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2.6.1 XML Representation of Resources

Implementable Technology Specifications Work Group

Maturity Level: 5

Ballot Status: Trial Use

The XML representation for a resource is described using this format:

 <name xmlns="http://hl7.org/fhir" (attrA="value")>   
   <nameA><!--  1..1 type description of content  --><nameA>
   <nameB[x]><!-- 0..1 type1|type1 description  --></nameB>
   <nameC> <!--  1..* -->
     <nameD><!-- 1..1 type>Relevant elements  --></nameD>
   </nameC>
 <name>

Using this format:

• To build a valid XML instance of a resource, simply replace the contents of the elements and attributes with valid content as described by the cardinality, type rules and content description found in the comment in each element
• Resource and Element names are case-sensitive (though duplicates that differ only in case are never defined)
• Elements must always appear in the order documented
• When an element is allowed to repeat, the elements are ordered, and the technical infrastructure needs to be able to access the items in the right order (see also Cardinality Rules for a further description of elements with cardinality > 1)
• A few properties are represented as attributes: values of primitive types in a value attribute, extension URLs in the url attribute on an extension, and the id property on elements (but not on resources, where the resource id is an element)
• Any of the XML elements may have an id attribute to serve as the target of an internal reference. The id attribute is not shown in this format
• FHIR elements are always in the namespace http://hl7.org/fhir . This is usually specified as the default namespace on the root element. The only other namespace that occurs in FHIR resources is the XHTML namespace (XHTML is found in most resources)
• Infrastructural elements that are common to all resources are not shown in the xml representation. These must appear prior to any other defined child elements in the following order:
  • First, the elements from the base resource, in order
  • Second, the elements from the domain resource, in order
• FHIR elements are never empty. If an element is present in the resource, it SHALL have either a value attribute, child elements as defined for its type, or 1 or more extensions
• Attributes canno be empty. Either they are absent, or they are present with at least one character of non-whitespace content
• Implementers SHOULD trim leading and trailing whitespace before writing and SHOULD trim leading and trailing whitespace when reading attribute values
• The lock icon () denotes that an element defines or is affected by additional rules that control its presence and/or content
• XML comments, processing instructions and formatting are not part of the contents of a resource
• There SHALL be no DTD references in FHIR resources (because of the XXE security exploit )
• XML resources SHALL be exchanged using UTF-8 encoding. Specifying the character encoding using a <?xml encoding="UTF-8" ?> processing instruction is optional but recommended
• Other processing instructions SHOULD not be included, and SHALL NOT be required to properly understand and/or present the data or narrative of the resource. Applications MAY preserve processing instructions when handling resources, but are not required to do so
• The MIME-type for this format is application/fhir+xml.

2.6.1.1 XML Schema and Schematron

This specification provides schema definitions for all the content models it describes.

The base schema is called "fhir-base.xsd" and defines all the datatypes and base infrastructure types. In addition, there is a schema for each resource and a common schema fhir-all.xsd that includes all the resource schemas. For schema processors that do not like circular includes, there is a single schema that contains everything.

In addition to the w3c schema files, this specification also provides Schematron files that enforce the various constraints defined for the datatypes and resources. These are packaged as files for each resource.

XML that is exchanged SHALL be valid against the w3c schema and Schematron, though being valid against the schema and Schematron is not sufficient to be a conformant instance: this specification makes several rules that cannot be checked by either mechanism. Operational systems may choose to use schema tools to check validation, but are not required to do so. Exchanged content SHALL NOT specify the schema or even contain the schema instance namespace in the resource itself.

Given the way extensions work, applications reading XML resources will never encounter unknown elements. However, once an application starts trading with other applications that conform to later versions of this specification, unknown XML elements may be encountered. Applications MAY choose to ignore unknown elements to foster forwards compatibility in this regard, but may also choose not to - which would be the normal behavior for schema generated applications. Applications declare their unknown elements behavior using CapabilityStatement.acceptUnknown.

2.6.1.2 Code Generation Schema

In addition to the validation schema, this specification provides a set of schemas suitable for code generation. These schemas describe the same XML syntax, but apply less validation to create schemas that work with more code generation tooling.

Specifically, these schemas are generated without any xsd:choice elements, for code generators that don't deal with choices well. Implementers that use these schemas will need to enforce the correct usage of the choice elements without schema support.

Implementers making use of schema driven code generation tooling need to consider how to handle the decimal data type. The decimal data type is defined to be precision aware - that is, that implementers need to preserve the difference between "2.0" and "2.00" - this is ubiquitously considered important in handling observed data in healthcare. Both schemas map this data type to xsd:decimal, but the base W3C schema decimal type is specified not to be precision aware. Schema driven implementations vary as to how precision is handled. Implementers will need to determine how their generated code handles decimals and consider changing the type for decimal in the schema from xsd:decimal to xsd:string. Specifically, implementers may wish to change

  <xs:simpleType name="decimal-primitive">
    <xs:restriction base="xs:decimal">
      <xs:pattern value="-?([0]|([1-9][0-9]*))(\.[0-9]+)?"/>
    </xs:restriction>
  </xs:simpleType>

to this:

  <xs:simpleType name="decimal-primitive">
    <xs:restriction base="xs:string">
      <xs:pattern value="-?([0]|([1-9][0-9]*))(\.[0-9]+)?"/>
    </xs:restriction>
  </xs:simpleType>

Alternatively, if supported, implementers may wish to use the precisionDecimal from the XSD 1.1 framework.

Note that most code generation frameworks ignore the pattern restriction.

2.6.1.3 Canonical XML

Resources and/or Bundles may be digitally signed (see Bundle and Provenance).

This specification defines the following method for canonicalizing FHIR resources, when represented as xml:

• No whitespace other than single spaces in attribute values and in the XHTML in the Narrative
• Use default namespaces for the FHIR and XHTML namespaces
• Omit all elements that have a default value, if a default value is defined
• Omit all comments
• Always use the unicode character representation for any XML entities (e.g. ' instead of ")
• Include the XML processing instruction <?xml version="1.0" encoding="UTF-8"?>
• Using the XML canonical method Canonical XML 1.1 (http://www.w3.org/2006/12/xml-c14n11)

This canonicalization method is identified by the URL http://hl7.org/fhir/canonicalization/xml. The following additional canonicalization URLS are also defined:

These canonicalization methods allow systems the flexibility to sign the various portions of the resource that matter for the workflow the signature serves.

Note: One consequence of signing the document is that URLs, identifiers and internal references are frozen and cannot be changed. This might be a desired feature, but it may also cripple interoperability between closed ecosystems where re-identification frequently occurs. For this reason, it is recommended that systems consider carefully the impact of any signature processes. The impact of signatures on Document bundles and their related processes is the most well understood use of digital signatures.