Release 4

This page is part of the FHIR Specification (v4.0.1: R4 - Mixed Normative and STU) in it's permanent home (it will always be available at this URL). The current version which supercedes this version is 5.0.0. For a full list of available versions, see the Directory of published versions . Page versions: R5 R4B R4 R3 R2

2.6.0 Resource Formats

FHIR Infrastructure Work GroupMaturity Level: NormativeStandards Status: Normative
This page has been approved as part of an ANSI standard. See the Infrastructure Package for further details.

This page documents how the content of the resources are described. In actual exchange, resources can be represented in the following formats: XML, JSON and Turtle. Additional Bulk Data Formats are also undergoing exploration. Other representations are allowed, but are not described by this specification (though see link below .

The resources are described in several different ways:

  • a hierarchical table that presents a logical view of the content
  • a UML diagram that summarizes the content graphically
  • a pseudo-XML syntax that provides a visual sense of what the end resource instances will look like in XML
  • a pseudo-JSON syntax that provides a visual sense of what the end resource instances will look like in JSON
  • a pseudo-Turtle syntax that provides a visual sense of what the end resource instances will look like in Turtle

In addition to this descriptive syntax, other definitional forms are available, including W3C schema, Schematron, JSON Schema, and the StructureDefinition syntax defined internally.

The Logical View shows the resources as a tree structure with the following columns:

Column Content
Name The name of the element in the resource (manifests as XML element name or JSON or RDF property name). Some names finish with [x] - the meaning of this is discussed below. In addition, this column contains an icon that denotes the underlying type of the content. The icons are described below
Flags A set of information about the element that impacts how implementers handle them. The flags are described below
Card. Cardinality: the lower and upper bounds on how many times this element is allowed to appear in the resource
Type The type of the element (hyperlinked to the definition of the type). Note that the type of the element has one of two meanings, depending on whether the element has defined children. If the element has children, then the element has an anonymous type that specializes the given type. If the element has no children, then the element has properties and children as specified by the nominated type
Description & Constraints A description of the element, and details about constraints that are applied to it. Particularly, for coded elements, information about which codes can be used. The description comes from ElementDefinition.short

Here's an example:

Name Flags Card. Type Description & Constraints
.. Resource Name Base Type Definition
... nameA Σ 1..1 TypeA description of content
... nameB[x] ?! Σ 0..1 description
SHALL at least have a value
.... nameBType1 0..1 TypeB
.... nameBType2 I 0..1 typeC
... nameC 1..* BackboneElement Definition
.... nameD 1..1 TypeD Relevant Records

Key to Type Icons

  • .: The base element for a resource (see Resources)
  • .: An element that is part of the resource and has elements within it defined in the same resource or profile
  • .: An element which can have one of several different types (see below)
  • .: An element of a data type which describes an element that has a value attribute/property. These are also known as primitive types. All primitive type names start with a lower case letter
  • .: An element of a data type which describes an element that has other elements. These are known as complex types. All complex type names defined in this specification start with an uppwer case letter
  • .: An element that contains a reference to another resource (see references)
  • .: This element has the same content as another element defined within this resource or profile
  • .: Introduction of a set of slices (see Slicing)
  • .: A complex extension - one with nested extensions (see Extensibility)
  • .: An extension that has a value and no nested extensions (see Extensibility)
  • .: A complex modifier extension - one with nested extensions (see Extensibility)
  • .: A modifier extension that has a value and no nested extensions (see Extensibility)
  • .: The root of a logical profile

Key to Flags


  • Resource and Element names are case-sensitive (though duplicates that differ only in case are never defined)
  • Any elements that have a primitive type will have a value attribute/property to contain the actual value of the element
  • This value attribute/property can never be empty. Either it is absent, or it is present with at least one character of non-whitespace content
  • Elements are assigned a cardinality that specifies how many times the element may or must appear.
  • Unless elements have children defined directly (as nameC does above) they are assigned one or more types (see the data types). All the type names are hyperlinked to the source definition
  • Element reuse: Some data types that have children have the same set of children as some other element defined in the resource. In that case, the type of that element has a "see [name]" where [name] is the name of the element that has the defined children
  • Each element name is also a hyperlink to the formal definition of the element in the data dictionary that underlies the exchange formats.
  • Any of the elements may have an id attribute to serve as the target of an internal reference. The id attribute is not shown in this format. Extensions are not always shown, but may appear except where the flag NE appears
  • FHIR elements can never be empty. If an element is present in the resource, it SHALL have either a value, child elements as defined for its type, or 1 or more extensions
  • Infrastructural elements that are common to all resources are not shown in the logical representation. These are described in the common base classes Resource, and DomainResource

The data type for a particular element is typically expressed as the name of the specified type with a hyperlink to its definition. However, there are two exceptions:

  • If the element supports multiple types (name ends with [x]), then the type will be a list of data type options, each separated by "|"
  • If one of the types is Reference or canonical, the data type might be followed by a list of allowed targets the reference is allowed to be. These might be resource names, data type names, or profile URLs, depending on the context. As well, the following symbols may appear that represent expectations of where the referenced resource is located:
    • b: Resource must appear within the same Bundle;
    • c: Resource must be sent as a contained resource;
    • r: Resource is a non-contained reference - i.e. to a resource within the same Bundle or to an external resource

In profiles, references to types may be profiled - i.e. Instances of the element must comply with a specified profile or one of a list of profiles. The canonical URLs of any applicable profiles are listed inside {}.

Where an element can have a choice of data types, or is a Reference these are represented by showing the common type (Reference or Type), and then showing the applicable data type names or resource types in a stereotype, separated by the | character. Type is not formally otherwise defined by this specification, but is a super type of all the data types.

A few elements have a choice of more than one data type for their content. All such elements have a name that takes the form nnn[x]. The "nnn" part of the name is constant, and the "[x]" is replaced with the title-cased name of the type that is actually used. The table view shows each of these names explicitly.

Elements that have a choice of data type cannot repeat - they must have a maximum cardinality of 1. When constructing an instance of an element with a choice of types, the authoring system must create a single element with a data type chosen from among the list of permitted data types.

Note: In object-orientated implementations, this is naturally represented as a polymorphic property. However this is not necessary and the correct implementation varies according to the particular features of the language. In XML schema, these become an xs:choice of element. To help with code generation, a list of choice elements is published.

The UML diagrams represent the same content as a series of classes that represent the elements of a resource.

NameA Documentationelement : [type] [0..*] DocumentationnameB : CodeableConcept [0..1] « Value Set Description (Strength=Preferred)Value Set Name? » NameC Documentationvalue[x] : Type [0..1] « Type1|Type2|Type3 » Docuementationreference : Reference [0..1] « Resource1|Resource2 » DocumentationnameC [0..1]

The elements and the data types are hyperlinks to the formal definitions of the parts. The UML diagrams also show the vocabulary bindings. These are hyperlinks to the value set details.

The actual order of the elements in XML cannot be determined from the diagram, nor whether a UML property becomes an element or an attribute in the XML representation.

Bindings to value sets are indicated by a stereotype on the element. The stereotype has 2 parts: the value set name, and a symbol that denotes the strength of the binding:

This specification defines the following ways to represent resources when they are exchanged:

Systems SHALL declare which format(s) they support in their Capability Statement. If a server receives a request for its Capability Statement in a format it does not otherwise support, it SHALL return a 406 Not Acceptable. Note: 406 is the appropriate response when the Accept header requests a format that the server does not support, and 415 Unsupported Media Type when the client posts a format that is not supported to the server.

Clients and servers can choose what syntax(s) to implement. In the interests of interoperability, servers SHOULD support both the XML and JSON formats, which have the same functionality, for different technical stacks. The RDF format has quite different benefits - primarily around data analysis rather than exchange.

Unlike this rest of this page, the bulk use formats are draft until further experience is gained with their use. Their status will be reviewed in a future version of FHIR.

The XML and JSON formats are designed to support typical system process-based data exchange uses. FHIR is also used to exchange large amounts of data- 1000s of records, or more (up to billions). The formats above can be used for this, but more suitable formats exist. This specification documents (or is exploring documenting) the following formats:

  • ND-Json (New line delimited JSON)
  • Google Protobuf (under consideration)
  • Apache Parquet/Avro (bulk data formats under consideration)