diff --git a/src/main/java/org/apache/gossip/GossipService.java b/src/main/java/org/apache/gossip/GossipService.java
index f32eb35..f216c33 100644
--- a/src/main/java/org/apache/gossip/GossipService.java
+++ b/src/main/java/org/apache/gossip/GossipService.java
@@ -131,4 +131,5 @@ public class GossipService {
   public SharedGossipDataMessage findSharedData(String key){
     return getGossipManager().findSharedGossipData(key);
   }
+  
 }
diff --git a/src/main/java/org/apache/gossip/crdt/Crdt.java b/src/main/java/org/apache/gossip/crdt/Crdt.java
new file mode 100644
index 0000000..8edfa8c
--- /dev/null
+++ b/src/main/java/org/apache/gossip/crdt/Crdt.java
@@ -0,0 +1,39 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.gossip.crdt;
+/**
+ * 
+ * Immutable type 
+ *
+ * @param <SetType>
+ * @param <MergeReturnType>
+ */
+public interface Crdt<SetType, MergeReturnType extends Crdt<SetType, MergeReturnType>> {
+
+ 
+  MergeReturnType merge(MergeReturnType other);
+  SetType value();
+  /**
+   * Called to self optimize. Some CRDTs may use some mechanism to clean up be 
+   * removing obsolete data outside the scope of merging. IE this could clean up 
+   * temporal values, old copies etc. 
+   * @return the Crdt structure optimized 
+   */
+  MergeReturnType optimize();
+
+}
diff --git a/src/main/java/org/apache/gossip/crdt/CrdtBiFunctionMerge.java b/src/main/java/org/apache/gossip/crdt/CrdtBiFunctionMerge.java
new file mode 100644
index 0000000..1ac7a30
--- /dev/null
+++ b/src/main/java/org/apache/gossip/crdt/CrdtBiFunctionMerge.java
@@ -0,0 +1,55 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.gossip.crdt;
+
+import java.util.function.BiFunction;
+
+@SuppressWarnings("rawtypes")
+public class CrdtBiFunctionMerge implements BiFunction<Crdt,Crdt,Crdt> {
+
+  @SuppressWarnings("unchecked")
+  @Override
+  public Crdt apply(Crdt t, Crdt u) {
+    if (t == null && u == null){
+      return null;
+    } else if (t == null){
+      return u;
+    } else if (u == null){
+      return t;
+    }
+    if (! u.getClass().equals(t.getClass())){
+      throw new IllegalArgumentException( "Can not merge " + t.getClass() + " "+ u.getClass());
+    }
+    return t.merge(u);
+  }
+
+  @SuppressWarnings("unchecked")
+  public static Crdt applyStatic(Crdt t, Crdt u){
+    if (t == null && u == null){
+      return null;
+    } else if (t == null){
+      return u;
+    } else if (u == null){
+      return t;
+    }
+    if (! u.getClass().equals(t.getClass())){
+      throw new IllegalArgumentException( "Can not merge " + t.getClass() + " "+ u.getClass());
+    }
+    return t.merge(u);
+  }
+}
diff --git a/src/main/java/org/apache/gossip/crdt/CrdtSet.java b/src/main/java/org/apache/gossip/crdt/CrdtSet.java
new file mode 100644
index 0000000..3a5fbca
--- /dev/null
+++ b/src/main/java/org/apache/gossip/crdt/CrdtSet.java
@@ -0,0 +1,26 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.gossip.crdt;
+
+import java.util.Set;
+
+public interface CrdtSet<ElementType, SetType extends Set<ElementType>, R extends CrdtSet<ElementType, SetType, R>>
+extends Crdt<SetType, R>, Set<ElementType> {
+
+}
+
diff --git a/src/main/java/org/apache/gossip/crdt/GrowOnlySet.java b/src/main/java/org/apache/gossip/crdt/GrowOnlySet.java
new file mode 100644
index 0000000..0b1771b
--- /dev/null
+++ b/src/main/java/org/apache/gossip/crdt/GrowOnlySet.java
@@ -0,0 +1,168 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.gossip.crdt;
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.LinkedHashSet;
+import java.util.Set;
+
+public class GrowOnlySet<ElementType> implements CrdtSet<ElementType, Set<ElementType>, GrowOnlySet<ElementType>>{
+
+  private final Set<ElementType> hidden = new LinkedHashSet<>();
+  
+  @SuppressWarnings("unused")
+  /*
+   * Used by SerDe
+   */
+  private GrowOnlySet(){
+    
+  }
+  
+  public GrowOnlySet(Set<ElementType> c){
+    hidden.addAll(c);
+  }
+  
+  public GrowOnlySet(Collection<ElementType> c){
+    hidden.addAll(c);
+  }
+  
+  public GrowOnlySet(GrowOnlySet<ElementType> first, GrowOnlySet<ElementType> second){
+    hidden.addAll(first.value());
+    hidden.addAll(second.value());
+  }
+  
+  @Override
+  public GrowOnlySet<ElementType> merge(GrowOnlySet<ElementType> other) {
+    return new GrowOnlySet<>(this, other);
+  }
+
+  @Override
+  public Set<ElementType> value() {
+    Set<ElementType> copy = new LinkedHashSet<>();
+    copy.addAll(hidden);
+    return Collections.unmodifiableSet(copy);
+  }
+  
+  @Override
+  public GrowOnlySet<ElementType> optimize() {
+    return new GrowOnlySet<>(hidden);
+  }
+
+  @Override
+  public int size() {
+    return hidden.size();
+  }
+
+  @Override
+  public boolean isEmpty() {
+    return hidden.isEmpty();
+  }
+
+  @Override
+  public boolean contains(Object o) {
+    return hidden.contains(o);
+  }
+
+  @Override
+  public Iterator<ElementType> iterator() {
+    Set<ElementType> copy = new HashSet<>();
+    copy.addAll(hidden);
+    return copy.iterator();
+  }
+
+  @Override
+  public Object[] toArray() {
+    return hidden.toArray();
+  }
+
+  @Override
+  public <T> T[] toArray(T[] a) {
+    return hidden.toArray(a);
+  }
+
+  @Override
+  public boolean add(ElementType e) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public boolean remove(Object o) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public boolean containsAll(Collection<?> c) {
+    return hidden.containsAll(c);
+  }
+
+  @Override
+  public boolean addAll(Collection<? extends ElementType> c) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public boolean retainAll(Collection<?> c) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public boolean removeAll(Collection<?> c) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public void clear() {
+    throw new UnsupportedOperationException();
+    
+  }
+
+  @Override
+  public String toString() {
+    return "GrowOnlySet [hidden=" + hidden + "]";
+  }
+
+  @Override
+  public int hashCode() {
+    final int prime = 31;
+    int result = 1;
+    result = prime * result + ((hidden == null) ? 0 : hidden.hashCode());
+    return result;
+  }
+
+  @Override
+  public boolean equals(Object obj) {
+    if (this == obj)
+      return true;
+    if (obj == null)
+      return false;
+    if (getClass() != obj.getClass())
+      return false;
+    @SuppressWarnings("rawtypes")
+    GrowOnlySet other = (GrowOnlySet) obj;
+    if (hidden == null) {
+      if (other.hidden != null)
+        return false;
+    } else if (!hidden.equals(other.hidden))
+      return false;
+    return true;
+  }
+
+}
diff --git a/src/main/java/org/apache/gossip/manager/GossipCore.java b/src/main/java/org/apache/gossip/manager/GossipCore.java
index 403acf4..de54597 100644
--- a/src/main/java/org/apache/gossip/manager/GossipCore.java
+++ b/src/main/java/org/apache/gossip/manager/GossipCore.java
@@ -23,6 +23,7 @@ import com.codahale.metrics.MetricRegistry;
 import org.apache.gossip.GossipMember;
 import org.apache.gossip.LocalGossipMember;
 import org.apache.gossip.RemoteGossipMember;
+import org.apache.gossip.crdt.Crdt;
 import org.apache.gossip.event.GossipState;
 import org.apache.gossip.model.*;
 import org.apache.gossip.udp.Trackable;
@@ -111,15 +112,27 @@ public class GossipCore implements GossipCoreConstants {
     } 
   }
 
-  public void addSharedData(SharedGossipDataMessage message){
-     SharedGossipDataMessage previous = sharedData.get(message.getKey());
-     if (previous == null){
-       sharedData.putIfAbsent(message.getKey(), message);
-     } else {
-       if (previous.getTimestamp() < message.getTimestamp()){
-         sharedData.replace(message.getKey(), previous, message);
-       }
-     }
+  @SuppressWarnings({ "unchecked", "rawtypes" })
+  public void addSharedData(SharedGossipDataMessage message) {
+    SharedGossipDataMessage previous = sharedData.get(message.getKey());
+    if (previous == null) {
+      sharedData.putIfAbsent(message.getKey(), message);
+    } else {
+      if (message.getPayload() instanceof Crdt){
+        SharedGossipDataMessage m = sharedData.get(message.getKey());
+        SharedGossipDataMessage merged = new SharedGossipDataMessage();
+        merged.setExpireAt(message.getExpireAt());
+        merged.setKey(m.getKey());
+        merged.setNodeId(message.getNodeId());
+        merged.setTimestamp(message.getTimestamp());
+        merged.setPayload( ((Crdt) message.getPayload()).merge((Crdt)m.getPayload()));
+        sharedData.put(m.getKey(), merged);
+      } else {
+        if (previous.getTimestamp() < message.getTimestamp()) {
+          sharedData.replace(message.getKey(), previous, message);
+        }
+      }
+    }
   }
 
   public void addPerNodeData(GossipDataMessage message){
@@ -345,4 +358,25 @@ public class GossipCore implements GossipCoreConstants {
           "Dead " + gossipManager.getDeadMembers()+ "\n" +
           "=======================");
   }
+
+  @SuppressWarnings("rawtypes")
+  public Crdt merge(SharedGossipDataMessage message) {
+    for (;;){
+      SharedGossipDataMessage ret = sharedData.putIfAbsent(message.getKey(), message);
+      if (ret == null){
+        return (Crdt) message.getPayload();
+      }
+      SharedGossipDataMessage copy = new SharedGossipDataMessage();
+      copy.setExpireAt(message.getExpireAt());
+      copy.setKey(message.getKey());
+      copy.setNodeId(message.getNodeId());
+      @SuppressWarnings("unchecked")
+      Crdt merged = ((Crdt) ret.getPayload()).merge((Crdt) message.getPayload());
+      message.setPayload(merged);
+      boolean replaced = sharedData.replace(message.getKey(), ret, copy);
+      if (replaced){
+        return merged;
+      }
+    }
+  }
 }
diff --git a/src/main/java/org/apache/gossip/manager/GossipManager.java b/src/main/java/org/apache/gossip/manager/GossipManager.java
index ab8e4ae..0140f00 100644
--- a/src/main/java/org/apache/gossip/manager/GossipManager.java
+++ b/src/main/java/org/apache/gossip/manager/GossipManager.java
@@ -22,6 +22,7 @@ import com.fasterxml.jackson.databind.ObjectMapper;
 import org.apache.gossip.GossipMember;
 import org.apache.gossip.GossipSettings;
 import org.apache.gossip.LocalGossipMember;
+import org.apache.gossip.crdt.Crdt;
 import org.apache.gossip.event.GossipListener;
 import org.apache.gossip.event.GossipState;
 import org.apache.gossip.manager.handlers.MessageInvoker;
@@ -291,6 +292,31 @@ public abstract class GossipManager {
     gossipCore.addSharedData(message);
   }
   
+
+  @SuppressWarnings("rawtypes")
+  public Crdt findCrdt(String key){
+    SharedGossipDataMessage l = gossipCore.getSharedData().get(key);
+    if (l == null){
+      return null;
+    }
+    if (l.getExpireAt() < clock.currentTimeMillis()){
+      return null;
+    } else {
+      return (Crdt) l.getPayload();
+    }
+  }
+  
+  @SuppressWarnings("rawtypes")
+  public Crdt merge(SharedGossipDataMessage message){
+    Objects.nonNull(message.getKey());
+    Objects.nonNull(message.getTimestamp());
+    Objects.nonNull(message.getPayload());
+    message.setNodeId(me.getId());
+    if (! (message.getPayload() instanceof Crdt)){
+      throw new IllegalArgumentException("Not a subclass of CRDT " + message.getPayload());
+    }
+    return gossipCore.merge(message);
+  }
   public GossipDataMessage findPerNodeGossipData(String nodeId, String key){
     ConcurrentHashMap<String, GossipDataMessage> j = gossipCore.getPerNodeData().get(nodeId);
     if (j == null){
diff --git a/src/test/java/org/apache/gossip/DataTest.java b/src/test/java/org/apache/gossip/DataTest.java
index b5fa705..f05636b 100644
--- a/src/test/java/org/apache/gossip/DataTest.java
+++ b/src/test/java/org/apache/gossip/DataTest.java
@@ -22,12 +22,13 @@ import java.net.URI;
 import java.net.URISyntaxException;
 import java.net.UnknownHostException;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.HashMap;
 import java.util.List;
 import java.util.UUID;
-import java.util.concurrent.Callable;
 import java.util.concurrent.TimeUnit;
 
+import org.apache.gossip.crdt.GrowOnlySet;
 import org.apache.gossip.model.GossipDataMessage;
 import org.apache.gossip.model.SharedGossipDataMessage;
 import org.junit.Test;
@@ -58,43 +59,60 @@ public class DataTest {
       clients.add(gossipService);
       gossipService.start();
     }
-    TUnit.assertThat(new Callable<Integer> (){
-      public Integer call() throws Exception {
-        int total = 0;
-        for (int i = 0; i < clusterMembers; ++i) {
-          total += clients.get(i).getGossipManager().getLiveMembers().size();
-        }
-        return total;
-      }}).afterWaitingAtMost(20, TimeUnit.SECONDS).isEqualTo(2);
+    TUnit.assertThat(() -> {
+      int total = 0;
+      for (int i = 0; i < clusterMembers; ++i) {
+        total += clients.get(i).getGossipManager().getLiveMembers().size();
+      }
+      return total;
+    }).afterWaitingAtMost(20, TimeUnit.SECONDS).isEqualTo(2);
     clients.get(0).gossipPerNodeData(msg());
     clients.get(0).gossipSharedData(sharedMsg());
 
-    TUnit.assertThat(new Callable<Object>() {
-      public Object call() throws Exception {
-        GossipDataMessage x = clients.get(1).findPerNodeData(1 + "", "a");
-        if (x == null)
-          return "";
-        else
-          return x.getPayload();
-      }
+    TUnit.assertThat(()-> {      
+      GossipDataMessage x = clients.get(1).findPerNodeData(1 + "", "a");
+      if (x == null)
+        return "";
+      else
+        return x.getPayload();
     }).afterWaitingAtMost(20, TimeUnit.SECONDS).isEqualTo("b");
     
-    TUnit.assertThat(new Callable<Object>() {
-      public Object call() throws Exception {
-        SharedGossipDataMessage x = clients.get(1).findSharedData("a");
-        if (x == null)
-          return "";
-        else
-          return x.getPayload();
-      }
+    TUnit.assertThat(() ->  {    
+      SharedGossipDataMessage x = clients.get(1).findSharedData("a");
+      if (x == null)
+        return "";
+      else
+        return x.getPayload();
     }).afterWaitingAtMost(20, TimeUnit.SECONDS).isEqualTo("c");
     
+    givenDifferentDatumsInSet(clients);
+    assertThatListIsMerged(clients);
     
     for (int i = 0; i < clusterMembers; ++i) {
       clients.get(i).shutdown();
     }
   }
   
+  private void givenDifferentDatumsInSet(final List<GossipService> clients){
+    clients.get(0).getGossipManager().merge(CrdtMessage("1"));
+    clients.get(1).getGossipManager().merge(CrdtMessage("2"));
+  }
+  
+  private void assertThatListIsMerged(final List<GossipService> clients){
+    TUnit.assertThat(() ->  {
+      return clients.get(0).getGossipManager().findCrdt("cr");
+    }).afterWaitingAtMost(10, TimeUnit.SECONDS).equals(new GrowOnlySet<String>(Arrays.asList("1","2")));
+  }
+  
+  private SharedGossipDataMessage CrdtMessage(String item){
+    SharedGossipDataMessage d = new SharedGossipDataMessage();
+    d.setKey("cr");
+    d.setPayload(new GrowOnlySet<String>( Arrays.asList(item)));
+    d.setExpireAt(Long.MAX_VALUE);
+    d.setTimestamp(System.currentTimeMillis());
+    return d;  
+  }
+  
   private GossipDataMessage msg(){
     GossipDataMessage g = new GossipDataMessage();
     g.setExpireAt(Long.MAX_VALUE);
diff --git a/src/test/java/org/apache/gossip/crdt/GrowOnlySetTest.java b/src/test/java/org/apache/gossip/crdt/GrowOnlySetTest.java
new file mode 100644
index 0000000..d4f12b6
--- /dev/null
+++ b/src/test/java/org/apache/gossip/crdt/GrowOnlySetTest.java
@@ -0,0 +1,38 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.gossip.crdt;
+
+import java.util.Arrays;
+import java.util.concurrent.ConcurrentHashMap;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class GrowOnlySetTest {
+
+  @SuppressWarnings("rawtypes")
+  @Test
+  public void mergeTest(){
+    ConcurrentHashMap<String, Crdt> a = new ConcurrentHashMap<>();
+    GrowOnlySet<String> gset = new GrowOnlySet<>(Arrays.asList("a", "b"));
+    Assert.assertEquals(gset, a.merge("a", gset, new CrdtBiFunctionMerge()));
+    GrowOnlySet<String> over = new GrowOnlySet<>(Arrays.asList("b", "d"));
+    Assert.assertEquals(new GrowOnlySet<>(Arrays.asList("a", "b", "d")), 
+            a.merge("a", over, CrdtBiFunctionMerge::applyStatic));
+  }
+}