diff --git a/arangod/Pregel/Aggregator.h b/arangod/Pregel/Aggregator.h
index 2a8045811f..f425809db3 100644
--- a/arangod/Pregel/Aggregator.h
+++ b/arangod/Pregel/Aggregator.h
@@ -43,14 +43,18 @@ class IAggregator {
   IAggregator() {}
   virtual ~IAggregator() {}
 
-  /// @brief Value from superstep S-1 supplied by the conductor
+  /// @brief Used when updating aggregator value locally
   virtual void aggregate(void const* valuePtr) = 0;
+  /// @brief Used when updating aggregator value from remote
+  virtual void parseAggregate(VPackSlice const& slice) = 0;
+  
+  virtual void const* getAggregatedValue() const = 0;
+  /// @brief Value from superstep S-1 supplied by the conductor
+  virtual void setAggregatedValue(VPackSlice const& slice) = 0;
+  
+  virtual void serialize(std::string const& key, VPackBuilder &builder) const = 0;
 
-  virtual void const* getValue() const = 0;
-  virtual VPackValue vpackValue() const = 0;
-  virtual void parse(VPackSlice slice) = 0;
-
-  virtual void reset(bool force) = 0;
+  virtual void reset() = 0;
   virtual bool isConverging() const = 0;
 };
 
@@ -58,26 +62,34 @@ template <typename T>
 struct NumberAggregator : public IAggregator {
   static_assert(std::is_arithmetic<T>::value, "Type must be numeric");
 
-  NumberAggregator(T init, bool perm = false, bool conv = false)
-      : _value(init), _initial(init), _permanent(perm), _converging(conv) {}
-
-  void const* getValue() const override { return &_value; };
-  VPackValue vpackValue() const override { return VPackValue(_value); };
-  void parse(VPackSlice slice) override {
+  NumberAggregator(T neutral, bool perm = false, bool conv = false)
+      : _value(neutral), _neutral(neutral), _permanent(perm), _converging(conv) {}
+  
+  void parseAggregate(VPackSlice const& slice) override {
     T f = slice.getNumber<T>();
     aggregate((void const*)(&f));
+  };
+  
+  void const* getAggregatedValue() const override { return &_value; };
+  
+  void setAggregatedValue(VPackSlice const& slice) override {
+    _value = slice.getNumber<T>();
   }
+  
+  void serialize(std::string const& key, VPackBuilder &builder) const override {
+    builder.add(key, VPackValue(_value));
+  };
 
-  void reset(bool force) override {
-    if (!_permanent || force) {
-      _value = _initial;
+  void reset() override {
+    if (!_permanent) {
+      _value = _neutral;
     }
   }
 
   bool isConverging() const override { return _converging; }
 
  protected:
-  T _value, _initial;
+  T _value, _neutral;
   bool _permanent, _converging;
 };
 
@@ -109,36 +121,46 @@ struct SumAggregator : public NumberAggregator<T> {
   void aggregate(void const* valuePtr) override {
     this->_value += *((T*)valuePtr);
   };
-  void parse(VPackSlice slice) override {
+  void parseAggregate(VPackSlice const& slice) override {
     this->_value += slice.getNumber<T>();
   }
 };
 
+/// Aggregator that stores a value that is overwritten once another value is aggregated.
+/// This aggregator is useful for one-to-many communication from master.compute() or from a special vertex.
+/// In case multiple vertices write to this aggregator, its behavior is non-deterministic.
 template <typename T>
-struct ValueAggregator : public NumberAggregator<T> {
-  ValueAggregator(T val, bool perm = false)
+struct OverwriteAggregator : public NumberAggregator<T> {
+  OverwriteAggregator(T val, bool perm = false)
       : NumberAggregator<T>(val, perm, true) {}
 
   void aggregate(void const* valuePtr) override {
     this->_value = *((T*)valuePtr);
   };
-  void parse(VPackSlice slice) override { this->_value = slice.getNumber<T>(); }
+  void parseAggregate(VPackSlice const& slice) override { this->_value = slice.getNumber<T>(); }
 };
 
 /// always initializes to true.
 struct BoolOrAggregator : public IAggregator {
   BoolOrAggregator(bool perm = false) : _permanent(perm) {}
-
-  void const* getValue() const override { return &_value; };
-  VPackValue vpackValue() const override { return VPackValue(_value); };
-
+  
   void aggregate(void const* valuePtr) override {
     _value = _value || *((bool*)valuePtr);
   };
-  void parse(VPackSlice slice) override { _value = _value || slice.getBool(); }
+  
+  void parseAggregate(VPackSlice const& slice) override { _value = _value || slice.getBool(); }
+  
+  void const* getAggregatedValue() const override { return &_value; };
+  void setAggregatedValue(VPackSlice const& slice) override {
+    _value = slice.getBool();
+  }
+  
+  void serialize(std::string const& key, VPackBuilder &builder) const override {
+    builder.add(key, VPackValue(_value));
+  };
 
-  void reset(bool force) override {
-    if (!_permanent || force) {
+  void reset() override {
+    if (!_permanent) {
       _value = false;
     }
   }
diff --git a/arangod/Pregel/AggregatorHandler.cpp b/arangod/Pregel/AggregatorHandler.cpp
index ca8349be30..ff413842a9 100644
--- a/arangod/Pregel/AggregatorHandler.cpp
+++ b/arangod/Pregel/AggregatorHandler.cpp
@@ -37,7 +37,7 @@ AggregatorHandler::~AggregatorHandler() {
   _values.clear();
 }
 
-IAggregator* AggregatorHandler::_get(AggregatorID const& name) {
+IAggregator* AggregatorHandler::getAggregator(AggregatorID const& name) {
   {
     READ_LOCKER(guard, _lock);
     auto it = _values.find(name);
@@ -59,46 +59,57 @@ IAggregator* AggregatorHandler::_get(AggregatorID const& name) {
 
 void AggregatorHandler::aggregate(AggregatorID const& name,
                                   const void* valuePtr) {
-  IAggregator* agg = _get(name);
+  IAggregator* agg = getAggregator(name);
   if (agg) {
     agg->aggregate(valuePtr);
   }
 }
 
-const void* AggregatorHandler::getAggregatedValue(AggregatorID const& name) {
-  IAggregator* agg = _get(name);
-  return agg != nullptr ? agg->getValue() : nullptr;
-}
-
-void AggregatorHandler::resetValues(bool force) {
-  for (auto& it : _values) {
-    it.second->reset(force);
-  }
-}
-
 void AggregatorHandler::aggregateValues(AggregatorHandler const& workerValues) {
   for (auto const& pair : workerValues._values) {
     AggregatorID const& name = pair.first;
-    IAggregator* agg = _get(name);
+    IAggregator* agg = getAggregator(name);
     if (agg) {
-      agg->aggregate(pair.second->getValue());
+      agg->aggregate(pair.second->getAggregatedValue());
     }
   }
 }
 
-bool AggregatorHandler::parseValues(VPackSlice data) {
-  VPackSlice values = data.get(Utils::aggregatorValuesKey);
-  if (values.isObject() == false) {
-    return false;
-  }
-  for (auto const& keyValue : VPackObjectIterator(values)) {
-    AggregatorID name = keyValue.key.copyString();
-    IAggregator* agg = _get(name);
-    if (agg) {
-      agg->parse(keyValue.value);
+void AggregatorHandler::aggregateValues(VPackSlice  const& workerValues) {
+  VPackSlice values = workerValues.get(Utils::aggregatorValuesKey);
+  if (values.isObject()) {
+    for (auto const& keyValue : VPackObjectIterator(values)) {
+      AggregatorID name = keyValue.key.copyString();
+      IAggregator* agg = getAggregator(name);
+      if (agg) {
+        agg->parseAggregate(keyValue.value);
+      }
     }
   }
-  return true;
+}
+
+void AggregatorHandler::setAggregatedValues(VPackSlice const& workerValues) {
+  VPackSlice values = workerValues.get(Utils::aggregatorValuesKey);
+  if (values.isObject()) {
+    for (auto const& keyValue : VPackObjectIterator(values)) {
+      AggregatorID name = keyValue.key.copyString();
+      IAggregator* agg = getAggregator(name);
+      if (agg) {
+        agg->setAggregatedValue(keyValue.value);
+      }
+    }
+  }
+}
+
+const void* AggregatorHandler::getAggregatedValue(AggregatorID const& name) {
+  IAggregator* agg = getAggregator(name);
+  return agg != nullptr ? agg->getAggregatedValue() : nullptr;
+}
+
+void AggregatorHandler::resetValues() {
+  for (auto& it : _values) {
+    it.second->reset();
+  }
 }
 
 bool AggregatorHandler::serializeValues(VPackBuilder& b,
@@ -107,9 +118,10 @@ bool AggregatorHandler::serializeValues(VPackBuilder& b,
   bool hasValues = false;
   b.add(Utils::aggregatorValuesKey, VPackValue(VPackValueType::Object));
   for (auto const& pair : _values) {
+    AggregatorID const& name = pair.first;
     IAggregator* agg = pair.second;
     if (!onlyConverging || agg->isConverging()) {
-      b.add(pair.first, agg->vpackValue());
+      agg->serialize(name, b);
       hasValues = true;
     }
   }
diff --git a/arangod/Pregel/AggregatorHandler.h b/arangod/Pregel/AggregatorHandler.h
index 77e213a653..b0fe895d9f 100644
--- a/arangod/Pregel/AggregatorHandler.h
+++ b/arangod/Pregel/AggregatorHandler.h
@@ -42,28 +42,28 @@ class AggregatorHandler {
   std::map<std::string, IAggregator*> _values;
   mutable basics::ReadWriteLock _lock;
 
-  IAggregator* _get(std::string const& name);
 
  public:
   AggregatorHandler(IAlgorithm const* c) : _algorithm(c) {}
   ~AggregatorHandler();
-
-  void registerAggregator(std::string const& name, IAggregator* aggregator);
+  
+  IAggregator* getAggregator(std::string const& name);
 
   /// aggregate this value
   void aggregate(std::string const& name, const void* valuePtr);
+  /// aggregates all values from this aggregator
+  void aggregateValues(AggregatorHandler const& workerValues);
+  /// aggregates all values from this aggregator
+  void aggregateValues(VPackSlice const& workerValues);
+  
+  /// return true if there are values in this Slice
+  void setAggregatedValues(VPackSlice const& workerValues);
 
   /// get the pointer to an aggregator value
   const void* getAggregatedValue(std::string const& name);
-
+  
   /// calls reset on every aggregator
-  void resetValues(bool force = false);
-
-  /// aggregates all values from this aggregator
-  void aggregateValues(AggregatorHandler const& workerValues);
-
-  /// return true if there are values in this Slice
-  bool parseValues(VPackSlice workerValues);
+  void resetValues();
 
   /// return true if there values in this aggregator which were serialized
   bool serializeValues(VPackBuilder& b, bool onlyConverging = false) const;
diff --git a/arangod/Pregel/Algorithm.h b/arangod/Pregel/Algorithm.h
index 3f2c980394..d262d4b5fb 100644
--- a/arangod/Pregel/Algorithm.h
+++ b/arangod/Pregel/Algorithm.h
@@ -86,7 +86,7 @@ template <typename V, typename E, typename M>
 struct Algorithm : IAlgorithm {
  public:
   virtual WorkerContext* workerContext(VPackSlice userParams) const {
-    return new WorkerContext(userParams);
+    return new WorkerContext();
   }
   virtual GraphFormat<V, E>* inputFormat() const = 0;
   virtual MessageFormat<M>* messageFormat() const = 0;
diff --git a/arangod/Pregel/Algos/AsyncSCC.cpp b/arangod/Pregel/Algos/AsyncSCC.cpp
index c05daf352d..103594919e 100644
--- a/arangod/Pregel/Algos/AsyncSCC.cpp
+++ b/arangod/Pregel/Algos/AsyncSCC.cpp
@@ -254,7 +254,7 @@ MasterContext* AsyncSCC::masterContext(VPackSlice userParams) const {
 
 IAggregator* AsyncSCC::aggregator(std::string const& name) const {
   if (name == kPhase) {  // permanent value
-    return new ValueAggregator<uint32_t>(SCCPhase::TRANSPOSE, true);
+    return new OverwriteAggregator<uint32_t>(SCCPhase::TRANSPOSE, true);
   } else if (name == kFoundNewMax) {
     return new BoolOrAggregator(false);  // non perm
   } else if (name == kConverged) {
diff --git a/arangod/Pregel/Algos/DMID.cpp b/arangod/Pregel/Algos/DMID.cpp
index f143af33a3..f998adab58 100644
--- a/arangod/Pregel/Algos/DMID.cpp
+++ b/arangod/Pregel/Algos/DMID.cpp
@@ -29,6 +29,7 @@
 #include "Pregel/IncomingCache.h"
 #include "Pregel/MasterContext.h"
 #include "Pregel/VertexComputation.h"
+#include "Pregel/Algos/DMID/VertexSumAggregator.h"
 
 using namespace arangodb;
 using namespace arangodb::pregel;
@@ -115,7 +116,7 @@ struct DMIDComputation
       superstep4(vertex, messages);
     }
     
-    if (getSuperstep() == rwFinished +1) {
+    if (globalSuperstep() == rwFinished +1) {
       /**
        * Superstep 0 and RW_ITERATIONBOUND + 5 are identical. Therefore
        * call superstep0
@@ -123,38 +124,36 @@ struct DMIDComputation
       superstep0(vertex, messages);
     }
     
-    if (getSuperstep() == rwFinished+2) {
+    if (globalSuperstep() == rwFinished+2) {
       superstep6(vertex, messages);
     }
     
-    if (getSuperstep() == rwFinished + 3) {
+    if (globalSuperstep() == rwFinished + 3) {
       superstep7(vertex, messages);
       
     }
     
-    LongWritable iterationCounter = getAggregatedValue(ITERATION_AGG);
-    double it = iterationCounter.get();
+    int64_t iterationCounter = getAggregatedValue<int64_t>(ITERATION_AGG);
     
     if (getSuperstep() >= rwFinished +4
-        && (it % 3 == 1 )) {
-      superstep8(vertex, messages);
+        && (iterationCounter % 3 == 1 )) {
+      superstep8(messages);
     }
     if (getSuperstep() >= rwFinished +5
-        && (it % 3 == 2 )) {
-      superstep9(vertex, messages);
+        && (iterationCounter % 3 == 2 )) {
+      superstep9(messages);
     }
     if (getSuperstep() >= rwFinished +6
-        && (it % 3 == 0 )) {
-      superstep10(vertex, messages);
+        && (iterationCounter % 3 == 0 )) {
+      superstep10(messages);
     }
   }
       
-      
+     /**
       * SUPERSTEP 0: send a message along all outgoing edges. Message contains
       * own VertexID and the edge weight.
       */
-      private void superstep0(
-                              Vertex<LongWritable, DMIDVertexValue, DoubleWritable> vertex,
+      void superstep0(Vertex<LongWritable, DMIDVertexValue, DoubleWritable> vertex,
                               Iterable<LongDoubleMessage> messages) {
         
         long vertexID = vertex.getId().get();
@@ -171,7 +170,7 @@ struct DMIDComputation
        * the form (ID,weightedInDegree) along all incoming edges (send every node
        * a reply)
        */
-      private void superstep1(
+      void superstep1(
                               Vertex<LongWritable, DMIDVertexValue, DoubleWritable> vertex,
                               Iterable<LongDoubleMessage> messages) {
         
@@ -208,7 +207,7 @@ struct DMIDComputation
        * Save the column as a part of the vertexValue. Aggregate DA with value 1/N
        * to initialize the Random Walk.
        */
-      private void superstep2(
+      void superstep2(
                               Vertex<LongWritable, DMIDVertexValue, DoubleWritable> vertex,
                               Iterable<LongDoubleMessage> messages) {
         
@@ -264,7 +263,7 @@ struct DMIDComputation
        * SUPERSTEP 3 - RW_ITERATIONBOUND+3: Calculate entry DA^(t+1)_ownID using
        * DA^t and disCol. Save entry in the DA aggregator.
        */
-      private void superstepRW(
+      void superstepRW(
                                Vertex<LongWritable, DMIDVertexValue, DoubleWritable> vertex,
                                Iterable<LongDoubleMessage> messages) {
         
@@ -614,10 +613,6 @@ struct DMIDValueMasterContext : public MasterContext {
   DMIDValueMasterContext() {}  // TODO use _threashold
   void preGlobalSuperstep() override {
     
-  };
-  
-  @Override
-  public void compute() {
     /**
      * setAggregatorValue sets the value for the aggregator after master
      * compute, before starting vertex compute of the same superstep. Does
@@ -625,31 +620,26 @@ struct DMIDValueMasterContext : public MasterContext {
      */
     
     
-    LongWritable iterCount = getAggregatedValue(DMIDComputation.ITERATION_AGG);
-    
-    boolean hasCascadingStarted = false;
-    LongWritable newIterCount = new LongWritable((iterCount.get() + 1));
-    
-    
-    if (iterCount.get() != 0) {
+    int64_t* iterCount = getAggregatedValue<int64_t>(ITERATION_AGG);
+    int64_t newIterCount = *iterCount + 1;
+    bool hasCascadingStarted = false;
+    if (*iterCount != 0) {
       /** Cascading behavior started increment the iteration count */
-      setAggregatedValue(DMIDComputation.ITERATION_AGG, newIterCount);
+      aggregateValue<int64_t>(ITERATION_AGG, newIterCount);
       hasCascadingStarted = true;
     }
     
-    if (getSuperstep() ==  DMIDComputation.RW_ITERATIONBOUND+ 8) {
-      setAggregatedValue(DMIDComputation.NEW_MEMBER_AGG,
-                         new BooleanWritable(false));
-      setAggregatedValue(DMIDComputation.NOT_ALL_ASSIGNED_AGG,
-                         new BooleanWritable(true));
-      setAggregatedValue(DMIDComputation.ITERATION_AGG, new LongWritable(1));
+    if (getSuperstep() ==  RW_ITERATIONBOUND+ 8) {
+      aggregateValue<bool>(NEW_MEMBER_AGG, false);
+      aggregateValue<bool>(NOT_ALL_ASSIGNED_AGG, true);
+      aggregateValue<int64_t>(ITERATION_AGG, 1);
       hasCascadingStarted = true;
       initializeGL();
     }
-    if (hasCascadingStarted && (newIterCount.get() % 3 == 1)) {
+    if (hasCascadingStarted && (newIterCount % 3 == 1)) {
       /** first step of one iteration */
-      LongWritable restartCountWritable = getAggregatedValue(RESTART_COUNTER_AGG);
-      Long restartCount=restartCountWritable.get();
+      int64_t* restartCountWritable = getAggregatedValue<int64_t>(RESTART_COUNTER_AGG);
+      Long restartCount = restartCountWritable.get();
       BooleanWritable newMember = getAggregatedValue(DMIDComputation.NEW_MEMBER_AGG);
       BooleanWritable notAllAssigned = getAggregatedValue(DMIDComputation.NOT_ALL_ASSIGNED_AGG);
       
@@ -672,17 +662,15 @@ struct DMIDValueMasterContext : public MasterContext {
       
     }
     
-    if (hasCascadingStarted && (iterCount.get() % 3 == 2)) {
+    if (hasCascadingStarted && (*iterCount % 3 == 2)) {
       /** Second step of one iteration */
       /**
        * Set newMember aggregator and notAllAssigned aggregator back to
        * initial value
        */
       
-      setAggregatedValue(DMIDComputation.NEW_MEMBER_AGG,
-                         new BooleanWritable(false));
-      setAggregatedValue(DMIDComputation.NOT_ALL_ASSIGNED_AGG,
-                         new BooleanWritable(false));
+      setAggregatedValue<bool>(NEW_MEMBER_AGG, false);
+      setAggregatedValue<bool>(NOT_ALL_ASSIGNED_AGG, false);
     }
     
     if (LOG_AGGS) {
@@ -719,10 +707,10 @@ struct DMIDValueMasterContext : public MasterContext {
    * Initilizes the global leader aggregator with 1 for every vertex with a
    * higher number of followers than the average.
    */
-  private void initializeGL() {
+  void initializeGL() {
     DoubleSparseVector initGL = new DoubleSparseVector(
                                                        (int) getTotalNumVertices());
-    DoubleSparseVector vecFD = getAggregatedValue(DMIDComputation.FD_AGG);
+    VertexSumAggregator::VertexMap const* vecFD = getAggregatedValue<VertexSumAggregator::VertexMap>(FD_AGG);
     
     double averageFD = 0.0;
     int numLocalLeader = 0;
@@ -740,7 +728,7 @@ struct DMIDValueMasterContext : public MasterContext {
     if (LOG_AGGS) {
       System.out.print("Global Leader:");
     }
-    for (int i = 0; i < getTotalNumVertices(); ++i) {
+    for (int i = 0; i < vertexCount(); ++i) {
       if (vecFD.get(i) > averageFD) {
         initGL.set(i, 1.0);
         if (LOG_AGGS) {
@@ -768,43 +756,24 @@ MasterContext* SCC::masterContext(VPackSlice userParams) const {
 
 IAggregator* SCC::aggregator(std::string const& name) const {
   if (name == DA_AGG) {  // permanent value
-    return new ValueAggregator<uint32_t>(SCCPhase::TRANSPOSE, true);
-  } else if (name == kFoundNewMax) {
-    return new BoolOrAggregator(false);  // non perm
-  } else if (name == kConverged) {
-    return new BoolOrAggregator(false);  // non perm
+    return new VertexSumAggregator(false);// non perm
+  } else if (name == LS_AGG) {
+    return new VertexSumAggregator(true);// perm
+  } else if (name == FD_AGG) {
+    return new VertexSumAggregator(true);// perm
+  } else if (name == GL_AGG) {
+    return new VertexSumAggregator(true);// perm
+  } else if (name == NEW_MEMBER_AGG) {
+    return new BooleanOrAggregator(false); // non perm
+  } else if (name == NOT_ALL_ASSIGNED_AGG) {
+    return new BooleanOrAggregator(false); // non perm
+  } else if (name == ITERATION_AGG) {
+    return new MaxAggregator<int64_t>(0, true); // perm
+  } else if (name == PROFITABILITY_AGG) {
+    return new MaxAggregator<double>(0.5, true); // perm
+  } else if (name == RESTART_COUNTER_AGG) {
+    return new MaxAggregator<int64_t>(1, true); // perm
   }
   
-  registerAggregator(DMIDComputation.DA_AGG,
-                     DoubleDenseVectorSumAggregator.class);
-		registerPersistentAggregator(DMIDComputation.LS_AGG,
-                                 DoubleDenseVectorSumAggregator.class);
-		registerPersistentAggregator(DMIDComputation.FD_AGG,
-                                 DoubleSparseVectorSumAggregator.class);
-		registerPersistentAggregator(DMIDComputation.GL_AGG,
-                                 DoubleSparseVectorSumAggregator.class);
-  
-		registerAggregator(DMIDComputation.NEW_MEMBER_AGG,
-                       BooleanOrAggregator.class);
-		registerAggregator(DMIDComputation.NOT_ALL_ASSIGNED_AGG,
-                       BooleanOrAggregator.class);
-  
-		registerPersistentAggregator(DMIDComputation.ITERATION_AGG,
-                                 LongMaxAggregator.class);
-  
-		registerPersistentAggregator(DMIDComputation.PROFITABILITY_AGG,
-                                 DoubleMaxAggregator.class);
-		registerPersistentAggregator(RESTART_COUNTER_AGG,
-                                 LongMaxAggregator.class);
-		//registerAggregator(DMIDComputation.RW_INFINITYNORM_AGG,
-    //	DoubleMaxAggregator.class);
-    //registerAggregator(DMIDComputation.RW_FINISHED_AGG,
-				//LongMaxAggregator.class);
-  
-		setAggregatedValue(DMIDComputation.PROFITABILITY_AGG,
-                       new DoubleWritable(0.5));
-		setAggregatedValue(RESTART_COUNTER_AGG, new LongWritable(1));
-  setAggregatedValue(DMIDComputation.ITERATION_AGG, new LongWritable(0));
-  
   return nullptr;
 }
diff --git a/arangod/Pregel/Algos/DMID/VertexSumAggregator.h b/arangod/Pregel/Algos/DMID/VertexSumAggregator.h
new file mode 100644
index 0000000000..9214656b6d
--- /dev/null
+++ b/arangod/Pregel/Algos/DMID/VertexSumAggregator.h
@@ -0,0 +1,132 @@
+////////////////////////////////////////////////////////////////////////////////
+/// DISCLAIMER
+///
+/// Copyright 2016 ArangoDB GmbH, Cologne, Germany
+///
+/// Licensed 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.
+///
+/// Copyright holder is ArangoDB GmbH, Cologne, Germany
+///
+/// @author Simon Grätzer
+////////////////////////////////////////////////////////////////////////////////
+
+#include <vector>
+#include <velocypack/Builder.h>
+#include <velocypack/Iterators.h>
+#include <velocypack/Slice.h>
+#include <velocypack/velocypack-aliases.h>
+#include "Pregel/Graph.h"
+
+#ifndef ARANGODB_PREGEL_AGG_DENSE_VECTOR_H
+#define ARANGODB_PREGEL_AGG_DENSE_VECTOR_H 1
+
+namespace arangodb {
+namespace pregel {
+
+struct VertexSumAggregator : public IAggregator {
+  static_assert(std::is_arithmetic<T>::value, "Type must be numeric");
+  typedef std::map<prgl_shard_t, std::unordered_map<std::string, double>> VertexMap;
+
+  VertexSumAggregator(bool perm = false)
+  : _empty(empty), _permanent(perm) {}
+  
+  // if you use this from a vertex I will end you
+  void aggregate(void const* valuePtr) {
+    VertexMap const* map = (VertexMap)valuePtr;
+    for (auto pair1 const& : map) {
+      for (auto pair2 const& : it.second) {
+        _entries[pair1.first][pair2.first] += pair2.second;
+      }
+    }
+  };
+  
+  void parseAggregate(VPackSlice slice) override {
+    for (auto const& pair: VPackObjectIterator(slice)) {
+      prgl_shard_t shard = it.key.getUInt();
+      std::string key;
+      VPackLength i = 0;
+      for (VPackSlice const& val : VPackArrayIterator(pair.value)) {
+        if (i % 2 == 0) {
+          key = val.copyString();
+        } else {
+          entries[shard][key] += val.getNumber<double>();
+        }
+        i++;
+      }
+    }
+  }
+  
+  void const* getAggregatedValue() const override { return &_entries; };
+  
+  void setAggregatedValue(VPackSlice slice) override {
+    for (auto const& pair: VPackObjectIterator(slice)) {
+      prgl_shard_t shard = it.key.getUInt();
+      std::string key;
+      VPackLength i = 0;
+      for (VPackSlice const& val : VPackArrayIterator(pair.value)) {
+        if (i % 2 == 0) {
+          key = val.copyString();
+        } else {
+          entries[shard][key] = val.getNumber<double>();
+        }
+        i++;
+      }
+    }
+  }
+  
+  void serialize(std::string const& key, VPackBuilder &builder) const override {
+    builder.add(key, VPackValueType::Array);
+    for (T const& e : _entries) {
+      builder.add(VPackValue(e));
+    }
+    builder.close();
+  };
+
+  void reset() override {
+    if (!_permanent) {
+      _entries.clear();
+    }
+  }
+  
+  double getAggregatedValue(prgl_shard_t shard, std::string const& key) {
+    auto const& it1 = _entries.find(shard);
+    if (it1 != _entries.end()) {
+      auto const& it2 = it1.second.find(key);
+      if (it2 != it1.second.end()) {
+        return it2.second;
+      }
+    }
+    return _empty;
+  }
+  
+  void setValue(prgl_shard_t shard, std::string const& key, double val) {
+    _entries[shard][key] = val;
+  }
+
+  void aggregate(prgl_shard_t shard, std::string const& key, double val) {
+    _entries[shard][key] += val;
+  }
+  
+  void setEmptyValue(double empty) {
+    _empty = empty;
+  }
+  
+  bool isConverging() const override { return false; }
+
+ protected:
+  VertexMap _entries;
+  double _empty = 0;
+  bool _permanent;
+};
+}
+#endif
diff --git a/arangod/Pregel/Algos/LineRank.cpp b/arangod/Pregel/Algos/LineRank.cpp
index 571354f96e..410190d78e 100644
--- a/arangod/Pregel/Algos/LineRank.cpp
+++ b/arangod/Pregel/Algos/LineRank.cpp
@@ -27,6 +27,7 @@
 #include "Pregel/MasterContext.h"
 #include "Pregel/Utils.h"
 #include "Pregel/VertexComputation.h"
+#include "Pregel/WorkerContext.h"
 
 #include "Cluster/ClusterInfo.h"
 #include "Utils/OperationCursor.h"
@@ -49,16 +50,29 @@ LineRank::LineRank(arangodb::velocypack::Slice params)
   //_threshold = t.isNumber() ? t.getNumber<float>() : 0.000002f;
 }
 
+struct LRWorkerContext : WorkerContext {
+  float startAtNodeProb = 0;
+  
+  void preApplication() override {
+    startAtNodeProb = 1.0f / edgeCount();
+  };
+};
+
+WorkerContext* LineRank::workerContext(VPackSlice params) const {
+  return new LRWorkerContext();
+}
+
 // github.com/JananiC/NetworkCentralities/blob/master/src/main/java/linerank/LineRank.java
 struct LRComputation : public VertexComputation<float, float, float> {
   LRComputation() {}
   void compute(MessageIterator<float> const& messages) override {
-    float startAtNodeProb = 1.0f / context()->edgeCount();
+    LRWorkerContext *ctx = (LRWorkerContext *)context();
+    
     float* vertexValue = mutableVertexData();
     RangeIterator<Edge<float>> edges = getEdges();
 
     if (*vertexValue < 0.0f) {
-      *vertexValue = startAtNodeProb;
+      *vertexValue = ctx->startAtNodeProb;
       aggregate(kMoreIterations, true);
     } else {
       float newScore = 0.0f;
@@ -76,7 +90,7 @@ struct LRComputation : public VertexComputation<float, float, float> {
         } else {
           newScore /= edges.size();
           newScore =
-              startAtNodeProb * RESTART_PROB + newScore * (1.0 - RESTART_PROB);
+              ctx->startAtNodeProb * RESTART_PROB + newScore * (1.0 - RESTART_PROB);
         }
 
         float diff = fabsf(newScore - *vertexValue);
@@ -91,6 +105,8 @@ struct LRComputation : public VertexComputation<float, float, float> {
   }
 };
 
+
+
 VertexComputation<float, float, float>* LineRank::createComputation(
     WorkerConfig const* config) const {
   return new LRComputation();
@@ -98,7 +114,7 @@ VertexComputation<float, float, float>* LineRank::createComputation(
 
 IAggregator* LineRank::aggregator(std::string const& name) const {
   if (name == kMoreIterations) {
-    return new ValueAggregator<bool>(false, false);  // non perm
+    return new OverwriteAggregator<bool>(false, false);  // non perm
   }
   return nullptr;
 }
diff --git a/arangod/Pregel/Algos/LineRank.h b/arangod/Pregel/Algos/LineRank.h
index e8eee4a1d6..18a09c1e43 100644
--- a/arangod/Pregel/Algos/LineRank.h
+++ b/arangod/Pregel/Algos/LineRank.h
@@ -51,9 +51,12 @@ struct LineRank : public SimpleAlgorithm<float, float, float> {
   MessageCombiner<float>* messageCombiner() const override {
     return new SumCombiner<float>();
   }
+  
+  WorkerContext* workerContext(VPackSlice params) const override;
 
   VertexComputation<float, float, float>* createComputation(
       WorkerConfig const*) const override;
+  
   IAggregator* aggregator(std::string const& name) const override;
 };
 }
diff --git a/arangod/Pregel/Algos/PageRank.h b/arangod/Pregel/Algos/PageRank.h
index b6787ca680..f07a2d939b 100644
--- a/arangod/Pregel/Algos/PageRank.h
+++ b/arangod/Pregel/Algos/PageRank.h
@@ -50,6 +50,7 @@ struct PageRank : public SimpleAlgorithm<float, float, float> {
 
   VertexComputation<float, float, float>* createComputation(
       WorkerConfig const*) const override;
+  
   IAggregator* aggregator(std::string const& name) const override;
 
   MasterContext* masterContext(VPackSlice userParams) const override;
diff --git a/arangod/Pregel/Algos/RecoveringPageRank.cpp b/arangod/Pregel/Algos/RecoveringPageRank.cpp
index 96c49f8a10..2eaac255b7 100644
--- a/arangod/Pregel/Algos/RecoveringPageRank.cpp
+++ b/arangod/Pregel/Algos/RecoveringPageRank.cpp
@@ -87,9 +87,9 @@ IAggregator* RecoveringPageRank::aggregator(std::string const& name) const {
   } else if (name == kRank) {
     return new SumAggregator<float>(0);
   } else if (name == kStep) {
-    return new ValueAggregator<uint32_t>(0);
+    return new OverwriteAggregator<uint32_t>(0);
   } else if (name == kScale) {
-    return new ValueAggregator<float>(-1);
+    return new OverwriteAggregator<float>(-1);
   }
   return nullptr;
 }
diff --git a/arangod/Pregel/Algos/SCC.cpp b/arangod/Pregel/Algos/SCC.cpp
index ce59920e2a..000574cf43 100644
--- a/arangod/Pregel/Algos/SCC.cpp
+++ b/arangod/Pregel/Algos/SCC.cpp
@@ -237,7 +237,7 @@ MasterContext* SCC::masterContext(VPackSlice userParams) const {
 
 IAggregator* SCC::aggregator(std::string const& name) const {
   if (name == kPhase) {  // permanent value
-    return new ValueAggregator<uint32_t>(SCCPhase::TRANSPOSE, true);
+    return new OverwriteAggregator<uint32_t>(SCCPhase::TRANSPOSE, true);
   } else if (name == kFoundNewMax) {
     return new BoolOrAggregator(false);  // non perm
   } else if (name == kConverged) {
diff --git a/arangod/Pregel/Conductor.cpp b/arangod/Pregel/Conductor.cpp
index 57b17af0f1..858f6c0ab2 100644
--- a/arangod/Pregel/Conductor.cpp
+++ b/arangod/Pregel/Conductor.cpp
@@ -142,7 +142,7 @@ bool Conductor::_startGlobalStep() {
   _totalEdgesCount = 0;
   for (auto const& req : requests) {
     VPackSlice payload = req.result.answer->payload();
-    _aggregators->parseValues(payload);
+    _aggregators->aggregateValues(payload);
     _statistics.accumulateActiveCounts(payload);
     _totalVerticesCount += payload.get(Utils::vertexCountKey).getUInt();
     _totalEdgesCount += payload.get(Utils::edgeCountKey).getUInt();
@@ -267,17 +267,16 @@ VPackBuilder Conductor::finishedWorkerStep(VPackSlice const& data) {
   } else if (_statistics.clientCount() < _dbServers.size() ||  // no messages
              !_statistics.allMessagesProcessed()) {  // haven't received msgs
     VPackBuilder response;
-    if (_aggregators->parseValues(data)) {
-      if (_masterContext) {
-        _masterContext->postLocalSuperstep();
-      }
-      response.openObject();
-      _aggregators->serializeValues(response);
-      if (_masterContext && _masterContext->_enterNextGSS) {
-        response.add(Utils::enterNextGSSKey, VPackValue(true));
-      }
-      response.close();
+    _aggregators->aggregateValues(data);
+    if (_masterContext) {
+      _masterContext->postLocalSuperstep();
     }
+    response.openObject();
+    _aggregators->serializeValues(response);
+    if (_masterContext && _masterContext->_enterNextGSS) {
+      response.add(Utils::enterNextGSSKey, VPackValue(true));
+    }
+    response.close();
     return response;
   }
 
@@ -312,7 +311,7 @@ void Conductor::finishedRecoveryStep(VPackSlice const& data) {
   }
 
   // the recovery mechanism might be gathering state information
-  _aggregators->parseValues(data);
+  _aggregators->aggregateValues(data);
   if (_respondedServers.size() != _dbServers.size()) {
     return;
   }
diff --git a/arangod/Pregel/Graph.h b/arangod/Pregel/Graph.h
index e280a0950e..04a1a392e2 100644
--- a/arangod/Pregel/Graph.h
+++ b/arangod/Pregel/Graph.h
@@ -157,4 +157,23 @@ class VertexEntry {
 };*/
 }
 }
+/*
+namespace std {
+  template <>
+  struct hash<arangodb::pregel::PregelID> {
+    std::size_t operator()(const PregelID& k) const {
+      using std::size_t;
+      using std::hash;
+      using std::string;
+      
+      // Compute individual hash values for first,
+      // second and third and combine them using XOR
+      // and bit shifting:
+      std::size_t h1 = std::hash<string>()(k.key);
+      std::size_t h2 = std::hash<prgl_shard_t>()(k.shard);
+      return h1 ^ (h2 << 1);
+    }
+  };
+}*/
+
 #endif
diff --git a/arangod/Pregel/MasterContext.h b/arangod/Pregel/MasterContext.h
index 9987edeffb..48469159e6 100644
--- a/arangod/Pregel/MasterContext.h
+++ b/arangod/Pregel/MasterContext.h
@@ -77,6 +77,7 @@ class MasterContext {
 
   /// Called when a worker send updated aggregator values.
   /// Only called in async mode, never called after a global superstep
+  /// Can be used to decide to enter the next phase
   virtual void postLocalSuperstep(){};
 
   /// should indicate if compensation is supposed to start by returning true
diff --git a/arangod/Pregel/VertexComputation.h b/arangod/Pregel/VertexComputation.h
index 414e52c444..5c5e592d6e 100644
--- a/arangod/Pregel/VertexComputation.h
+++ b/arangod/Pregel/VertexComputation.h
@@ -37,7 +37,7 @@ namespace pregel {
 
 template <typename V, typename E, typename M>
 class Worker;
-class Aggregator;
+class IAggregator;
 
 template <typename V, typename E, typename M>
 class VertexContext {
@@ -47,8 +47,8 @@ class VertexContext {
   uint64_t _lss = 0;
   WorkerContext* _context;
   GraphStore<V, E>* _graphStore;
-  AggregatorHandler* _conductorAggregators;
-  AggregatorHandler* _workerAggregators;
+  AggregatorHandler* _readAggregators;
+  AggregatorHandler* _writeAggregators;
   VertexEntry* _vertexEntry;
 
  public:
@@ -57,12 +57,16 @@ class VertexContext {
   template <typename T>
   inline void aggregate(std::string const& name, T const& value) {
     T const* ptr = &value;
-    _workerAggregators->aggregate(name, ptr);
+    _writeAggregators->aggregate(name, ptr);
   }
 
   template <typename T>
   inline const T* getAggregatedValue(std::string const& name) {
-    return (const T*)_conductorAggregators->getAggregatedValue(name);
+    return (const T*)_readAggregators->getAggregatedValue(name);
+  }
+  
+  IAggregator* getAggregator(std::string const& name) {
+    return _writeAggregators->getAggregator(name);
   }
 
   inline WorkerContext const* context() { return _context; }
diff --git a/arangod/Pregel/Worker.cpp b/arangod/Pregel/Worker.cpp
index f451a6cc2d..4ddc49b46a 100644
--- a/arangod/Pregel/Worker.cpp
+++ b/arangod/Pregel/Worker.cpp
@@ -201,8 +201,8 @@ VPackBuilder Worker<V, E, M>::prepareGlobalStep(VPackSlice const& data) {
 
   // initialize worker context
   if (_workerContext && gss == 0 && _config.localSuperstep() == 0) {
-    _workerContext->_conductorAggregators = _conductorAggregators.get();
-    _workerContext->_workerAggregators = _workerAggregators.get();
+    _workerContext->_readAggregators = _conductorAggregators.get();
+    _workerContext->_writeAggregators = _workerAggregators.get();
     _workerContext->_vertexCount = data.get(Utils::vertexCountKey).getUInt();
     _workerContext->_edgeCount = data.get(Utils::edgeCountKey).getUInt();
     _workerContext->preApplication();
@@ -295,9 +295,8 @@ void Worker<V, E, M>::startGlobalStep(VPackSlice const& data) {
     THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_BAD_PARAMETER, "Wrong GSS");
   }
 
-  _workerAggregators->resetValues(true);
-  _conductorAggregators->resetValues(true);
-  _conductorAggregators->parseValues(data);
+  _workerAggregators->resetValues();
+  _conductorAggregators->setAggregatedValues(data);
   // execute context
   if (_workerContext) {
     _workerContext->_vertexCount = data.get(Utils::vertexCountKey).getUInt();
@@ -359,7 +358,7 @@ void Worker<V, E, M>::_initializeVertexContext(VertexContext<V, E, M>* ctx) {
   ctx->_lss = _config.localSuperstep();
   ctx->_context = _workerContext.get();
   ctx->_graphStore = _graphStore.get();
-  ctx->_conductorAggregators = _conductorAggregators.get();
+  ctx->_readAggregators = _conductorAggregators.get();
 }
 
 // internally called in a WORKER THREAD!!
@@ -381,7 +380,7 @@ bool Worker<V, E, M>::_processVertices(size_t threadId,
   std::unique_ptr<VertexComputation<V, E, M>> vertexComputation(
       _algorithm->createComputation(&_config));
   _initializeVertexContext(vertexComputation.get());
-  vertexComputation->_workerAggregators = &workerAggregator;
+  vertexComputation->_writeAggregators = &workerAggregator;
   vertexComputation->_cache = outCache;
   if (!_config.asynchronousMode()) {
     // Should cause enterNextGlobalSuperstep to do nothing
@@ -531,13 +530,15 @@ void Worker<V, E, M>::_finishedProcessing() {
   }
 
   if (_config.asynchronousMode()) {
-    bool proceed = true;
+    bool proceed = false;
     // if the conductor is unreachable or has send data (try to) proceed
     std::unique_ptr<ClusterCommResult> result = _callConductorWithResponse(
         Utils::finishedWorkerStepPath, package.slice());
     if (result->status == CL_COMM_RECEIVED) {
       VPackSlice data = result->answer->payload();
-      if ((proceed = _conductorAggregators->parseValues(data))) {
+      if (data.isObject()) {
+        proceed = true;
+        _conductorAggregators->aggregateValues(data);// only aggregate values
         VPackSlice nextGSS = data.get(Utils::enterNextGSSKey);
         if (nextGSS.isBool()) {
           _requestedNextGSS = nextGSS.getBool();
@@ -643,8 +644,7 @@ void Worker<V, E, M>::compensateStep(VPackSlice const& data) {
   MUTEX_LOCKER(guard, _commandMutex);
 
   _workerAggregators->resetValues();
-  _conductorAggregators->resetValues();
-  _conductorAggregators->parseValues(data);
+  _conductorAggregators->setAggregatedValues(data);
 
   ThreadPool* pool = PregelFeature::instance()->threadPool();
   pool->enqueue([this] {
@@ -657,7 +657,7 @@ void Worker<V, E, M>::compensateStep(VPackSlice const& data) {
     std::unique_ptr<VertexCompensation<V, E, M>> vCompensate(
         _algorithm->createCompensation(&_config));
     _initializeVertexContext(vCompensate.get());
-    vCompensate->_workerAggregators = _workerAggregators.get();
+    vCompensate->_writeAggregators = _workerAggregators.get();
 
     size_t i = 0;
     for (VertexEntry* vertexEntry : vertexIterator) {
diff --git a/arangod/Pregel/WorkerContext.h b/arangod/Pregel/WorkerContext.h
index 09c9bbc675..b73fadf781 100644
--- a/arangod/Pregel/WorkerContext.h
+++ b/arangod/Pregel/WorkerContext.h
@@ -37,19 +37,19 @@ class WorkerContext {
   friend class Worker;
 
   uint64_t _vertexCount, _edgeCount;
-  AggregatorHandler* _conductorAggregators;
-  AggregatorHandler* _workerAggregators;
+  AggregatorHandler* _readAggregators;
+  AggregatorHandler* _writeAggregators;
 
  protected:
   template <typename T>
   inline void aggregate(std::string const& name, T const& value) {
     T const* ptr = &value;
-    _workerAggregators->aggregate(name, ptr);
+    _writeAggregators->aggregate(name, ptr);
   }
 
   template <typename T>
   inline const T* getAggregatedValue(std::string const& name) {
-    return (T*)_conductorAggregators->getAggregatedValue(name);
+    return (T*)_readAggregators->getAggregatedValue(name);
   }
 
   virtual void preApplication(){};
@@ -58,7 +58,7 @@ class WorkerContext {
   virtual void postApplication(){};
 
  public:
-  WorkerContext(VPackSlice params) {}
+  WorkerContext() {}
   virtual ~WorkerContext() {}
 
   inline uint64_t vertexCount() const { return _vertexCount; }