Advanced .NET API (Ewout)

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Advanced
.NET API
Ewout Kramer
FHIR Developer Days
November 17, 2016

Who am I?
 Name: Ewout Kramer
 Company: Furore
 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

BEYOND THE POCO
3

Why?
 No need to parse whole POCO’s in memory
 Generic clients/servers not needing pre-
compiled model classes
 Tools that can parse (partly) invalid FHIR
data
 Accessing DSTU2 and STU3 data
4

“Logical” structure of
FHIR data
5

IElementNavigator
6
 Represents a position in a tree of FHIR data
 Has all the aspects of a node (from last slide)
 Path: Patient.name[1].family[0]

INavigator<T>
7
GetChildrenByName(this IElementNavigator navigator,
string name)
IEnumerable<object> Values(this IElementNavigator navigator)

Caveat
 We are showing you work in progress
 Interfaces may change
 Will be implemented for DSTU2 and STU3
 Currently prackaged as part of the
FluentPath .NET Core library
(HL7.FluentPath)
 Use the latest 0.4.x (alpha) releases
 Only available implementation is the
PocoNavigator8

GETTING TO METADATA

Conformance Resources
 Provide “metadata” about
 Model Definitions (StructureDefinition)
 Operations (OperationDefinition)
 Search parameters (SearchParameter)
 List of codes (ValueSet)
 Identified by a “canonical url” that SHOULD
resolve, e.g.
“http://hl7.org/fhir/StructureDef
inition/Patient”

Resolution
 Directly using a FHIR REST call to the url
 More likely:
 As part of a “snapshot”/zip of files delivered with
your app
 Compiled into your app
 In a database
11

IResourceResolver
 Concrete implementations in API:
 DirectorySource, ZipSource
 WebSource
 CachedResolver (wraps another resolver)
 MultiResolver (tries a list of resolvers)12

Combine at will
13
MyDbResolver
ZipSource WebResolver
MultiResolver
CachedResolverMultiResolver

Practicalities
 Packaged in Hl7.Fhir.Specification assembly
 Namespace Hl7.Fhir.Specification.Source
 Includes useful extension methods like:
 FindStructureDefinition(this IResourceResolver resolver, string uri,
bool requireSnapshot = false)
 FindStructureDefinitionForCoreType(this IResourceResolver
resolver, FHIRDefinedType type)
 FindExtensionDefinition(this IResourceResolver resolver, string
uri, bool requireSnapshot = false)
 Data for base spec is in “specification.zip”, easy to get
to using ZipSource.CreateValidationSource()
14

TERMINOLOGY

FHIR Resources
 CodingSystem resource (STU3)
 A dictionary of concepts (possibly huge!)
 ValueSet (DSTU2, STU3)
 A (use-case specific) selection of concepts from
1..* CodingSystems
 May be directly enumerated (‘extensional’)
 May be composed using filters (‘intensional’)
 “all the LOINC codes in LOINC Part Concept Cholesterol |
Bld-Ser-Plas (LP43571-6), except for 5932-9 Cholesterol
[Presence] in Blood by Test strip”
 May be composed from other ValueSets

Main functionality
 Expand an intensional ValueSet
 Determine whether some code is member of
a ValueSet
 Do a code-lookup
 Find details like alternative designation
 Could be done “in-memory” (i.e. using an
expanded ValueSet)
 By calling the FHIR terminology operations17

In-memory
 Expand using the ValueSetExpander
class in
Hl7.Fhir.Specification.Terminology
 Set limits on expansion size
 Uses IResourceResolver to locate
ValueSets
 Current limitations:
 No filters
 No imports of whole CodeSystems
18

Working with the
expansion
 Useful extension methods on ValueSet
 HasExpansion()
 ExpansionSize()
 FindInExpansion(String code, string system)
 bool CodeInExpansion(String code, string system)
 Note: no automatic expansion, use
ValueSetExpander
19

Terminology operations
 https://www.hl7.org/FHIR/valueset-
operations.html
 $expand, $lookup, $validate-code
 class FhirClient
 ExpandValueSet()
 ConceptLookup()
 ValidateCode()
20

ITerminologyService
 Currently available: LocalTerminologyServer
 Does an in-memory expand & lookup
 Planned: try cache - then the
LocalTerminologyServer - then the operations on
a “real” terminology server.
 Note: returns OperationOutcome
21

FHIRPATH SUPPORT
22

FhirPath in .NET
 First, the name
 FhirPath -> FluentPath -> FhirPath
 Available as a .NET Core library
Hl7.FluentPath on NuGet (0.4.x)
 Is built on top of IElementNavigator – so
it could work on *any* object model

Compiling FhirPath
 Advantage: Compile once, run many (fast)
 class FluentPathCompiler
 public CompiledExpression
Compile(string expression)
 CompiledExpression is a native Lambda that
will run the statement against an
IElementNavigator
 You can also compile to an Expression tree
(for debug/display purposes)
24

Using
CompiledExpression
 First, compile the statement
 var ce =
compiler.Compile(“Patient.name”);
 Then:
 var result = ce(…)
 result is a set of IElementNavigators
 Or:
 object s = ce.Scalar(…);
 bool p = ce.Predicate(…);
 bool b = ce.IsBoolean(true, …);
25

Convenience methods
 Comparable methods exist on
IElementNavigator so you can directly
query a source of data
 Example:
IElementNavigator nav =
new PocoNavigator(myPatient);
object cnt =
nav.Scalar(“Patient.name.count()”);
 Compiles & caches last 500 expressions for
you26

On POCO’s directly…
 The Hl7.Fhir.Core assembly has extension
methods in Hl7.Fhir.FluentPath for working
directly on FHIR POCO’s
Patient p = …;
object cnt = p.Scalar(“Patient.name.count()”);
27

VALIDATION

The Big Picture
FHIR Data
<Patient>
</Patient>
IElementNavigator FHIR Definitions
StructureDefinition
OperationDefinition
ValueSet
IResource
Resolver
FindStructure
Definition()
ValueSetExpander
LocalTerminology
Server
ITerminologyService
Validator
FluentPathCompiler
FHIR Poco
OperationOutcome
(with IssueComponents)

Practicalities
 class Validator in Hl7.Fhir.Validation namespace
(Hl7.Fhir.Specification assembly)
 Configure: new Validator(settings)
 Resolver, terminology service to use
 Validate: call one of the overloads:
 Validate(IElementNavigator)
 Validate(Base)
 Validate(XmlReader)
 Result is an OperationOutcome
30

Caveats
 Validator is still being developed as we
speak
 Support most features already, except for
slicing – only discriminator-less slicing is
supported
 Other improvements: e.g. loop-detection
 Almost daily updates with more functionality
31

OperationOutcome
- on steroids
 Lots of helper methods
 Properties:
 Success (!information,!warning)
 Fatals, Errors, Warnings properties
 Extension methods (in Hl7.Fhir.Support)
 ErrorsAt(string path)
 Where(severity, type, …)
 Set/GetHierachyLevel()
 Include(outcome), Add(outcome)
32

QUESTIONS?
33

HANDS-ON TRACK
Suggestions for a pleasurable afternoon

IResourceResolver
 Try to use the ZipSource with the
specification.zip to get some core resource
definitions
 Write your own IResourceResolver that
resolves from a .NET compiled-in resource
(not a FHIR resource ;-)

ITerminologyService
 Use your new IResourceResolver to resolve
a ValueSet & expand it.
 Use ValueSet’s FindInExpansion to verify
whether you succeeded
 Look at the C# implementation of
LocalTerminologyService. Could you make
one that calls the FhirClient.ValidateCode()
instead?36

Play with the validator
 Check out
https://github.com/ewoutkramer/Furore.Fhir.ValidationDe
mo & compile
 Try to incorporate your resolver and terminology service
(if any)
 Try running the examples at
https://github.com/ewoutkramer/fhir-net-
api/tree/develop/src/Hl7.Fhir.Specification.Tests/TestData
/validation
 Alter the examples so they trigger your code (change a
binding…)
37

HIDDEN GEMS

ResourceIdentity
 class ResoureIdentity : Uri
 Builds FHIR RESTful Uri’s
 Factory methods Build():
 .Build(“Patient”, “4E75”, vid: “4”)
 “/Patient/4E75/_history/4”
 StructureDefinition URLs for core types
 .Core(FHIRDefinedType.HumanName)
 “http://hl7.org/fhir/StructureDefiniti
on/HumanName”

ResourceIdentity (2)
 Even more useful: parsing RESTful Urls
 var ep = “http://server.org/fhir/Patient/4”
var u = new ResourceIdentity(ep);
 u.ResourceType
 “Patient”
 u.Id
 “4”
40

Extension Manipulations
41

TransactionBuilder
42

SnapshotGenerator
43

Advanced .NET API (Ewout)

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