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Q: When should I use an XSD facet like maxLength, and when should I use the DFDL length property?

  Here's part of an example from the DFDL tutorial of a street address:

  <xs:element name="houseNumber" type="xs:string" dfdl:lengthKind="explicit" dfdl:length="6"/>

  Note that the length of the house number is constrained with DFDL.  XSD can also be used to constrain lengths.

  When should you used XSD to do this, and when should you use DFDL?  Should you ever use both?

A: You must use the dfdl:length property, because it can't parse the data without it. You may use the XSD facets to check further, and it often makes sense to use both.

Consider 

  <xs:element name="article" type="xs:string" dfdl:length="{ ../header/articleLength }" dfdl:lengthKind='explicit'/>

Now the length is coming from a field someplace at runtime. Validating that it is within some additional constraints on maxLength might be very valuable. To do that you nave to write the more verbose:

<xs:element name="article" dfdl:length="{ ../header/articleLength }" dfdl:lengthKind='explicit'>
  <xs:simpleType>
    <xs:restriction base="xs:string">
      <xs:maxLength value="140"/>
    </xs:restriction>
  </xs:simpleType>
</xs:element>

Not too bad actually. And if you can reuse some simple type definitions it's not bad at all.

One further point. Suppose you want to parse the string using the header-supplied length, but it's flat out a parse error if the length turns out to be greater than 140. You can ask the DFDL processor to check the facet maxLength at parse time using an assertion like this:

<xs:element name="article" dfdl:length="{ ../header/articleLength }" dfdl:lengthKind='explicit'>
  <xs:simpleType>
    <xs:annotation><xs:appinfo source="http://www.ogf.org/dfdl/dfdl-1.0">
       <dfdl:assert>{ dfdl:checkConstraints() }</dfdl:assert>
    </xs:appinfo></xs:annotation>
    <xs:restriction base="xs:string">
      <xs:maxLength value="140"/>
    </xs:restriction>
  </xs:simpleType>
</xs:element>

The dfdl:assert statement annotation calls a built-in DFDL function called dfdl:checkConstraints, which tells DFDL to test the facet constraints and issue a parse error if they are not satisfied. This is particularly useful for enumeration constraints where an element value is an identifier of some sort.

 

 

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