The `resourceDefinition` attribute is a feature of the XML format, and is not defined anywhere in the structural definitions of FHIR (in the same way that the base name is not). The resourceDefinition attribute is not present if the resource is one defined in this specification.
Also, see note about the schema and additional resources below.
### XML Representation for repeating elements
An element that has a maximum cardinality of >1 (e.g., `x..*` in the definitions) may occur more than once in the instance. In XML, this is simply done by repeating the XML element multiple times.
So a [CodeableConcept](datatypes#CodeableConcept) is represented in XML like this:
For comparison, In JSON, this is done by using an array type, also using the singular name:
{
"coding": \[
{
"system" : "http://snomed.info/sct",
"code" : "104934005"
},
{
"system" : "http://loinc.org",
"code" : "2947-0"
}
\]
}
### XML representation of primitive elements
FHIR elements with primitive datatypes are represented in by an XML element with a value attribute.
<**code** value="abc"/>
<**date** value="1972-11-30"/>
<**deceased** value="false" />
<**count** value="23" />
For comparison, this is represented in JSON as
"code" : "abc",
"date" : "1972-11-30",
"deceased" : false,
"count" : 23
If the value has an id attribute, or extensions, then these are represented as follows:
<**birthDate** id="314159" value="1970-03-30" >
The JSON representation is more complex:
"birthDate": "1970-03-30",
"\_birthDate": {
"id": "314159",
"extension" : \[ {
"url" : "http://example.org/fhir/StructureDefinition/text",
"valueString" : "Easter 1970"
}\]
}
### XML representation of Complex Datatypes and XHTML
Complex [datatypes](datatypes) (types that contain named elements of other types) are represented using an element that containing an element for each element in the datatype. Complex datatypes can have ids, which are represented as id attributes, in the same manner as described for primitives. For example:
For comparison, this is represented in JSON as:
{
"resourceType" : "Patient",
"name" : \[{
"id" : "f2",
"use" : "official" ,
"given" : \[ "Karen" \],
"family" : "Van",
"\_family" : {"id" : "a2"}
}\],
"text" : {
"status" : "generated" ,
"div" : ""
}
}
Things to note here are:
- All elements are in the `http://hl7.org/fhir` namespace, except for the XHTML content
- The XHTML content in the `div` element which is in the Narrative element `text` is represented directly as xhtml, with a root element in the xhtml namespace
### XML Schema and Schematron
This specification provides schema definitions for all the resource and datatype content models it describes.
The base schema is called "[fhir-base.xsd](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](fhir-all.xsd) that includes all the resource schemas. For schema processors that do not like circular includes, there is [a single schema](fhir-single.xsd) that contains everything.
The base schema only describes the resources defined in this specification; additional resources are not included in the schema. If enough implementers identify a need for this, HL7 may choose to provide a tool that stitches a new schema for a given set of additional resources.
In addition to the w3c schema files, this specification also provides Schematron files that enforce most of the constraints defined for the datatypes and resources, though some are only expressible and validatable using [FHIRPath](fhirpath). 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](extensibility) 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.
### 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 better with 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](formats#choice) without schema support.
Implementers making use of schema-driven code generation tooling need to consider how to handle the [decimal](datatypes#decimal) datatype. The decimal datatype 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 datatype to a union of `xsd:decimal` and `xsd:double`, but the base [W3C schema decimal type](http://www.w3.org/TR/xmlschema-2/#decimal) 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/doubles and consider changing the type for decimal in their schema from `xsd:decimal/double` to `xsd:string`. Specifically, implementers may wish to change:
to this:
Note that most code generation frameworks ignore the pattern restriction.
### Canonical XML
See [Signatures](signatures#xml) for details.
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