Composing terminology services

Real applications often combine terminology services: consult a fast, local service first and fall back to an external server, or route specific value sets to a service that specializes in them. MultiTerminologyService composes several ITerminologyServices for exactly this — and because it is itself an ITerminologyService, the combination is used just like any single service.

Fallback

MultiTerminologyService tries its services in the order given. The first service that comes back with a result — success or failure — wins; if a service is indecisive (it throws a FhirOperationException), the next service is consulted.

var local = new LocalTerminologyService(ZipSource.CreateValidationSource());

var client = new FhirClient("https://someterminologyserver.org/fhir");
var external = new ExternalTerminologyService(client);

var multi = new MultiTerminologyService(local, external);

Here the local service is always consulted first; only when it cannot decide does the request go to the external server.

Routing

Sometimes you already know which service should handle certain value sets — for example, a local service that owns all of your custom value sets. Routing sends matching value sets to a chosen service first. Provide a TerminologyServiceRoutingSettings per service:

var local = new LocalTerminologyService(ZipSource.CreateValidationSource());
var localRouting = new TerminologyServiceRoutingSettings(local)
{
    PreferredValueSets = new string[]{"http://fire.ly/ValueSet/*"}
};

var client = new FhirClient("https://someterminologyserver.org/fhir");
var external = new ExternalTerminologyService(client);
var externalRouting = new TerminologyServiceRoutingSettings(external)
{
    PreferredValueSets = new string[]{"http://hl7.fhir.org/ValueSet/*"}
};

var multi = new MultiTerminologyService(localRouting, externalRouting);

Note

You can use * as a wildcard in the routing patterns.

Value sets under http://fire.ly/ValueSet/ now go to the local service first and those under http://hl7.fhir.org/ValueSet/ to the external one; anything else follows the order the services were passed to the constructor — here local, then external.