This document provides an overview of StructureDefinition resources in FHIR. StructureDefinition resources describe the definition and validation rules for core FHIR resources, data types, logical models, and extensions. They contain metadata and constraints that are used to validate conformance to FHIR standards. The speaker discusses the key components of StructureDefinition resources, including differentials, snapshots, references between definitions, and slices.

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Amsterdam, 15-17 November | @fhir_furore | #fhirdevdays17 | www.fhirdevdays.com
StructureDefinition 101
Ewout Kramer, Furore Health Informatics

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Who am I?
 Name: Ewout Kramer
 Company: Furore Health Informatics
 Background:
 Computer Science (operating systems)
 In Health IT since 1999
 FHIR Core team
 Lead dev on the .NET API
 e.kramer@furore.com, @ewoutkramer
 http://thefhirplace.com2

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Context...
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StructureDefinition is one of
the “conformance” resources
They contain (meta)data about
FHIR itself, not patient data

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StructureDefinition basics
 It’s a resource
 It describes the definition (structure+validation rules) for
 The core resources and datatypes in the spec
 Logical models
 It describes constraints on an existing StructureDefinition
 Profiles
 Extensions
 Constrained logical models
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Where to find them?
 For the core spec: under the “tree” of any single resource
 For the core spec: all of them combined
 http://hl7.org/fhir/downloads.html - “FHIR Definitions”
 profiles-types.xml, profiles-resources.xml
 On simplifier.net / registry.fhir.org
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What’s in a StructureDefinition?
Metadata
(author, version, etc)
Differential
Constraints on the base -
only the differences - as
authored by the creator
Snapshot
Constraints on the base
+
Unconstrained parts - filled
out from the (unchanged)
base elements
When describing constraints**:
** Contents for definition of resources/datatypes/logical models are slightly different

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Metadata + differential
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Unique URI for identification
and referencing (“canonical”)
Constrains “Patient”
Just the differential –
only the parts that the author
wants to change!

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A view of the snapshot...
 490kb versus 1kb
 All elements from Patient +
DomainResource +
Resource
 All constraints info for each
element
 Definitions, comments
 Min/max
 Invariants, fixed values
 Note: Hierarchy is flattened
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This was the base...
 Note how Patient originally just
had Patient.identifier (of type
Identifier)
 The differential has “walked
into” Patient.identifier.use”
 The whole Identifier type
becomes expanded in the
snapshot (previous slide)
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Combine type profile +
constraint
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Quiz question: How will the snapshot be composed?

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Schematic representation...
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StructureDefinition “Patient”
Patient.identifier : Identifier
StructureDefinition “PatientEK”
Patient.identifier : IdentifierNL
constraints on:
Identifier.use
StructureDefinition “IdentifierNL”
constraints on:
Identifier.use
Identifier.system

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So, why a snapshot?
 Snapshot can be derived from base
snap = gen_snap(
gen_snap(
gen_snap(core,differental1) , differential2), differential3) ....
 All elements my refer to other types and walk into types
 Possibly requires reaching out to core spec, registries –
need access to all StructureDefinitions you depend on
 Might not be accessible at all times!
 Has “all you need” to render, validate, ... in 1 place
 ... or pack everything with your app!12

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ElementDefinition
 Differential/Snapshot are collections of ElementDefinitions (a
datatype)
 Most important parts:
 path (the dotted part expressing hierarchy: “Patient.identifier.use”)
 short/definition
 min/max (cardinality)
 fixed/pattern
 typeRef (base data type of element)
 binding
 (fhirpath) invariants13

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Pattern...the unloved feature
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value[x] and references
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typeRef details
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In R4, this will change!

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Quiz
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profile + targetProfile combined..... what does that do?
1. “targetProfile” Profiles what the refence may point to
2. “profile” Profiles what the reference itself looks like
(e.g. must have reference.display)

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Summary of references
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StructureDefinition “Patient”
http://hl7.org/fhir/StructureDefinitio
n/Patient
StructureDefinition “PatientEK”
http://example.org/fhir/StructureDefin
ition/PatientEK
StructureDefinition “WeightQuantity”
http://example.org/fhir/StructureDefiniti
on/WeightQuantity
StructureDefinition for “nl-core-
patient”
http://fhir.nl/fhir/StructureDefinitio
n/nl-core-patient
ValueSet “observation-status”
http://hl7.org/fhir/ValueSet/observati
on-status
baseDefinition
binding.valueSet[x]
typeRef.profile
ElementDefinition typeRef.targetProfile
All of them use the
canonical url
(StructureDefinition/ValueSet.url)

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Extensions and StructureDefs
 Extensions are defined using StructureDefinitions
 In fact, they are constraints on the Extension datatype!
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Fix this URL to the extension’s url
Limit to just “Boolean” [1..1]
Don’t allow nested extensions

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Finally....slices (only a bit)
 So, is every path unique within a StructureDefinition?
 Yes, in the core types. No (not necessarily), within a profile
 If you see more than one element with the same path => it’s
a slice
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A slicing group
• 1x slice intro (info about the whole slice:
cardinalities, open/closed/ordered etc)
• Followed by one repeat for each slice

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Questions?
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