private List<Entry> appendResults(
      ByteBuffer key,
      ByteBuffer columnStart,
      ByteBuffer columnEnd,
      List<Entry> entries,
      LimitTracker limit,
      TransactionHandle txh) {
    if (limit.limitExhausted()) return null;
    List<Entry> results = null;

    for (int readAttempt = 0; readAttempt < maxReadRetryAttempts; readAttempt++) {
      try {
        results = edgeStore.getSlice(key, columnStart, columnEnd, limit.getLimit(), txh);
        break;
      } catch (StorageException e) {
        if (e instanceof TemporaryStorageException) {
          if (readAttempt < maxReadRetryAttempts - 1) temporaryStorageException(e);
          else throw readException(e, maxReadRetryAttempts);
        } else throw readException(e);
      }
    }

    limit.retrieved(results.size());

    if (entries == null) return results;
    else {
      entries.addAll(results);
      return entries;
    }
  }
  @Override
  public long[] indexRetrieval(Object key, TitanKey pt, InternalTitanTransaction tx) {
    Preconditions.checkArgument(
        pt.isSimple(), "Currently, only simple properties are supported for index retrieval");
    Preconditions.checkArgument(
        pt.hasIndex(), "Cannot retrieve for given property key - it does not have an index");

    long[] vertices = null;

    Preconditions.checkArgument(
        pt.getDataType().isInstance(key),
        "Specified object is incompatible with property data type [" + pt.getName() + "]");

    for (int readAttempt = 0; readAttempt < maxReadRetryAttempts; readAttempt++) {
      try {
        if (pt.isUnique()) {
          ByteBuffer value =
              propertyIndex.get(getIndexKey(key), getKeyedIndexColumn(pt), tx.getTxHandle());
          if (value != null) {
            vertices = new long[1];
            vertices[0] = VariableLong.readPositive(value);
          }
        } else {
          ByteBuffer startColumn = VariableLong.positiveByteBuffer(pt.getID());
          List<Entry> entries =
              propertyIndex.getSlice(
                  getIndexKey(key),
                  startColumn,
                  ByteBufferUtil.nextBiggerBuffer(startColumn),
                  tx.getTxHandle());
          vertices = new long[entries.size()];
          int i = 0;
          for (Entry ent : entries) {
            vertices[i++] = VariableLong.readPositive(ent.getValue());
          }
        }

        break;
      } catch (StorageException e) {
        if (e instanceof TemporaryStorageException) {
          if (readAttempt < maxReadRetryAttempts - 1) temporaryStorageException(e);
          else throw readException(e, maxReadRetryAttempts);
        } else throw readException(e);
      }
    }

    if (vertices == null) return new long[0];
    else return vertices;
  }
  @Override
  public boolean containsVertexID(long id, InternalTitanTransaction tx) {
    log.trace("Checking node existence for {}", id);

    for (int readAttempt = 0; readAttempt < maxReadRetryAttempts; readAttempt++) {
      try {
        return edgeStore.containsKey(IDHandler.getKey(id), tx.getTxHandle());
      } catch (StorageException e) {
        if (e instanceof TemporaryStorageException) {
          if (readAttempt < maxReadRetryAttempts - 1) temporaryStorageException(e);
          else throw readException(e, maxReadRetryAttempts);
        } else throw readException(e);
      }
    }
    throw new AssertionError("Illegal program state");
  }
  @Override
  public RecordIterator<Long> getVertexIDs(final InternalTitanTransaction tx) {
    if (!(edgeStore instanceof ScanKeyColumnValueStore))
      throw new UnsupportedOperationException(
          "The configured storage backend does not support global graph operations - use Faunus instead");

    for (int readAttempt = 0; readAttempt < maxReadRetryAttempts; readAttempt++) {
      try {
        final RecordIterator<ByteBuffer> keyiter =
            ((ScanKeyColumnValueStore) edgeStore).getKeys(tx.getTxHandle());
        return new RecordIterator<Long>() {

          @Override
          public boolean hasNext() throws StorageException {
            return keyiter.hasNext();
          }

          @Override
          public Long next() throws StorageException {
            return IDHandler.getKeyID(keyiter.next());
          }

          @Override
          public void close() throws StorageException {
            keyiter.close();
          }
        };
      } catch (StorageException e) {
        if (e instanceof TemporaryStorageException) {
          if (readAttempt < maxReadRetryAttempts - 1) temporaryStorageException(e);
          else throw readException(e, maxReadRetryAttempts);
        } else throw readException(e);
      }
    }
    throw new AssertionError("Illegal program state");
  }
  @Override
  public void save(
      final Collection<InternalRelation> addedRelations,
      final Collection<InternalRelation> deletedRelations,
      final InternalTitanTransaction tx)
      throws StorageException {
    // Setup
    log.debug(
        "Saving transaction. Added {}, removed {}", addedRelations.size(), deletedRelations.size());
    final Map<TitanType, TypeSignature> signatures = new HashMap<TitanType, TypeSignature>();
    final TransactionHandle txh = tx.getTxHandle();

    final StoreMutator mutator = getStoreMutator(txh);
    final boolean acquireLocks = tx.getTxConfiguration().hasAcquireLocks();

    // 1. Assign TitanVertex IDs
    assignIDs(addedRelations, tx);

    for (int saveAttempt = 0; saveAttempt < maxWriteRetryAttempts; saveAttempt++) {
      //        while (true) { //Indefinite loop, broken if no exception occurs, otherwise retried
      // or failed immediately
      try {
        // 2. Collect deleted edges
        ListMultimap<InternalTitanVertex, InternalRelation> mutations = ArrayListMultimap.create();
        if (deletedRelations != null && !deletedRelations.isEmpty()) {
          for (InternalRelation del : deletedRelations) {
            assert del.isRemoved();
            for (int pos = 0; pos < del.getArity(); pos++) {
              InternalTitanVertex node = del.getVertex(pos);
              if (pos == 0 || !del.isUnidirected()) {
                mutations.put(node, del);
              }
              if (pos == 0
                  && acquireLocks
                  && del.getType().isFunctional()
                  && ((InternalTitanType) del.getType()).isFunctionalLocking()) {
                Entry entry = getEntry(tx, del, node, signatures);
                mutator.acquireEdgeLock(
                    IDHandler.getKey(node.getID()), entry.getColumn(), entry.getValue());
              }
            }
            if (acquireLocks && del.isProperty()) {
              lockKeyedProperty((TitanProperty) del, mutator);
            }
          }
        }

        ListMultimap<InternalTitanType, InternalRelation> simpleEdgeTypes = null;
        ListMultimap<InternalTitanType, InternalRelation> otherEdgeTypes = null;

        // 3. Sort Added Edges
        for (InternalRelation edge : addedRelations) {
          if (edge.isRemoved()) continue;
          assert edge.isNew();

          TitanType et = edge.getType();

          // Give special treatment to edge type definitions
          if (SystemTypeManager.prepersistedSystemTypes.contains(et)) {
            assert edge.getVertex(0) instanceof InternalTitanType;
            InternalTitanType node = (InternalTitanType) edge.getVertex(0);
            assert node.hasID();
            if (node.isSimple()) {
              if (simpleEdgeTypes == null) simpleEdgeTypes = ArrayListMultimap.create();
              simpleEdgeTypes.put(node, edge);
            } else {
              if (otherEdgeTypes == null) otherEdgeTypes = ArrayListMultimap.create();
              otherEdgeTypes.put(node, edge);
            }
          } else { // STANDARD TitanRelation
            assert (edge.getArity() == 1 && edge.isProperty())
                || (edge.getArity() == 2 && edge.isEdge());
            for (int pos = 0; pos < edge.getArity(); pos++) {
              InternalTitanVertex node = edge.getVertex(pos);
              assert node.hasID();
              if (pos == 0 || !edge.isUnidirected()) {
                mutations.put(node, edge);
              }
              if (pos == 0
                  && acquireLocks
                  && edge.getType().isFunctional()
                  && !node.isNew()
                  && ((InternalTitanType) edge.getType()).isFunctionalLocking()) {
                Entry entry = getEntry(tx, edge, node, signatures, true);
                mutator.acquireEdgeLock(IDHandler.getKey(node.getID()), entry.getColumn(), null);
              }
            }
          }
          if (acquireLocks && edge.isProperty()) {
            lockKeyedProperty((TitanProperty) edge, mutator);
          }
        }

        // 3. Persist
        if (simpleEdgeTypes != null) persist(simpleEdgeTypes, signatures, tx, mutator);
        if (otherEdgeTypes != null) persist(otherEdgeTypes, signatures, tx, mutator);
        mutator.flush();

        // Commit saved EdgeTypes to TypeManager
        if (simpleEdgeTypes != null) commitEdgeTypes(simpleEdgeTypes.keySet());
        if (otherEdgeTypes != null) commitEdgeTypes(otherEdgeTypes.keySet());

        if (!mutations.isEmpty()) persist(mutations, signatures, tx, mutator);
        mutator.flush();

        // Successfully completed - return to break out of loop
        break;
      } catch (Throwable e) {
        if (e instanceof TemporaryStorageException) {
          if (saveAttempt < maxWriteRetryAttempts - 1) temporaryStorageException(e);
          else
            throw new PermanentStorageException(
                "Tried committing "
                    + maxWriteRetryAttempts
                    + " times on temporary exception without success",
                e);
        } else if (e instanceof StorageException) {
          throw (StorageException) e;
        } else {
          throw new PermanentStorageException(
              "Unidentified exception occurred during persistence", e);
        }
      }
    }
  }