Data Format Description Language

Data Format Description Language (DFDL, often pronounced daff-o-dil) is a modeling language from the Open Grid Forum for describing general text and binary data. A DFDL model or schema allows any text or binary data to be read (or "parsed") from its native format and to be presented as an instance of an information set. The same DFDL schema also allows data to be taken from an instance of an information set and written out (or "serialized") to its native format.

DFDL achieves this by building upon the facilities of W3C XML Schema 1.0. A subset of XML Schema is used, enough to enable the modeling of non-XML data. One of the results of this is that is very easy to use DFDL to convert general text and binary data, via a DFDL information set, into a corresponding XML document.

DFDL is descriptive and not prescriptive. DFDL is not a data format, nor does it impose the use of any particular data format. DFDL allows an application to design an appropriate data representation according to its requirements, and for that format to be described in a standard way so that multiple programs can directly interchange the data.

History

DFDL was created in response to a need for grid APIs to be able to understand data regardless of source. A language was needed capable of modeling a wide variety of existing text and binary data formats. A working group was established at the Global Grid Forum (which later became the Open Grid Forum) in 2003 to create a specification for such a language.

A decision was made early on to base the language on a subset of W3C XML Schema, using <xs:appinfo> annotations to carry the extra information necessary to describe non-XML physical representations. This is an established approach that is already being used today in commercial systems. DFDL takes this approach and evolves it into an open standard capable of describing many text or binary data formats.

Work continued on the specification, culminating in the publication of DFDL 1.0 ^[1] as an OGF Proposed Recommendation in January 2011. A summary of DFDL and its features is available at the OGF site.

Implementations of DFDL processors that can parse and serialize data using DFDL schemas are in progress. Any issues with the specification that are encountered during implementation work are being tracked in an errata document.

Example

Take as an example the following text data stream which gives the name, age and location of a person:

Joe Bloggs,46,Hampshire,England

The logical model for this data can be described by the following fragment of an XML Schema document. The order, names, types and cardinality of the fields are expressed by the XML schema model.

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" ...>
 
<xs:complexType name="person_type">
  <xs:sequence>
    <xs:element name="name" type="xs:string"/>
    <xs:element name="age" type="xs:short"/>
    <xs:element name="county" type="xs:string"/>
    <xs:element name="country" type="xs:string"/>
  </xs:sequence>
</xs:complexType>
 
</xs:schema>

To additionally model the physical representation of the data stream, DFDL augments the XML schema fragment with annotations on the xs:element and xs:sequence objects, as follows:

<xs:schema xmlns:dfdl="http://www.ogf.org/dfdl/dfdl-1.0/" xmlns:xs="http://www.w3.org/2001/XMLSchema" ...>
 
<xs:complexType name="person_type">
  <xs:sequence>
    <xs:annotation><xs:appinfo source="http://www.ogf.org/dfdl/">
        <dfdl:sequence encoding="ASCII" sequenceKind="ordered" 
                       separator="," separatorType="infix" separatorPolicy="required"/>                   
    </xs:appinfo></xs:annotation>
    <xs:element name="name" type="xs:string">
      <xs:annotation><xs:appinfo source="http://www.ogf.org/dfdl/">
        <dfdl:element lengthKind="delimited" encoding="ASCII"/>                   
      </xs:appinfo></xs:annotation>
    </xs:element>
    <xs:element name="age" type="xs:short">
      <xs:annotation><xs:appinfo source="http://www.ogf.org/dfdl/">
        <dfdl:element representation="text" lengthKind="delimited" encoding="ASCII"
                      textNumberRep="standard" textNumberPattern="#0" textNumberBase="10"/>                   
      </xs:appinfo></xs:annotation>
    </xs:element>
    <xs:element name="county" type="xs:string">
      <xs:annotation><xs:appinfo source="http://www.ogf.org/dfdl/">
        <dfdl:element lengthKind="delimited" encoding="ASCII"/>                   
      </xs:appinfo></xs:annotation>
    </xs:element>
    <xs:element name="country" type="xs:string">
      <xs:annotation><xs:appinfo source="http://www.ogf.org/dfdl/">
        <dfdl:element lengthKind="delimited" encoding="ASCII"/>                   
      </xs:appinfo></xs:annotation>
    </xs:element>
  </xs:sequence>
</xs:complexType>
 
</xs:schema>

The property attributes on these DFDL annotations express that the data are represented in an ASCII text format with fields being of variable length and delimited by commas.

An alternative, more compact syntax is also provided, where DFDL properties are carried as non-native attributes on the XML Schema objects themselves.

<xs:schema xmlns:dfdl="http://www.ogf.org/dfdl/dfdl-1.0/" xmlns:xs="http://www.w3.org/2001/XMLSchema" ...>
 
<xs:complexType name="person_type">
  <xs:sequence dfdl:encoding="ASCII" dfdl:sequenceKind="ordered" 
               dfdl:separator="," dfdl:separatorType="infix" dfdl:separatorPolicy="required">
    <xs:element name="name" type="xs:string"
                dfdl:lengthKind="delimited" dfdl:encoding="ASCII"/>                   
    <xs:element name="age" type="xs:short"
                dfdl:representation="text" dfdl:lengthKind="delimited" dfdl:encoding="ASCII"
                dfdl:textNumberRep="standard" dfdl:textNumberPattern="##0" dfdl:textNumberBase="10"/>                   
    <xs:element name="county" type="xs:string"
                dfdl:lengthKind="delimited" dfdl:encoding="ASCII"/>                   
    <xs:element name="country" type="xs:string"
                dfdl:lengthKind="delimited" dfdl:encoding="ASCII"/>                   
  </xs:sequence>
</xs:complexType>
 
</xs:schema>

Features

The goal of DFDL is to provide a rich modeling language capable of representing any text or binary data format. The 1.0 release is a major step towards this goal. The capability includes support for:

• Language structures such as COBOL, C and PL/1
• Industry standards such as CSV, SWIFT, FIX, HL7, X12, HL7, HIPAA, EDIFACT, ISO8583
• Data delimited by text or binary markup
• Physical data types including text strings, text numbers, binary two's complement integers, BCD, mainframe zoned and packed decimals, IEEE and mainframe floats, text and binary calendars, text and binary Booleans
• Any encoding and endian-ness
• Bi-directional text
• Bit data of arbitrary length
• Pattern languages for text numbers and calendars
• Ordered and unordered content
• Default values on parsing and serializing
• Nil values capability for handling out-of-band data
• A built-in expression language including variables to model dynamic data
• Mechanisms to resolve choices and optionality
• Fixed and variable arrays
• Hiding elements in the data from the information set
• Calculating element values for the information set
• Validation to XML Schema 1.0 rules
• A scoping mechanism that allows common property values to be applied at multiple annotation points

Future releases are anticipated in which it is hoped to include support for:

• Direct access by offset
• True multi-dimensional arrays
• Embedded comments
• Custom language extensions

See also

• Open Grid Forum
• W3C XML Schema

References

1. ^ DFDL 1.0

External links

• Open Grid Forum
• DFDL home page
• W3C XML Schema