--- a/platform/collections/src/main/java/com/redhat/thermostat/collections/graph/Graph.java	Fri May 13 12:28:07 2016 -0400
+++ b/platform/collections/src/main/java/com/redhat/thermostat/collections/graph/Graph.java	Tue May 17 14:45:47 2016 +0200
@@ -83,4 +83,10 @@
      */
     @Override
     Iterator<Node> iterator();
+
+    /**
+     * Clear the graph by removing all its content. The running time of this
+     * method is implementation dependent.
+     */
+    void clear();
 }

--- a/platform/collections/src/main/java/com/redhat/thermostat/collections/graph/HashGraph.java	Fri May 13 12:28:07 2016 -0400
+++ b/platform/collections/src/main/java/com/redhat/thermostat/collections/graph/HashGraph.java	Tue May 17 14:45:47 2016 +0200
@@ -112,4 +112,10 @@
     public Iterator<Node> iterator() {
         return relationships.keySet().iterator();
     }
+
+    @Override
+    public void clear() {
+        relationships.clear();
+        size = 0;
+    }
 }

--- a/platform/collections/src/test/java/com/redhat/thermostat/collections/graph/GraphTest.java	Fri May 13 12:28:07 2016 -0400
+++ b/platform/collections/src/test/java/com/redhat/thermostat/collections/graph/GraphTest.java	Tue May 17 14:45:47 2016 +0200
@@ -41,6 +41,7 @@
 import java.util.Set;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 
 /**
@@ -129,4 +130,30 @@
         assertEquals(3, graph.size());
         assertEquals(2, graph.order());
     }
+
+    @Test
+    public void testClear() {
+        Graph graph = new HashGraph();
+        assertEquals(0, graph.size());
+        assertEquals(0, graph.order());
+
+        Node a = new Node("A");
+        Node b = new Node("B");
+
+        graph.addRelationship(a, "knows", b);
+
+        assertEquals(1, graph.size());
+        assertEquals(2, graph.order());
+
+        graph.clear();
+        assertEquals(0, graph.size());
+        assertEquals(0, graph.order());
+
+        Set<Relationship> rel = graph.getRelationships(a);
+        assertNull(rel);
+
+        graph.addRelationship(a, "knows", b);
+        assertEquals(1, graph.size());
+        assertEquals(2, graph.order());
+    }
 }