diff --git a/src/main/java/org/gcube/informationsystem/discovery/knowledge/AddElementToTreeAction.java b/src/main/java/org/gcube/informationsystem/discovery/knowledge/AddElementToTreeAction.java
index dee5550..4f98e17 100644
--- a/src/main/java/org/gcube/informationsystem/discovery/knowledge/AddElementToTreeAction.java
+++ b/src/main/java/org/gcube/informationsystem/discovery/knowledge/AddElementToTreeAction.java
@@ -3,9 +3,9 @@
  */
 package org.gcube.informationsystem.discovery.knowledge;
 
-import org.gcube.informationsystem.base.reference.AccessType;
 import org.gcube.informationsystem.base.reference.Element;
 import org.gcube.informationsystem.discovery.DiscoveredElementAction;
+import org.gcube.informationsystem.model.reference.entities.Resource;
 import org.gcube.informationsystem.tree.Tree;
 import org.gcube.informationsystem.types.TypeMapper;
 import org.gcube.informationsystem.types.reference.Type;
@@ -18,6 +18,25 @@ public class AddElementToTreeAction implements DiscoveredElementAction<Element>
 
 	protected Tree<Class<Element>> tree;
 	
+	protected UsageKnowledge facetKnowledge;
+	protected UsageKnowledge resourceKnowledge;
+	
+	public UsageKnowledge getFacetKnowledge() {
+		return facetKnowledge;
+	}
+
+	public void setFacetKnowledge(UsageKnowledge facetKnowledge) {
+		this.facetKnowledge = facetKnowledge;
+	}
+
+	public UsageKnowledge getResourceKnowledge() {
+		return resourceKnowledge;
+	}
+
+	public void setResourceKnowledge(UsageKnowledge resourceKnowledge) {
+		this.resourceKnowledge = resourceKnowledge;
+	}
+
 	public AddElementToTreeAction(Tree<Class<Element>> tree) {
 		this.tree = tree;
 	}
@@ -25,19 +44,20 @@ public class AddElementToTreeAction implements DiscoveredElementAction<Element>
 	@Override
 	public void analizeElement(Class<Element> e) throws Exception {
 		tree.addNode(e);
-		Type type = TypeMapper.createTypeDefinition(e);
-		createUsageKnowledge(type);
+		createUsageKnowledge(e);
 	}
 	
-	protected void createUsageKnowledge(Type type) {
-		if (type.getAccessType() != AccessType.RESOURCE) {
+	protected void createUsageKnowledge(Class<Element> e) {
+		if (e.isAssignableFrom(Resource.class)) {
 			return;
 		}
-		ResourceType rt = (ResourceType) type;
-		UsageKnowledge fk = UsageKnowledge.getFacetKnowledge();
-		fk.addAll(rt.getFacets());
-		UsageKnowledge rk = UsageKnowledge.getResourceKnowledge();
-		rk.addAll(rt.getResources());
+		
+		if(facetKnowledge!=null && resourceKnowledge!=null) {
+			Type type = TypeMapper.createTypeDefinition(e);
+			ResourceType rt = (ResourceType) type;
+			facetKnowledge.addAll(rt.getFacets());
+			resourceKnowledge.addAll(rt.getResources());
+		}
 	}
 
 }
diff --git a/src/main/java/org/gcube/informationsystem/discovery/knowledge/ModelKnowledge.java b/src/main/java/org/gcube/informationsystem/discovery/knowledge/ModelKnowledge.java
index 8ca876c..eb5a6e1 100644
--- a/src/main/java/org/gcube/informationsystem/discovery/knowledge/ModelKnowledge.java
+++ b/src/main/java/org/gcube/informationsystem/discovery/knowledge/ModelKnowledge.java
@@ -1,7 +1,10 @@
 package org.gcube.informationsystem.discovery.knowledge;
 
+import java.util.ArrayList;
+import java.util.Collection;
 import java.util.HashMap;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Map;
 import java.util.ServiceLoader;
 import java.util.Set;
@@ -22,19 +25,49 @@ public class ModelKnowledge {
 	
 	private static final Logger logger = LoggerFactory.getLogger(ModelKnowledge.class);
 	
+	private static ModelKnowledge modelKnowledge;
+	
+	public static ModelKnowledge getGlobalModelKnowledgeInstance() {
+		if(modelKnowledge==null) {
+			modelKnowledge = new ModelKnowledge();
+			ServiceLoader<? extends RegistrationProvider> registrationProviders = ServiceLoader
+					.load(RegistrationProvider.class);
+			for(RegistrationProvider rp : registrationProviders) {
+				modelKnowledge.addRegistrationProvider(rp);
+			}
+		}
+		return modelKnowledge;
+	}
+	
 	protected boolean created;
 	
-	public static ModelKnowledge getInstance() {
-		return new ModelKnowledge();
-	}
+	protected Map<AccessType,UsageKnowledge> usageKnowledges;
 	
 	protected Map<AccessType, Tree<Class<Element>>> trees;
 	protected Map<AccessType, Discovery<? extends Element>> discoveries;
+	protected List<RegistrationProvider> registrationProviders;
 	
-	protected ModelKnowledge() {
+	public ModelKnowledge() {
 		this.trees = new HashMap<>();
 		this.discoveries = new HashMap<>();
 		this.created = false;
+		this.usageKnowledges = new HashMap<>();
+		this.registrationProviders = new ArrayList<>();
+	}
+	
+	public void addRegistrationProvider(RegistrationProvider rp) {
+		registrationProviders.add(rp);
+	}
+	
+	public void addRegistrationProviders(Collection<RegistrationProvider> rps) {
+		registrationProviders.addAll(rps);
+	}
+	
+	public UsageKnowledge getUsageKnowledge(AccessType accessType) {
+		if(!created) {
+			throw new RuntimeException("You must invoke createKnowledge() first");
+		}
+		return usageKnowledges.get(accessType);
 	}
 	
 	public synchronized void createKnowledge(boolean force) throws Exception {
@@ -53,15 +86,14 @@ public class ModelKnowledge {
 			return;
 		}
 		
-		AccessType[] modelTypes = AccessType.getModelTypes();
-		
 		Set<Package> allPackages = new HashSet<>();
-		ServiceLoader<? extends RegistrationProvider> registrationProviders = ServiceLoader
-				.load(RegistrationProvider.class);
 		for(RegistrationProvider rp : registrationProviders) {
 			allPackages.addAll(rp.getPackagesToRegister());
 		}
 		
+		
+		AccessType[] modelTypes = AccessType.getModelTypes();
+		
 		ClassInformation classInformation = new ClassInformation();
 		
 		for(AccessType accessType : modelTypes) {
@@ -70,6 +102,15 @@ public class ModelKnowledge {
 			Tree<Class<Element>> tree = new Tree<>(clz, classInformation);
 			trees.put(accessType, tree);
 			AddElementToTreeAction aetta = new AddElementToTreeAction(tree);
+			if(accessType == AccessType.RESOURCE) {
+				UsageKnowledge facetKnowledge = new UsageKnowledge();
+				this.usageKnowledges.put(AccessType.FACET, facetKnowledge);
+				aetta.setFacetKnowledge(facetKnowledge);
+				
+				UsageKnowledge resourceKnowledge = new UsageKnowledge();
+				this.usageKnowledges.put(AccessType.RESOURCE, resourceKnowledge);
+				aetta.setResourceKnowledge(resourceKnowledge);
+			}
 			
 			Discovery<? extends Element> discovery = new Discovery<>(clz);
 			discoveries.put(accessType, discovery);
diff --git a/src/main/java/org/gcube/informationsystem/discovery/knowledge/UsageKnowledge.java b/src/main/java/org/gcube/informationsystem/discovery/knowledge/UsageKnowledge.java
index 511c1d3..a9d7a14 100644
--- a/src/main/java/org/gcube/informationsystem/discovery/knowledge/UsageKnowledge.java
+++ b/src/main/java/org/gcube/informationsystem/discovery/knowledge/UsageKnowledge.java
@@ -13,27 +13,9 @@ import org.gcube.informationsystem.types.reference.properties.LinkedEntity;
  */
 public class UsageKnowledge {
 	
-	private static UsageKnowledge facetKnowledge;
-	private static UsageKnowledge resourceKnowledge;
-	
-	public static UsageKnowledge getFacetKnowledge() {
-		if(UsageKnowledge.facetKnowledge==null) {
-			UsageKnowledge.facetKnowledge = new UsageKnowledge();
-		}
-		return UsageKnowledge.facetKnowledge;
-	}
-	
-	public static UsageKnowledge getResourceKnowledge() {
-		if(UsageKnowledge.resourceKnowledge==null) {
-			UsageKnowledge.resourceKnowledge = new UsageKnowledge();
-		}
-		return UsageKnowledge.resourceKnowledge;
-	}
-	
-	
 	protected Map<String, Set<LinkedEntity>> map;
 	
-	private UsageKnowledge(){
+	public UsageKnowledge(){
 		this.map = new LinkedHashMap<>();
 	}
 	
diff --git a/src/main/java/org/gcube/informationsystem/serialization/ElementMapper.java b/src/main/java/org/gcube/informationsystem/serialization/ElementMapper.java
index 7fe51c1..d6f7c8d 100644
--- a/src/main/java/org/gcube/informationsystem/serialization/ElementMapper.java
+++ b/src/main/java/org/gcube/informationsystem/serialization/ElementMapper.java
@@ -107,7 +107,7 @@ public abstract class ElementMapper {
 		}
 		
 		try {
-			ModelKnowledge.getInstance().createKnowledge();
+			ModelKnowledge.getGlobalModelKnowledgeInstance().createKnowledge();
 		}catch (Exception e) {
 			throw new RuntimeException(e);
 		}
diff --git a/src/test/java/org/gcube/informationsystem/discovery/DiscoveryTest.java b/src/test/java/org/gcube/informationsystem/discovery/DiscoveryTest.java
index 986f00a..2e1fd96 100644
--- a/src/test/java/org/gcube/informationsystem/discovery/DiscoveryTest.java
+++ b/src/test/java/org/gcube/informationsystem/discovery/DiscoveryTest.java
@@ -25,7 +25,7 @@ public class DiscoveryTest {
 	
 	@Test
 	public void testDiscovery() throws Exception {
-		ModelKnowledge modelKnowledge = ModelKnowledge.getInstance();
+		ModelKnowledge modelKnowledge = ModelKnowledge.getGlobalModelKnowledgeInstance();
 		modelKnowledge.createKnowledge();
 		
 		AccessType[] accessTypes = AccessType.getModelTypes();