Authoring FHIR Resources

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The technical parts of this documentation are being migrated to FHIR Guide to Authoring Resources - look there instead

Introduction

This document outlines the steps involved in developing a resource for FHIR. It covers both the technical steps required to produce a resource, and the governance over the process.

Creating a Resource

The first step is to get approval for the resource. Each resource that exists must first be approved by the FHIR governance committee (for now, the governance committee is the MnM project team, but eventually this will be a TSC sub-committee). A proposal to the committee must consider the following things:


  • Is the resource scope well described?
  • What is the scope of the resource? The scope should ideally broad enough to meet the potential needs of a wide spectrum of users (inpatient + community, individual & public health, human & veterinary, all countries & realms). If covering such a broad spectrum is not possible, the space needs to be segregated such that multiple non-overlapping resources can cover the space
  • What other resources will it depend on? What is not in scope for the resource?
  • Will other resources are envisaged to depend on it?
  • Does the resource overlap with other resources? (i.e. cover the same elements or be associated with the same extensions as)
  • What are the target market/implementation focuses for the resource
  • What is life cycle of the resource? Does it have a stable id?
  • How do the restful transactions [1] match the life cycle of the resource?
  • Does the proposed resource have a status field? How does the status field relate to the life cycle?
  • What message events [2] does this resource need defined, if any?
  • Is this resource an order or other intention that could be fulfilled by other resources? Which ones? What does the fulfillment cycle look like?
  • Which committee will be managing this resource?

Generally, the governance committee is looking for stability, for proof that the committee has thought through the implications of the multiple frameworks that FHIR supports, and to prevent the same content being modeled in different ways for no good reason. The committee is also responsible for ongoing QA of resources, and can withdraw permission to publish a non-normative resource, or reassign the ownership of a resource.

Also, from the clinical statement rules:

  • a concept must be associated with a patient in a manner which makes clear:
    • Its temporal context
    • Its relationship to the patient
    • In the case of an observation, its mood and presence, absence or value
    • In the case of a procedure, its mood and status
  • This clarity may be achieved by
    • Explicit representation; or
    • Implicit application of defaults ONLY where explicitly modeled rules state the appropriate defaults.

These rules also apply to resources, though mood would never surface directly in a resource

Once approval has been gained, the committee can build the resource definition.

Note that anyone can ask the management committee for a sandbox resource. This is a resource that is created in the local infrastructure, and can be treated like any other resource, but is omitted from any build that is posted to the HL7 website.

Tooling Requirements

The following tools are required to develop resources for FHIR:


  • A Java Runtime Environment 1.6 (to run the build process)
  • A spreadsheet editor to edit the definitions. Excel is used by the core team, but you can use openoffice instead
  • Enterprise Architect v9 to edit the UML diagrams (see FHIR UML Editing guide for how to set this up)
  • An XML editor. XMLSpy is recommended, or another schema driven editor is useful, but any XML editor can be used
  • A subversion client is required for version control
  • For FHIR tooling development, a Java development environment is required (e.g. Eclipse, which the team uses, but any other java ide such as netbeans will work) (see FHIR Java Setup for eclipse)
  • a text editor for the infrastructure pages. Most of the team use notepad++

In addition to these tooling prerequisites, to develop a resource, you will need a subversion username and password, and a resource name.


Note that there is a special variant of the build process for the HL7 web site (FHIR HL7 web site build notes), and this can only be run on windows with a copy of Microsoft Help Compiler installed. (todo: use DITA and replace this)

Resource definition

To define a resource, you define (where "[n]" is the lower case name of the resource):

  • A spreadsheet that contains the master resource definition ([n]-spreadsheet.xml)
    • A "Data Elements" sheet that defines the content of the resource
    • An "Invariants" sheet that defines the xpath conditions associated with the resource
    • a "Search" sheet that defines RESTful search parameters for the resource
    • An "Events" sheet that defines any messaging events associated with the resource
  • A UML class diagram that shows the class based definition of the resource ([n]-uml.xml)
  • An XHTML fragment that contains additional notes required to explain the resource definition ([n]-notes.xml)
  • [optional] An XHTML fragment that contains introductory notes required to explain the resource definition (will be created empty by build process anyway) ([n]-introduction.xml)
  • An XML example for the resource ([n]-example.xml)
  • Additional mapping files and/or translation files may also be created, but are not required.

In addition, the resource has to be integrated into the build. This will be done by the governance/management council, which must:

  • add the link to navigation.xml
  • add the link to fhir.ini

Getting started

  1. Check out org.hl7.fhir from the subversion repository at https://svn.codespaces.com/healthintersections/fhir/
  2. In the root directory, execute publish.bat to check that you have the required build environment - it should report that everything builds successfully (on unix systems [how to run publish process])
  3. Open /publish/index.htm and check that it displays ok
  4. edit the content of the files defined in the previous section
    1. remember to save the updated UML diagram to the images folder
  5. Execute publish.bat again, and check that the build process succeeds without error
  6. check that the content appears as you want


Development Process

The master definitions of the resource are found in the .csv file. This contains a series of columns that define the content of the resource.


Element The full path of the element (defines the element - see below)
Card. Minimum and maximum Cardinality. Elements that have a max cardinality > 1 should still

have a singular name. Note that the only valid cardinality combinations are 0..1, 1..1, 0..* and 1..* (rare). Other values for the lower and upper limit are allowed in profiles

Conf. Conformance. Possible values:
  • [blank] - this element doesn't need to have a conformance flag on it. This means that the element is there fore purely structural reasons, has no inherent meaning, and is optional
  • Mandatory - if this element is present, it may not have a dataAbsentReason on it (it's quite common to have min cardinality = 0, but to be mandatory)
  • Conditional - the element has a minimum cardinality of 0. If the condition is true, then it must be present (or at least one must be present), and it cannot have a dataAbsentReason on it
  • Optional - if the element is present, it may have a dataAbsentReason on it
  • Prohibited - (only in a profile) if the element is present, it must have a dataAbsentReason on it.

See FHIR conformance and cardinality for discussion

Type [Conditional] The type of a data element - may be from the data types [see below], or a Resource reference - e.g. Resource(Person), or a Narrative (though this is only for use on the text element in all resources).

A type must be assigned if the element has no children defined, else a type must not be assigned

Multiple types can be indicated, seperated by a "|". If more than one type is listed, the element name must terminate with "[x]". If the type is Resource, one or more resource types must be listed as a parameter, or "Any" for any kind of resource.

Binding [Conditional] If the type is code, Coding, or CodeableConcept, a binding name must be defined. See FHIR Vocabulary binding for Authors for further info.
Must Understand [Optional] Whether a receiving application must understand the element (note that must-understand isn't quite the same as must support - if an application understands the element by insisting on one fixed value, this is acceptable)
Short Name The definition used for the resource in the XML definitions - limited in length by the available space in the definitions
Definition Formal definition for the meaning of the element. This must follow definition good practices – See below.
Requirements [Optional] Underlying requirements that this element is defined to meet
Comments [Optional] Additional user focused comments - clarifications, explanation of how to use the element, notes on how not to use it.
Condition [Conditional] if the conformance code is "Conditional", this specifies what the condition is
RIM Mapping RIM mapping for the element. See below
v2 Mapping [Optional] where this element comes from in v2 messages. Multiple mappings may be provided
To Do [Optional] Committee notes to implementers about future things that are planned (published)
Committee Notes [Optional] Private committee notes - not published

The most important column is the first one, which defines the structure of the resource. The first row of the first column defines the formal title of the resource used throughout the FHIR build process. Every row from then on must start with [Title]. The second row is always [Title].id, which is the master identifier of the resource that never changes. From there on, the values of the rows take the form of [Title].x.y where the dotted notation indicates structural containment. For instance, [Title].id is the id element in the resource [Title], and [Title].section.name is a name element inside the section element of the [Title] resource element. A path can never be used without defining it first. So [Title].element.name can only be used after [Title].element. The last two elements in a resource are always [Title].extensions and [Title].text, as defined for all resources.

Names used for resource elements must follow the following rules and guidelines:

  1. use lowerCamelCase
  2. U.S. English (spelled correctly!)
  3. Expressed as a noun, with a preceding adjective where necessary to clarify the semantics
  4. Use the “most broadly recognized” industry term for the object that is unlikely to be mistaken for a different element.
  5. Use abbreviations only where extremely well understood by the entire target market. Only the first letter of an abbreviation should be uppercase
  6. Use different names for different elements within the hierarchy unless the semantic is the same in different context. (E.g. using “.name” for “sponsor.name” and “contact.name” is ok if the meaning of “name” is consistent for both sponsor and name.)
  7. Be concise (differentiation of semantics through the name is only necessary with other elements in the resource. Full semantic description is in the definitions.)

Elements should be ordered roughly in terms of importance, with related concepts grouped together.

The base spreadsheet only has columns for mapping to the RIM and v2. Many other mappings can be done, but might be found in other spreadsheets (details to be provided).

RIM mappings

Todo: methdology + grammar

Design Considerations

A core tenant of FHIR is that resources should be simple, stable, and easy to use. Generally, the following guidelines apply to defining resources:

  • Define the resource scope, and target market clearly
  • Resources are subject to ballot. All of FHIR is balloted as a single pack (a la v2). A difference analysis will be available to help with the ballot cycle
  • When designing resources, see FHIR Design Requirements Sources for pre-existing designs that should be consulted
  • Conciseness is a Primary virtue. Every word must count, since everyone has to read it.
  • The element definitions include a requirements slot. This is not mandatory, and can be hard work, but can also be used to make the resource more maintainable. Note that in FHIR, negative ballot comments can't be ruled out of scope if they challenge a missing or unclear requirements.
  • All elements and extensions defined by HL7 committees must have a genuine RIM mapping.
  • Structural re-arrangements to the resources are very difficult if not impossible (including cardinality - see below). Path based navigation must not be changed (=broken)). Therefore, considerable thought should go into data structure. Specific considerations are listed below
  • Event data that goes in the resource: enduring semantically significant event metadata i.e. pathologist approving lab report, prescriber of medication - need to be maintained in band with the data, not somewhere else
  • todo : status, request/fulfillment (there will be other "pointer" resources for request/fulfillment, + pattern for event history on resources that have events (and usually status)

A key part of the design process is that the resources should the define the data elements that everyone needs, and leave the rest of the data elements to extensions - the committee can define and publish extensions. The point of committees defining extensions is that uncommonly encountered use cases can be kept in extensions, and all implementers are not forced to deal with them (this is a big part of the perceived implementation difficulty associated with HL7 standards). Therefore one significant part of the process of building a resource is deciding what elements to put in the resource, and what to put in extensions. Data elements are candidates for being in the base resource if:

  • They are not specific to only one or two countries.
  • If the data elements are in the 80%: i.e. something that 80% of implementers will encounter and/or need - not 80% of the cases that occur to all implementers (this is about preventing work flow variations from complicating the model)
  • Any implementer must understand the content in order to understand the core problem (i.e. can't be ignored)
  • If modeled in all other relevant specifications (particularly, v2, v3, openEHR)

Extensions are a legitimate way for committees to define content. While committees should not define extensions that must be understood, there is no reason for committees not to define extensions. Note that in FHIR, ballot comments that something is in an extension and not a model can be ruled out of scope or non-persuasive if substantial evidence is not provided that the above criteria are met. Committees may subsequently elevate extensions to full resource content (implementation implications of this are discussed in the standard). It is expected that most, if not all, useful implementation profiles will include the use of extensions.


Definitions

todo: put something here from vocabulary. Specifically, we’re looking for: short, non-tautological using broadly understood terms, with examples (usually 3-4, unless it’s bound or something simple like a date). Definitions must clearly distinguish an element from other elements, including extension elements.


Choosing a data type

The following table defines the data types available, and provides some notes concerning their use.


Type Description v3 Equivalent Usage

string A simple string ST Should be known to be short, and no prospect of containing paragraphs or structure or formatting
Narrative xhtml structure N/A Don't use except where it appears in all resources
Attachment Multimedia Attachment ED For complex narrative that may have paragraphs, structure, or formatting, or not be text based at all (image, video etc)

code A simple code CS Simple code where the list is defined by the FHIR infrastructure, the resource definition, or by some fixed external reference (usually ISO or W3C)
Coding A defined code ~CV A direct reference to a concept in a terminology. Use where the content model is managing the issue of text equivalence, and translations across multiple fields directly
CodeableConcept A full concept ~CD Use for a simple equivalent of CD/CE
Choice A choice from a questionnare N/A a code from a code/description list, where there is no formal terminology underlying the list

instant a fixed instant TS System times, always known to the second
date / dateTime a date or a date and a time with variable precision TS Year, Year + month, Year + month + date, or a dateTime - use for any dates that might come from humans
Schedule A set of times ~GTS For specifying a schedule (a tamed GTS)

integer A simple integer INT only a simple integer - for structural values only (like query result count)
Quantity A measured/measurable amount ~PQ/CO/MO/IVL Use where there is a value with human units, that may be coded. It may be possible to use a more specific variant:
Duration An amount of time ~PQ) Fixes to units of time coded in UCUM
Money A currency amount ~MO Fixes units to a currency

Interval() An interval IVL/URG A range (low + high). If an element can be either a fixed interval or a duration, allow both types. Usual types parameters can be Quantity or humanDate, but integer, decimal, and dateTime can also be used
Ratio numerator/denominator RTO Same usage as v3 ratio. Usually just used as a choice of possible data type on observations

uri A uri ~URL A uri. (urn or URL). Usually used in the data types, not in resources
id A simple id This is not properly scoped. It's used where the scope implicitly implies the meaning - resource ids, or internal references. No use in general resource content
Identifier A known identifier II An identifier. Same scope as II
HumanId A human assigned identifier (see v3 identifier pattern) v2 CX - for v3, see Common Design Patterns In addition to the actual Id, has a type, a period, and an assigner. Use for human assigned identifiers
Resource() A reference to another resource You can specify one or more resource types in the parameter (or "Any")

HumanName A human assigned name EN use as for v3 EN / v2 XPN / XON
Address A postal address AD Use as for v3 AD / v2 XAD
Contact Contact details TEL Use as for v3 TEL / v2 XTN

In addition to these data types, there's a few other legal data types that are not expected to be used anywhere in a resource.

Notes

The information defined in the spreadsheet will all appear in the specification but are all specific to an element, and some are only published in the formal definitions.

To help the user with implementation, a set of concise additional notes is found under each resource definition. These notes may discuss how to use a resource, why it doesn't contain things, the relationship between fields, etc. these notes are defined by editing [n]-notes.xml. In addition, [n]-notes.xml contains further usage notes, and any search parameters that must defined for the resource.

It's also possible to define content that goes at the top before the resource is defined. For this, use [n]-introduction.xml

Example

The final part of building a resource is to hand edit an example. The data in the example should illustrate aspects of building and using the resource and is a useful way of double checking the resource definitions. Ideally it should be appropriate from a business/clinical perspective. However, fully exercising the resource data elements is more important.

This is edited by hand - preferably using a schema aware editor.

Message Events

Resources may also have specific message events defined for them. todo: document this

Known To Do for this guide

  • Need a section on how to author extensions, as that will need to happen as part of development. Hints on what sorts of extensions should be created – e.g. commonly used v2 elements, v3 elements, etc.
  • Need a section providing guidance on how to structure resources, particularly approaches that will simplify future growth and ability to attach extensions.
  • Need guidance on when selection of a vocabulary is appropriate.

To find a place

If the content of an element on a resource determines the meaning of and/or constraints on a substantial number of other elements in the resource, its probably a type-discriminator and you should investigate splitting up the resource in multiple resources based on the possible values of that discriminator element