an anonymous complexType.
- Avoid the use of mixed content.
- Only define root level
elements if the
element is capable of being the root
element in an XML document.
- Use consistent name space aliases
- Try to think about versioning early on in your schema design. If it is important for a new versions of a schema to be backwardly compatible, then all additions to the schema should be optional. If it is important that existing products should be able to read newer versions of a given document, then consider adding any and anyAttribute entries to the end of your definitions. See Versioning recommendations.
- Define a
targetNamespace in your schema. This better identifies your schema and can make things easier to modularize and re-use.
elementFormDefault="qualified" in the schema
element of your schema. This makes qualifying the namespaces in the resulting XML simpler (if not more verbose).