In the following, the focus is on server-side caching applied to search operations and how a client can influence it.

General Aspects of Interacting with Services

Caching of Search results

Caching in Telemedicine Solutions

Telemedicine Solutions caching of information is, as caching in general, a mechanism for achieving better performance although it must be applied with caution, especially when used with clinical content.

Telemedicine Solutions caching of information from the more stable Administrative Domain must be considered.

Caching in eHealth Services

It can generally be assumed that the caching is based on the principles described in https://hapifhir.io/hapi-fhir/docs/server_jpa/configuration.html#search-result-caching . When interacting with custom operations, caching is specific to the operation itself.

Patient Service

• Services cache search results for 1 minute

  • Services use the cache when identical searches (same search parameters) are performed

  • This means search results can be up to 1 minute stale!

• Search results for a particular bundle ID are cached in the database for 1 hour

  • This cache is used when paging through the result bundle of a particular search

• The max value for the max-results header is 1000

  • Cache-Control: nostore, max-results=1000

• The default page size is 20

• Maximum page size is 200

  • Telemedicine solutions can request a particular page size by setting the _count parameter

• Person$match does not employ any caching

  • internally, it first searches for a Patient with the provided CRN in the local database (does not use the search result cache, for technical reasons)

    • If a patient exists, the information from that patient is returned

    • If a patient does not exist, a request is sent to the CPR registry to look for the information

      • The result of this request is not cached or persisted

Disabling Server-side Caching on Searching

Telemedicine Solutions can selectively disable caching for an individual request using the Cache-Control header:

...

In a HAPI FHIR-based client, opt-out can be achieved like this:

...

Filtering on Searching with Included Resources

...

When a search operation includes references reference resources, however, this cannot be determined up frontupfront. In that case, access is checked for each included resource just before a page of search results is returned to the client. Previously, an error would be returned to the client in case of inadvertent security violations. This behavior behaviour has now been changed to a filtering mechanism instead. Any included resources that the user is not allowed to access are removed from the search result and returned to the client.

When resources have been removed due to access control, an OperationOutcome is included in the search result, where particular elements contains contain the following:

• OperationOutcome.issue.severity: warning

• OperationOutcome.issue.code: suppressed

• OperationOutcome.issue.diagnostics: Some included resources were filtered due to access constraints for the user

Interactions with the Terminology Service

Extracting Translations from ValueSet and CodeSystem

How to determine, say, Danish translations used in ValueSet? The ValueSet as a resource is likely not to contain a proper, displayable value in Danish as the ValueSet can be defined as include of other ValueSet and/or include of entire or partial CodeSystem, and as the display values in the ValueSet are for technical help primarily. Translations, if available, reside in the CodeSystem.

To get translations for at ValueSet, use $expand as follows:

1. Call $expand on the ValueSet

   1. GET <base-url>/fhir/ValueSet/$expand?url=<ValueSet url>&property=designation

2. Determine best translation:

   1. Choose designation with “da” (for Danish) if available, otherwise

   2. Choose display (as best alternative - this will be in whatever language the concept is defined with)

      1. In the example below, this is the case for code assessment

CodeSystems and designations

For supported operations on concept lookups and decomposition, please see the supported operations on CodeSystem at http://hl7.org/fhir/STU3/codesystem-operations.html. Designations targeted consumers/citizens will eg. contain the following data in the designation:

<designation>
<language value="da"></language>
<use>
<system value="http://terminology.hl7.org/CodeSystem/hl7TermMaintInfra"</system>>
<code value="consumer"></code>
</use>
<value value="Højde"></value>
</designation>

Determining the proper code, system and unit for Quantity

Given an observation codeand corresponding system from the ValueSet ehealth-observation-codes, how does one then determine how to specify an observed quantity or reference range? The following describes how to obtain values to set inQuantity.code, .system and .unit.

To get values forQuantity.codeandQuantity.system use the ConceptMap conceptmap-obs-code-to-ucum as follows:

1. Call $translate on the given code and system:

   1. GET <base-url>/fhir/ConceptMap/$translate?system=<system>&code=<code>

2. On match found, the values to use in Quantity.codeandQuantity.systemare the matching concept’s code and system, respectively, where the source ConceptMap is “http://ehealth.sundhed.dk/ConceptMap/conceptmap-obs-code-to-ucum” (see example response below):

To get the corresponding and printable value for Quantity.unit, use the ConceptMap conceptmap-ucum-to-printsymbol as follows:

1. Call $translate on the given system and code determined in the response above:

   1. GET <base-url>/fhir/ConceptMap/$translate?target=http://ehealth.sundhed.dk/vs/ehealth-ucum-printsymbol-supplement&code=<code>&system=<system>

2. On match found, extract the matching concept (see example response below).

3. Call $lookupon the matching (CodeSystem) system and code:

   1. GET <base-url>/fhir/CodeSystem/$lookup?system=http://ehealth.sundhed.dk/cs/ehealth-ucum-printsymbol-supplement&code=<code>

4. On match found, the value to use in Quantity.unitis the matching concept’s designation.valuewhere the designation.language=da(see example response below). If there is no such designation, the printable value in Quantity.unit shall be left empty.

Determining whether to provide measurement as integer or decimal

Given an observation codeand corresponding system from the ValueSet ehealth-observation-codes, how does one determine whether an observed quantity or reference range shall be stated as an integer or decimal? While a particular device might produce a decimal with a given precision for a measure, it is up to the client/solution to convert to the form expected by the infrastructure. Whether integer or decimal, the value is set inQuantity.value.

To determine whether to use integer or decimal, use the ConceptMap conceptmap-obs-code-to-value-type as follows:

1. Call $translate on the given code and system:

   1. GET <base-url>/fhir/ConceptMap/$translate?system=<system>&code=<code>&target=http://hl7.org/fhir/ValueSet/data-types

2. On match found, indication of whether to use integer or decimal are the matching concept’s code and system, respectively, where the source ConceptMap is “http://ehealth.sundhed.dk/ConceptMap/conceptmap-obs-code-to-value-type” (see example response below).

3. The codes integer and decimal in system http://hl7.org/fhir/data-types signify that the observed value or reference range shall be given as integer and decimal, respectively.

...

Example request and response on $translate on conceptmap-obs-code-to-value-type

Request:

GET <base-url>/fhir/ConceptMap/$translate?system=urn:oid:1.2.208.176.2.1&code=NPU03804&target=http://hl7.org/fhir/ValueSet/data-types

Response:

...