Subscription fhirpath criteria (#5975)

* Added fhirpath-criteria evaluation

* Added test for valid fhirpath that does not evaluate to boolean

* Added resolving the variables %current and %previous

* Added test using only FP criteria

* Added test cases for valid FhirPath expressions that return non-booleans

* Added use of central cache

* Added more elaborate tests for non-sunshine scenarios

* Added changelog

* CheckStyle'd errorcode added.

* Added spotless formatting and converted FhirPathR5 expression to be Android compatible

* Applied more spotless

hapi-fhir-base/src/main/java/ca/uhn/fhir/fhirpath/IFhirPathEvaluationContext.java

@@ -24,6 +24,8 @@ import jakarta.annotation.Nullable;
 import org.hl7.fhir.instance.model.api.IBase;
 import org.hl7.fhir.instance.model.api.IIdType;
 
+import java.util.List;
+
 public interface IFhirPathEvaluationContext {
 
 	/**
@@ -36,4 +38,15 @@ public interface IFhirPathEvaluationContext {
 	default IBase resolveReference(@Nonnull IIdType theReference, @Nullable IBase theContext) {
 		return null;
 	}
+
+	/**
+	 *
+	 * @param appContext
+	 * @param name The name of the constant(s) to be resolved
+	 * @param beforeContext
+	 * @return
+	 */
+	default List<IBase> resolveConstant(Object appContext, String name, boolean beforeContext) {
+		return null;
+	}
 }

hapi-fhir-docs/src/main/resources/ca/uhn/hapi/fhir/changelog/7_4_0/6031-add-support-for-fhirpath-criteria.yaml

@@ -0,0 +1,4 @@
+---
+type: add
+issue: 6031
+title: "Subscriptions now support the evaluation use of FhirPath criteria and the use of the variables %current and %previous. Thanks to Jens Villadsen (@jkiddo) for the contribution!"

hapi-fhir-jpaserver-subscription/src/main/java/ca/uhn/fhir/jpa/topic/SubscriptionTopicConfig.java

@@ -24,6 +24,7 @@ import ca.uhn.fhir.jpa.api.dao.DaoRegistry;
 import ca.uhn.fhir.jpa.searchparam.matcher.SearchParamMatcher;
 import ca.uhn.fhir.jpa.subscription.config.SubscriptionConfig;
 import ca.uhn.fhir.jpa.subscription.submit.interceptor.SubscriptionQueryValidator;
+import ca.uhn.fhir.jpa.util.MemoryCacheService;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
@@ -33,11 +34,12 @@ import org.springframework.context.annotation.Lazy;
 @Import(SubscriptionConfig.class)
 public class SubscriptionTopicConfig {
 	@Bean
-	SubscriptionTopicMatchingSubscriber subscriptionTopicMatchingSubscriber(FhirContext theFhirContext) {
+	SubscriptionTopicMatchingSubscriber subscriptionTopicMatchingSubscriber(
+			FhirContext theFhirContext, MemoryCacheService memoryCacheService) {
 		switch (theFhirContext.getVersion().getVersion()) {
 			case R5:
 			case R4B:
-				return new SubscriptionTopicMatchingSubscriber(theFhirContext);
+				return new SubscriptionTopicMatchingSubscriber(theFhirContext, memoryCacheService);
 			default:
 				return null;
 		}

hapi-fhir-jpaserver-subscription/src/main/java/ca/uhn/fhir/jpa/topic/SubscriptionTopicMatcher.java

@@ -21,6 +21,7 @@ package ca.uhn.fhir.jpa.topic;
 
 import ca.uhn.fhir.jpa.searchparam.matcher.InMemoryMatchResult;
 import ca.uhn.fhir.jpa.subscription.model.ResourceModifiedMessage;
+import ca.uhn.fhir.jpa.util.MemoryCacheService;
 import org.hl7.fhir.instance.model.api.IBaseResource;
 import org.hl7.fhir.r5.model.SubscriptionTopic;
 
@@ -29,10 +30,15 @@ import java.util.List;
 public class SubscriptionTopicMatcher {
 	private final SubscriptionTopicSupport mySubscriptionTopicSupport;
 	private final SubscriptionTopic myTopic;
+	private final MemoryCacheService myMemoryCacheService;
 
-	public SubscriptionTopicMatcher(SubscriptionTopicSupport theSubscriptionTopicSupport, SubscriptionTopic theTopic) {
+	public SubscriptionTopicMatcher(
+			SubscriptionTopicSupport theSubscriptionTopicSupport,
+			SubscriptionTopic theTopic,
+			MemoryCacheService memoryCacheService) {
 		mySubscriptionTopicSupport = theSubscriptionTopicSupport;
 		myTopic = theTopic;
+		myMemoryCacheService = memoryCacheService;
 	}
 
 	public InMemoryMatchResult match(ResourceModifiedMessage theMsg) {
@@ -43,7 +49,7 @@ public class SubscriptionTopicMatcher {
 		for (SubscriptionTopic.SubscriptionTopicResourceTriggerComponent next : triggers) {
 			if (resourceName.equals(next.getResource())) {
 				SubscriptionTriggerMatcher matcher =
-						new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, theMsg, next);
+						new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, theMsg, next, myMemoryCacheService);
 				InMemoryMatchResult result = matcher.match();
 				if (result.matched()) {
 					// as soon as one trigger matches, we're done

hapi-fhir-jpaserver-subscription/src/main/java/ca/uhn/fhir/jpa/topic/SubscriptionTopicMatchingSubscriber.java

@@ -27,6 +27,7 @@ import ca.uhn.fhir.jpa.searchparam.matcher.InMemoryMatchResult;
 import ca.uhn.fhir.jpa.subscription.model.ResourceModifiedJsonMessage;
 import ca.uhn.fhir.jpa.subscription.model.ResourceModifiedMessage;
 import ca.uhn.fhir.jpa.topic.filter.InMemoryTopicFilterMatcher;
+import ca.uhn.fhir.jpa.util.MemoryCacheService;
 import ca.uhn.fhir.rest.api.RestOperationTypeEnum;
 import ca.uhn.fhir.subscription.api.IResourceModifiedMessagePersistenceSvc;
 import ca.uhn.fhir.util.Logs;
@@ -67,8 +68,11 @@ public class SubscriptionTopicMatchingSubscriber implements MessageHandler {
 	@Autowired
 	private IResourceModifiedMessagePersistenceSvc myResourceModifiedMessagePersistenceSvc;
 
-	public SubscriptionTopicMatchingSubscriber(FhirContext theFhirContext) {
+	private MemoryCacheService myMemoryCacheService;
+
+	public SubscriptionTopicMatchingSubscriber(FhirContext theFhirContext, MemoryCacheService memoryCacheService) {
 		myFhirContext = theFhirContext;
+		this.myMemoryCacheService = memoryCacheService;
 	}
 
 	@Override
@@ -110,7 +114,8 @@ public class SubscriptionTopicMatchingSubscriber implements MessageHandler {
 
 		Collection<SubscriptionTopic> topics = mySubscriptionTopicRegistry.getAll();
 		for (SubscriptionTopic topic : topics) {
-			SubscriptionTopicMatcher matcher = new SubscriptionTopicMatcher(mySubscriptionTopicSupport, topic);
+			SubscriptionTopicMatcher matcher =
+					new SubscriptionTopicMatcher(mySubscriptionTopicSupport, topic, myMemoryCacheService);
 			InMemoryMatchResult result = matcher.match(theMsg);
 			if (result.matched()) {
 				int deliveries = deliverToTopicSubscriptions(theMsg, topic, result);

hapi-fhir-jpaserver-subscription/src/main/java/ca/uhn/fhir/jpa/topic/SubscriptionTriggerMatcher.java

@@ -19,17 +19,26 @@
  */
 package ca.uhn.fhir.jpa.topic;
 
+import ca.uhn.fhir.fhirpath.IFhirPath;
+import ca.uhn.fhir.fhirpath.IFhirPathEvaluationContext;
+import ca.uhn.fhir.i18n.Msg;
 import ca.uhn.fhir.jpa.api.dao.IFhirResourceDao;
 import ca.uhn.fhir.jpa.searchparam.matcher.InMemoryMatchResult;
 import ca.uhn.fhir.jpa.subscription.model.ResourceModifiedMessage;
+import ca.uhn.fhir.jpa.util.MemoryCacheService;
 import ca.uhn.fhir.rest.api.server.SystemRequestDetails;
 import ca.uhn.fhir.rest.server.messaging.BaseResourceMessage;
 import ca.uhn.fhir.storage.PreviousVersionReader;
 import ca.uhn.fhir.util.Logs;
+import com.google.common.base.Strings;
+import org.hl7.fhir.exceptions.FHIRException;
+import org.hl7.fhir.instance.model.api.IBase;
 import org.hl7.fhir.instance.model.api.IBaseResource;
+import org.hl7.fhir.r5.model.BooleanType;
 import org.hl7.fhir.r5.model.Enumeration;
 import org.hl7.fhir.r5.model.SubscriptionTopic;
 import org.slf4j.Logger;
+import org.slf4j.helpers.MessageFormatter;
 
 import java.util.List;
 import java.util.Optional;
@@ -45,11 +54,13 @@ public class SubscriptionTriggerMatcher {
 	private final IFhirResourceDao myDao;
 	private final PreviousVersionReader myPreviousVersionReader;
 	private final SystemRequestDetails mySrd;
+	private final MemoryCacheService myMemoryCacheService;
 
 	public SubscriptionTriggerMatcher(
 			SubscriptionTopicSupport theSubscriptionTopicSupport,
 			ResourceModifiedMessage theMsg,
-			SubscriptionTopic.SubscriptionTopicResourceTriggerComponent theTrigger) {
+			SubscriptionTopic.SubscriptionTopicResourceTriggerComponent theTrigger,
+			MemoryCacheService theMemoryCacheService) {
 		mySubscriptionTopicSupport = theSubscriptionTopicSupport;
 		myOperation = theMsg.getOperationType();
 		myResource = theMsg.getPayload(theSubscriptionTopicSupport.getFhirContext());
@@ -58,6 +69,7 @@ public class SubscriptionTriggerMatcher {
 		myTrigger = theTrigger;
 		myPreviousVersionReader = new PreviousVersionReader(myDao);
 		mySrd = new SystemRequestDetails();
+		myMemoryCacheService = theMemoryCacheService;
 	}
 
 	public InMemoryMatchResult match() {
@@ -66,21 +78,22 @@ public class SubscriptionTriggerMatcher {
 		if (SubscriptionTopicUtil.matches(myOperation, supportedInteractions)) {
 			SubscriptionTopic.SubscriptionTopicResourceTriggerQueryCriteriaComponent queryCriteria =
 					myTrigger.getQueryCriteria();
-			InMemoryMatchResult result = match(queryCriteria);
+			String fhirPathCriteria = myTrigger.getFhirPathCriteria();
-			if (result.matched()) {
+			return match(queryCriteria, fhirPathCriteria);
-				return result;
-			}
 		}
 		return InMemoryMatchResult.noMatch();
 	}
 
 	private InMemoryMatchResult match(
-			SubscriptionTopic.SubscriptionTopicResourceTriggerQueryCriteriaComponent theQueryCriteria) {
+			SubscriptionTopic.SubscriptionTopicResourceTriggerQueryCriteriaComponent theQueryCriteria,
+			String theFhirPathCriteria) {
 		String previousCriteria = theQueryCriteria.getPrevious();
 		String currentCriteria = theQueryCriteria.getCurrent();
 		InMemoryMatchResult previousMatches = InMemoryMatchResult.fromBoolean(previousCriteria == null);
 		InMemoryMatchResult currentMatches = InMemoryMatchResult.fromBoolean(currentCriteria == null);
 
+		InMemoryMatchResult fhirPathCriteriaEvaluationResult = evaluateFhirPathCriteria(theFhirPathCriteria);
+
 		// WIP STR5 implement fhirPathCriteria per https://build.fhir.org/subscriptiontopic.html#fhirpath-criteria
 		if (currentCriteria != null) {
 			currentMatches = matchResource(myResource, currentCriteria);
@@ -105,12 +118,89 @@ public class SubscriptionTriggerMatcher {
 		}
 		// WIP STR5 implement resultForCreate and resultForDelete
 		if (theQueryCriteria.getRequireBoth()) {
-			return InMemoryMatchResult.and(previousMatches, currentMatches);
+			return InMemoryMatchResult.and(
+					InMemoryMatchResult.and(previousMatches, currentMatches), fhirPathCriteriaEvaluationResult);
 		} else {
-			return InMemoryMatchResult.or(previousMatches, currentMatches);
+			return InMemoryMatchResult.and(
+					InMemoryMatchResult.or(previousMatches, currentMatches), fhirPathCriteriaEvaluationResult);
 		}
 	}
 
+	private InMemoryMatchResult evaluateFhirPathCriteria(String theFhirPathCriteria) {
+		if (!Strings.isNullOrEmpty(theFhirPathCriteria)) {
+			IFhirPath fhirPathEngine =
+					mySubscriptionTopicSupport.getFhirContext().newFhirPath();
+			fhirPathEngine.setEvaluationContext(new IFhirPathEvaluationContext() {
+
+				@Override
+				public List<IBase> resolveConstant(Object appContext, String name, boolean beforeContext) {
+					if ("current".equalsIgnoreCase(name)) return List.of(myResource);
+
+					if ("previous".equalsIgnoreCase(name)) {
+						Optional previousResource = myPreviousVersionReader.readPreviousVersion(myResource);
+						if (previousResource.isPresent()) return List.of((IBase) previousResource.get());
+					}
+
+					return null;
+				}
+			});
+			try {
+				IFhirPath.IParsedExpression expression = myMemoryCacheService.get(
+						MemoryCacheService.CacheEnum.FHIRPATH_EXPRESSION, theFhirPathCriteria, exp -> {
+							try {
+								return fhirPathEngine.parse(exp);
+							} catch (FHIRException e) {
+								throw e;
+							} catch (Exception e) {
+								throw new RuntimeException(Msg.code(2534) + e.getMessage(), e);
+							}
+						});
+
+				List<IBase> result = fhirPathEngine.evaluate(myResource, expression, IBase.class);
+
+				return parseResult(theFhirPathCriteria, result);
+
+			} catch (FHIRException fhirException) {
+				ourLog.warn(
+						"Subscription topic {} has a fhirPathCriteria that is not valid: {}",
+						myTrigger.getId(),
+						theFhirPathCriteria,
+						fhirException);
+				return InMemoryMatchResult.unsupportedFromReason(fhirException.getMessage());
+			}
+		}
+		return InMemoryMatchResult.fromBoolean(true);
+	}
+
+	private InMemoryMatchResult parseResult(String theFhirPathCriteria, List<IBase> result) {
+		if (result == null) {
+			return InMemoryMatchResult.unsupportedFromReason(MessageFormatter.format(
+							"FhirPath evaluation criteria '{}' from Subscription topic: '{}' resulted in null results.",
+							theFhirPathCriteria,
+							myTrigger.getId())
+					.getMessage());
+		}
+
+		if (result.size() != 1) {
+			return InMemoryMatchResult.unsupportedFromReason(MessageFormatter.arrayFormat(
+							"FhirPath evaluation criteria '{}' from Subscription topic: '{}' resulted in '{}' results. Expected 1.",
+							new String[] {theFhirPathCriteria, myTrigger.getId(), String.valueOf(result.size())})
+					.getMessage());
+		}
+
+		if (!(result.get(0) instanceof BooleanType)) {
+			return InMemoryMatchResult.unsupportedFromReason(MessageFormatter.arrayFormat(
+							"FhirPath evaluation criteria '{}' from Subscription topic: '{}' resulted in a non-boolean result: '{}'",
+							new String[] {
+								theFhirPathCriteria,
+								myTrigger.getId(),
+								result.get(0).getClass().getName()
+							})
+					.getMessage());
+		}
+		return InMemoryMatchResult.fromBoolean(((BooleanType) result.get(0)).booleanValue());
+	}
+
 	private InMemoryMatchResult matchResource(IBaseResource theResource, String theCriteria) {
 		InMemoryMatchResult result =
 				mySubscriptionTopicSupport.getSearchParamMatcher().match(theCriteria, theResource, mySrd);

hapi-fhir-jpaserver-subscription/src/test/java/ca/uhn/fhir/jpa/topic/SubscriptionTriggerMatcherTest.java

@@ -1,12 +1,15 @@
 package ca.uhn.fhir.jpa.topic;
 
 import ca.uhn.fhir.context.FhirContext;
+import ca.uhn.fhir.jpa.api.config.JpaStorageSettings;
 import ca.uhn.fhir.jpa.api.dao.DaoRegistry;
 import ca.uhn.fhir.jpa.api.dao.IFhirResourceDao;
 import ca.uhn.fhir.jpa.searchparam.matcher.InMemoryMatchResult;
 import ca.uhn.fhir.jpa.searchparam.matcher.SearchParamMatcher;
 import ca.uhn.fhir.jpa.subscription.model.ResourceModifiedMessage;
+import ca.uhn.fhir.jpa.util.MemoryCacheService;
 import org.hl7.fhir.r5.model.Encounter;
+import org.hl7.fhir.r5.model.Enumerations;
 import org.hl7.fhir.r5.model.IdType;
 import org.hl7.fhir.r5.model.SubscriptionTopic;
 import org.junit.jupiter.api.BeforeEach;
@@ -15,7 +18,10 @@ import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
@@ -30,11 +36,14 @@ class SubscriptionTriggerMatcherTest {
 	@Mock
 	SearchParamMatcher mySearchParamMatcher;
 
+	MemoryCacheService myMemoryCacheService;
+
 	private SubscriptionTopicSupport mySubscriptionTopicSupport;
 	private Encounter myEncounter;
 
 	@BeforeEach
 	public void before() {
+		myMemoryCacheService = new MemoryCacheService(new JpaStorageSettings());
 		mySubscriptionTopicSupport = new SubscriptionTopicSupport(ourFhirContext, myDaoRegistry, mySearchParamMatcher);
 		myEncounter = new Encounter();
 		myEncounter.setIdElement(new IdType("Encounter", "123", "2"));
@@ -48,7 +57,7 @@ class SubscriptionTriggerMatcherTest {
 		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
 
 		// run
-		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger);
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
 		InMemoryMatchResult result = svc.match();
 
 		// verify
@@ -65,7 +74,7 @@ class SubscriptionTriggerMatcherTest {
 		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.CREATE);
 
 		// run
-		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger);
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
 		InMemoryMatchResult result = svc.match();
 
 		// verify
@@ -82,7 +91,7 @@ class SubscriptionTriggerMatcherTest {
 		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
 
 		// run
-		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger);
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
 		InMemoryMatchResult result = svc.match();
 
 		// verify
@@ -99,7 +108,7 @@ class SubscriptionTriggerMatcherTest {
 		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
 
 		// run
-		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger);
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
 		InMemoryMatchResult result = svc.match();
 
 		// verify
@@ -124,11 +133,253 @@ class SubscriptionTriggerMatcherTest {
 		when(mySearchParamMatcher.match(any(), any(), any())).thenReturn(InMemoryMatchResult.successfulMatch());
 
 		// run
-		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger);
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
 		InMemoryMatchResult result = svc.match();
 
 		// verify
 		assertTrue(result.matched());
 	}
 
+	@Test
+	public void testFalseFhirPathCriteriaEvaluation() {
+		ResourceModifiedMessage msg = new ResourceModifiedMessage(ourFhirContext, myEncounter, ResourceModifiedMessage.OperationTypeEnum.UPDATE);
+
+		// setup
+		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
+		trigger.setResource("Encounter");
+		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
+		trigger.setFhirPathCriteria("false");
+
+		// run
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
+		InMemoryMatchResult result = svc.match();
+
+		// verify
+		assertFalse(result.matched());
+	}
+
+	@Test
+	public void testInvalidFhirPathCriteriaEvaluation() {
+		ResourceModifiedMessage msg = new ResourceModifiedMessage(ourFhirContext, myEncounter, ResourceModifiedMessage.OperationTypeEnum.UPDATE);
+
+		// setup
+		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
+		trigger.setResource("Encounter");
+		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
+		trigger.setFhirPathCriteria("random text");
+
+		// run
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
+		InMemoryMatchResult result = svc.match();
+
+		// verify
+		assertFalse(result.matched());
+		assertEquals("Error @1, 2: Premature ExpressionNode termination at unexpected token \"text\"", result.getUnsupportedReason());
+	}
+
+	@Test
+	public void testInvalidBooleanOutcomeOfFhirPathCriteriaEvaluation() {
+		ResourceModifiedMessage msg = new ResourceModifiedMessage(ourFhirContext, myEncounter, ResourceModifiedMessage.OperationTypeEnum.UPDATE);
+
+		// setup
+		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
+		trigger.setResource("Encounter");
+		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
+		trigger.setFhirPathCriteria("id");
+
+		// run
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
+		InMemoryMatchResult result = svc.match();
+
+		// verify
+		assertFalse(result.matched());
+	}
+
+	@Test
+	public void testValidFhirPathCriteriaEvaluation() {
+		ResourceModifiedMessage msg = new ResourceModifiedMessage(ourFhirContext, myEncounter, ResourceModifiedMessage.OperationTypeEnum.UPDATE);
+
+		// setup
+		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
+		trigger.setResource("Encounter");
+		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
+		trigger.setFhirPathCriteria("id = " + myEncounter.getIdElement().getIdPart());
+
+		// run
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
+		InMemoryMatchResult result = svc.match();
+
+		// verify
+		assertTrue(result.matched());
+	}
+
+	@Test
+	public void testValidFhirPathCriteriaEvaluationUsingCurrent() {
+		ResourceModifiedMessage msg = new ResourceModifiedMessage(ourFhirContext, myEncounter, ResourceModifiedMessage.OperationTypeEnum.UPDATE);
+
+		// setup
+		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
+		trigger.setResource("Encounter");
+		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
+		trigger.setFhirPathCriteria("%current.id = " + myEncounter.getIdElement().getIdPart());
+
+		// run
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
+		InMemoryMatchResult result = svc.match();
+
+		// verify
+		assertTrue(result.matched());
+	}
+
+	@Test
+	public void testValidFhirPathCriteriaEvaluationReturningNonBoolean() {
+		ResourceModifiedMessage msg = new ResourceModifiedMessage(ourFhirContext, myEncounter, ResourceModifiedMessage.OperationTypeEnum.UPDATE);
+
+		// setup
+		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
+		trigger.setId("1");
+		trigger.setResource("Encounter");
+		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
+		trigger.setFhirPathCriteria("%current.id");
+
+		// run
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
+		InMemoryMatchResult result = svc.match();
+
+		// verify
+		assertFalse(result.matched());
+		assertEquals("FhirPath evaluation criteria '%current.id' from Subscription topic: '1' resulted in a non-boolean result: 'org.hl7.fhir.r5.model.IdType'", result.getUnsupportedReason());
+	}
+
+	@Test
+	public void testValidFhirPathReturningCollection() {
+		ResourceModifiedMessage msg = new ResourceModifiedMessage(ourFhirContext, myEncounter, ResourceModifiedMessage.OperationTypeEnum.UPDATE);
+
+		// setup
+		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
+		trigger.setId("1");
+		trigger.setResource("Encounter");
+		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
+		trigger.setFhirPathCriteria("%current | %previous");
+
+		IFhirResourceDao mockEncounterDao = mock(IFhirResourceDao.class);
+		when(myDaoRegistry.getResourceDao("Encounter")).thenReturn(mockEncounterDao);
+		Encounter encounterPreviousVersion = new Encounter();
+		when(mockEncounterDao.read(any(), any(), eq(false))).thenReturn(encounterPreviousVersion);
+
+		// run
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
+		InMemoryMatchResult result = svc.match();
+
+		// verify
+		assertFalse(result.matched());
+		assertEquals("FhirPath evaluation criteria '%current | %previous' from Subscription topic: '1' resulted in '2' results. Expected 1.", result.getUnsupportedReason());
+	}
+
+	@Test
+	public void testUpdateWithPrevCriteriaMatchAndFailingFhirPathCriteria() {
+		ResourceModifiedMessage msg = new ResourceModifiedMessage(ourFhirContext, myEncounter, ResourceModifiedMessage.OperationTypeEnum.UPDATE);
+
+		// setup
+		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
+		trigger.setResource("Encounter");
+		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
+		trigger.getQueryCriteria().setPrevious("Encounter?status=in-progress");
+		trigger.setFhirPathCriteria("random text");
+
+
+		IFhirResourceDao mockEncounterDao = mock(IFhirResourceDao.class);
+		when(myDaoRegistry.getResourceDao("Encounter")).thenReturn(mockEncounterDao);
+		Encounter encounterPreviousVersion = new Encounter();
+		when(mockEncounterDao.read(any(), any(), eq(false))).thenReturn(encounterPreviousVersion);
+		when(mySearchParamMatcher.match(any(), any(), any())).thenReturn(InMemoryMatchResult.successfulMatch());
+
+		// run
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
+		InMemoryMatchResult result = svc.match();
+
+		// verify
+		assertFalse(result.matched());
+		assertEquals("Error @1, 2: Premature ExpressionNode termination at unexpected token \"text\"", result.getUnsupportedReason());
+	}
+
+	@Test
+	public void testUpdateWithPrevCriteriaMatchAndFhirPathCriteriaUsingPreviousVersion() {
+		myEncounter.setStatus(Enumerations.EncounterStatus.INPROGRESS);
+		ResourceModifiedMessage msg = new ResourceModifiedMessage(ourFhirContext, myEncounter, ResourceModifiedMessage.OperationTypeEnum.UPDATE);
+
+		// setup
+		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
+		trigger.setResource("Encounter");
+		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
+		trigger.getQueryCriteria().setPrevious("Encounter?status=in-progress");
+		trigger.setFhirPathCriteria("%current.status='in-progress' and %previous.status.exists().not()");
+
+
+		IFhirResourceDao mockEncounterDao = mock(IFhirResourceDao.class);
+		when(myDaoRegistry.getResourceDao("Encounter")).thenReturn(mockEncounterDao);
+		Encounter encounterPreviousVersion = new Encounter();
+		when(mockEncounterDao.read(any(), any(), eq(false))).thenReturn(encounterPreviousVersion);
+		when(mySearchParamMatcher.match(any(), any(), any())).thenReturn(InMemoryMatchResult.successfulMatch());
+
+		// run
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
+		InMemoryMatchResult result = svc.match();
+
+		// verify
+		assertTrue(result.matched());
+	}
+
+	@Test
+	public void testUpdateOnlyFhirPathCriteriaUsingPreviousVersion() {
+		myEncounter.setStatus(Enumerations.EncounterStatus.INPROGRESS);
+		ResourceModifiedMessage msg = new ResourceModifiedMessage(ourFhirContext, myEncounter, ResourceModifiedMessage.OperationTypeEnum.UPDATE);
+
+		// setup
+		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
+		trigger.setResource("Encounter");
+		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
+		trigger.setFhirPathCriteria("%current.status='in-progress' and %previous.status.exists().not()");
+
+
+		IFhirResourceDao mockEncounterDao = mock(IFhirResourceDao.class);
+		when(myDaoRegistry.getResourceDao("Encounter")).thenReturn(mockEncounterDao);
+		Encounter encounterPreviousVersion = new Encounter();
+		when(mockEncounterDao.read(any(), any(), eq(false))).thenReturn(encounterPreviousVersion);
+
+		// run
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
+		InMemoryMatchResult result = svc.match();
+
+		// verify
+		assertTrue(result.matched());
+	}
+
+	@Test
+	public void testCacheUsage() {
+		myEncounter.setStatus(Enumerations.EncounterStatus.INPROGRESS);
+		ResourceModifiedMessage msg = new ResourceModifiedMessage(ourFhirContext, myEncounter, ResourceModifiedMessage.OperationTypeEnum.UPDATE);
+
+		// setup
+		SubscriptionTopic.SubscriptionTopicResourceTriggerComponent trigger = new SubscriptionTopic.SubscriptionTopicResourceTriggerComponent();
+		trigger.setResource("Encounter");
+		trigger.addSupportedInteraction(SubscriptionTopic.InteractionTrigger.UPDATE);
+		String fhirPathCriteria = "%current.status='in-progress'";
+		trigger.setFhirPathCriteria(fhirPathCriteria);
+
+
+		IFhirResourceDao mockEncounterDao = mock(IFhirResourceDao.class);
+		when(myDaoRegistry.getResourceDao("Encounter")).thenReturn(mockEncounterDao);
+
+		assertNull(myMemoryCacheService.getIfPresent(MemoryCacheService.CacheEnum.FHIRPATH_EXPRESSION, fhirPathCriteria));
+
+		// run
+		SubscriptionTriggerMatcher svc = new SubscriptionTriggerMatcher(mySubscriptionTopicSupport, msg, trigger, myMemoryCacheService);
+		InMemoryMatchResult result = svc.match();
+
+
+		// verify
+		assertTrue(result.matched());
+		assertNotNull(myMemoryCacheService.getIfPresent(MemoryCacheService.CacheEnum.FHIRPATH_EXPRESSION, fhirPathCriteria));
+	}
 }

hapi-fhir-storage/src/main/java/ca/uhn/fhir/jpa/util/MemoryCacheService.java

@@ -77,6 +77,7 @@ public class MemoryCacheService {
 				case HISTORY_COUNT:
 				case TAG_DEFINITION:
 				case RESOURCE_CONDITIONAL_CREATE_VERSION:
+				case FHIRPATH_EXPRESSION:
 				default:
 					timeoutSeconds = SECONDS.convert(1, MINUTES);
 					maximumSize = 10000;
@@ -193,6 +194,7 @@ public class MemoryCacheService {
 		TAG_DEFINITION(TagDefinitionCacheKey.class),
 		RESOURCE_LOOKUP(String.class),
 		FORCED_ID_TO_PID(String.class),
+		FHIRPATH_EXPRESSION(String.class),
 		/**
 		 * Key type: {@literal Long}
 		 * Value type: {@literal Optional<String>}

hapi-fhir-structures-r5/src/main/java/org/hl7/fhir/r5/hapi/fhirpath/FhirPathR5.java

@@ -19,8 +19,10 @@ import org.hl7.fhir.r5.model.Base;
 import org.hl7.fhir.r5.model.IdType;
 import org.hl7.fhir.r5.model.ValueSet;
 
+import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
+import java.util.stream.Collectors;
 
 public class FhirPathR5 implements IFhirPath {
 
@@ -99,7 +101,11 @@ public class FhirPathR5 implements IFhirPath {
 					boolean beforeContext,
 					boolean explicitConstant)
 					throws PathEngineException {
-				return null;
+
+				return Collections.unmodifiableList(
+						theEvaluationContext.resolveConstant(appContext, name, beforeContext).stream()
+								.map(Base.class::cast)
+								.collect(Collectors.toList()));
 			}
 
 			@Override