5.9LastN Operation

 

HAPI FHIR 5.1.0 introduced preliminary support for the $lastn operation described here.

This implementation of the $lastn operation requires an external Elasticsearch server implementation which is used to implement the indexes required by this operation. The following sections describe the current functionality supported by this operation and the configuration needed to enable this operation.

5.9.1Functional Overview and Parameters

 

As described in the FHIR specification, the $lastn can be used to retrieve the most recent or last n=number of observations for one or more subjects. This implementation supports the following search parameters:

  • subject= or patient=: Identifier(s) of patient(s) to return Observation resources for. If not specified, returns most recent observations for all patients.
  • category=: One or more category code search parameters used to filter Observations.
  • Observation.code=: One or more Observation.code search parameters use to filter and group observations. If not specified, returns most recent observations for all Observation.code values.
  • date=: Date search parameters used to filter Observations by Observation.effectiveDtm.
  • max=: The maximum number of observations to return for each Observation.code. If not specified, returns only the most recent observation in each group.

5.9.2Limitations

 

Currently only Elasticsearch versions up to 6.5.4 are supported.

Search parameters other than those listed above are currently not supported.

The grouping of Observation resources by Observation.code means that the $lastn operation will not work in cases where Observation.code has more than one coding.

5.9.3Deployment and Configuration

 

The $lastn operation is disabled by default. The operation can be enabled by setting the DaoConfig#setLastNEnabled property (see JavaDoc).

In addition, the Elasticsearch client service, ElasticsearchSvcImpl will need to be instantiated with parameters specifying how to connect to the Elasticsearch server, for e.g.:

  @Bean()
  public ElasticsearchSvcImpl elasticsearchSvc() {
     String elasticsearchHost = "localhost";
     String elasticsearchUserId = "elastic";
     String elasticsearchPassword = "changeme";
     int elasticsearchPort = 9301;

     return new ElasticsearchSvcImpl(elasticsearchHost, elasticsearchPort, elasticsearchUserId, elasticsearchPassword);
  }

The Elasticsearch client service requires that security be enabled in the Elasticsearch clusters, and that an Elasticsearch user be available with permissions to create an index and to index, update and delete documents as needed.

See the JavaDoc for more information regarding the Elasticsearch client service.