diff --git a/AuraTrader/AuraTrader.dproj b/AuraTrader/AuraTrader.dproj
index e0cbb98..78cf432 100644
--- a/AuraTrader/AuraTrader.dproj
+++ b/AuraTrader/AuraTrader.dproj
@@ -4,7 +4,7 @@
         <ProjectVersion>20.3</ProjectVersion>
         <FrameworkType>FMX</FrameworkType>
         <Base>True</Base>
-        <Config Condition="'$(Config)'==''">Debug</Config>
+        <Config Condition="'$(Config)'==''">Release</Config>
         <Platform Condition="'$(Platform)'==''">Win64</Platform>
         <ProjectName Condition="'$(ProjectName)'==''">AuraTrader</ProjectName>
         <TargetedPlatforms>3</TargetedPlatforms>
diff --git a/AuraTrader/MainForm.fmx b/AuraTrader/MainForm.fmx
index 2606846..cbf3bbe 100644
--- a/AuraTrader/MainForm.fmx
+++ b/AuraTrader/MainForm.fmx
@@ -64,21 +64,21 @@ object Form1: TForm1
       JustifyLastLine = Left
       FlowDirection = LeftToRight
     end
-    object Button1: TButton
+    object ChartButton: TButton
       Position.X = 712.00000000000000000
       Position.Y = 72.00000000000000000
       TabOrder = 6
-      Text = 'Button1'
+      Text = 'ChartButton'
       TextSettings.Trimming = None
-      OnClick = Button1Click
+      OnClick = ChartButtonClick
     end
-    object Button2: TButton
+    object StopButton: TButton
       Position.X = 720.00000000000000000
       Position.Y = 96.00000000000000000
       TabOrder = 8
-      Text = 'Button2'
+      Text = 'StopButton'
       TextSettings.Trimming = None
-      OnClick = Button2Click
+      OnClick = StopButtonClick
     end
   end
   object LogMemo: TMemo
diff --git a/AuraTrader/MainForm.pas b/AuraTrader/MainForm.pas
index 5199b31..dd6fd7b 100644
--- a/AuraTrader/MainForm.pas
+++ b/AuraTrader/MainForm.pas
@@ -45,14 +45,14 @@ type
         SymbolsComboBox: TComboBox;
         LoadButton: TButton;
         FlowLayout: TFlowLayout;
-        Button1: TButton;
-        Button2: TButton;
+    ChartButton: TButton;
+    StopButton: TButton;
         procedure RandomButtonClick(Sender: TObject);
         procedure FormCreate(Sender: TObject);
         procedure FormDestroy(Sender: TObject);
         procedure LoadButtonClick(Sender: TObject);
-        procedure Button1Click(Sender: TObject);
-        procedure Button2Click(Sender: TObject);
+        procedure ChartButtonClick(Sender: TObject);
+        procedure StopButtonClick(Sender: TObject);
     private
         const
             cnt = 20;
@@ -83,84 +83,7 @@ implementation
 
 {$R *.fmx}
 
-procedure TForm1.Button1Click(Sender: TObject);
-begin
-    if SymbolsComboBox.ItemIndex < 0 then
-        exit;
-
-    var curr := TWriteable<TDataSeries<TAskBidItem>>.CreateProtected;
-    var Symbol := FSymbols.WaitFor[SymbolsComboBox.ItemIndex];
-
-    var currPathData := TWriteable<IObjectRef<TPathData>>.CreateProtected;
-
-    LogMemo.AddIdleHandler(curr.Changed, procedure begin LogMemo.Lines.Add(Symbol + ': ' + curr.Value.TotalCount.ToString); end);
-
-    var arr := TDataSeries<TAskBidItem>.CreateArray(3000);
-    var done :=
-        FServer.ProcessData(
-            Symbol,
-            FTerminate.Signal,
-            procedure(const Values: TArray<TDataPoint<TAskBidItem>>; const Terminated: TState)
-            begin
-                arr.Add(Values);
-                curr.Value := arr.Copy;
-
-                var PathData := TPathData.Create;
-                var Prices := arr;
-                if Prices.Count > 0 then
-                begin
-                    PathData.MoveTo(PointF(Prices.Count - 1, Prices[0].Data.Ask));
-                    for var i := 1 to Prices.Count - 1 do
-                        PathData.LineTo(PointF(Prices.Count - i - 1, Prices[i].Data.Ask));
-                end;
-
-                currPathData.Value := TObjectRef<TPathData>.Create(PathData);
-            end
-        );
-
-    FLoadDone := TState.All([FLoadDone, done]);
-
-    Path1.AddIdleHandler(
-        currPathData.Changed,
-        procedure
-        begin
-            if currPathData.Value<>nil then
-            begin
-                Path1.Data.Assign( currPathData.Value.Obj );
-                currPathData.Value := nil;
-            end;
-        end );
-
-
-(*
-    FLoadDone := TState.All([FLoadDone, done]);
-
-    Path1.AddIdleHandler(
-        curr.Changed,
-        procedure
-        begin
-            var Prices := curr.Value;
-
-            if Prices.Count > 0 then
-            begin
-                Path1.BeginUpdate;
-                try
-                    Path1.Data.Clear;
-                    Path1.Data.MoveTo(PointF(Path1.Width - 1, Prices[0].Data.Ask));
-                    for var i := 1 to Prices.Count - 1 do
-                    begin
-                        Path1.Data.LineTo(PointF(Path1.Width - i - 1, Prices[i].Data.Ask));
-                    end;
-                finally
-                    Path1.EndUpdate;
-                end;
-            end;
-        end
-    );
-*)
-end;
-
-procedure TForm1.Button2Click(Sender: TObject);
+procedure TForm1.StopButtonClick(Sender: TObject);
 begin
     FTerminate.Notify;
 end;
@@ -174,6 +97,8 @@ begin
     FServer := TAuraTABFileServer.Create('\\COFFEE\TickData\Pepperstone');
 
     SymbolsComboBox.Enabled := false;
+    ChartButton.Enabled := false;
+    LoadButton.Enabled := false;
 
     FSymbols := FServer.EnumerateSymbols;
 
@@ -190,6 +115,8 @@ begin
                     if SymbolsComboBox.ItemIndex < 0 then
                         SymbolsComboBox.ItemIndex := SymbolsComboBox.Items.IndexOf('GER40');
                     SymbolsComboBox.Enabled := true;
+                    ChartButton.Enabled := true;
+                    LoadButton.Enabled := true;
                 end;
             finally
                 SymbolsComboBox.EndUpdate;
@@ -200,6 +127,7 @@ end;
 
 procedure TForm1.FormDestroy(Sender: TObject);
 begin
+    FSymbols.WaitFor;
     FTerminate.Notify;
     TaskManager.WaitFor(FLoadDone);
     Application.OnIdle := nil;
@@ -279,4 +207,83 @@ begin
     Proc(FRandom.Count - 1);
 end;
 
+procedure TForm1.ChartButtonClick(Sender: TObject);
+begin
+    if SymbolsComboBox.ItemIndex < 0 then
+        exit;
+
+    var Symbol := FSymbols.WaitFor[SymbolsComboBox.ItemIndex];
+
+    var currPathData := TMutable<IObjectRef<TPathData>>.CreateProtected;
+    var currLog := TMutable<String>.CreateProtected;
+
+    const width = 1000;
+    const incsize = 100;
+    begin
+        var arr := TDataSeries<TAskBidItem>.CreateWriteable(width);
+        var done :=
+            FServer.ProcessData(
+                Symbol,
+                FTerminate.Signal,
+                procedure(const Values: TArray<TDataPoint<TAskBidItem>>; const Terminated: TState)
+                begin
+                    arr.Add(Values);
+                    currLog.Value := arr.TotalCount.ToString;
+
+                    var PathData := TPathData.Create;
+                    var Prices := arr;
+                    if Prices.Count > 0 then
+                    begin
+                        PathData.MoveTo(PointF(Prices.Count - 1, Prices[0].Data.Ask));
+                        for var i := 1 to Prices.Count - 1 do
+                            PathData.LineTo(PointF(Prices.Count - i - 1, Prices[i].Data.Ask));
+                    end;
+
+                    currPathData.Value := TObjectRef<TPathData>.Create(PathData);
+                end
+            );
+        FLoadDone := TState.All([FLoadDone, done]);
+    end;
+
+    LogMemo.AddIdleHandler(currLog.Changed, procedure begin LogMemo.Lines.Add(Symbol + ': ' + currLog.Value); end);
+
+    Path1.AddIdleHandler(
+        currPathData.Changed,
+        procedure
+        begin
+            if currPathData.Value<>nil then
+            begin
+                Path1.Data.Assign( currPathData.Value.Obj );
+            end;
+        end );
+
+
+(*
+    FLoadDone := TState.All([FLoadDone, done]);
+
+    Path1.AddIdleHandler(
+        curr.Changed,
+        procedure
+        begin
+            var Prices := curr.Value;
+
+            if Prices.Count > 0 then
+            begin
+                Path1.BeginUpdate;
+                try
+                    Path1.Data.Clear;
+                    Path1.Data.MoveTo(PointF(Path1.Width - 1, Prices[0].Data.Ask));
+                    for var i := 1 to Prices.Count - 1 do
+                    begin
+                        Path1.Data.LineTo(PointF(Path1.Width - i - 1, Prices[i].Data.Ask));
+                    end;
+                finally
+                    Path1.EndUpdate;
+                end;
+            end;
+        end
+    );
+*)
+end;
+
 end.
diff --git a/Src/Myc.Core.Lazy.pas b/Src/Myc.Core.Lazy.pas
index e93d468..8706a5e 100644
--- a/Src/Myc.Core.Lazy.pas
+++ b/Src/Myc.Core.Lazy.pas
@@ -10,9 +10,10 @@ uses
 
 type
     TMycNullMutable<T> = class(TInterfacedObject, TMutable<T>.IMutable)
-    protected
+    private
         function GetChanged: TSignal;
         function GetValue: T;
+        function GetWriter: TMutable<T>.IWriter;
     end;
 
     TMycMutableBase<T> = class(TInterfacedObject, TMutable<T>.IMutable)
@@ -20,6 +21,7 @@ type
         FChanged: TEvent;
         FChangeState: TSignal.TSubscription;
         function GetChanged: TSignal;
+        function GetWriter: TMutable<T>.IWriter;
     protected
         function GetValue: T; virtual; abstract;
     public
@@ -36,13 +38,14 @@ type
         constructor Create(const AChanged: TSignal; const AProc: TFunc<T>);
     end;
 
-    TMycWriteableMutable<T> = class(TInterfacedObject, TMutable<T>.IMutable, TWriteable<T>.IWriteable)
+    TMycWriteableMutable<T> = class(TInterfacedObject, TMutable<T>.IMutable, TMutable<T>.IWriter)
     private
         FValue: T;
         FChanged: TEvent;
     protected
         function GetChanged: TSignal;
         function GetValue: T;
+        function GetWriter: TMutable<T>.IWriter;
     public
         constructor Create(const AValue: T);
         procedure SetValue(const Value: T);
@@ -77,7 +80,7 @@ type
         constructor Create(const AChanged: TSignal.ISignal; const AProc: TFunc<T>);
     end;
 
-    TMycProtectedWriteableMutable<T> = class(TInterfacedObject, TMutable<T>.IMutable, TWriteable<T>.IWriteable)
+    TMycProtectedWriteableMutable<T> = class(TInterfacedObject, TMutable<T>.IMutable, TMutable<T>.IWriter)
     private
         FValue: T;
         FChanged: TEvent;
@@ -85,6 +88,7 @@ type
     protected
         function GetChanged: TSignal;
         function GetValue: T;
+        function GetWriter: TMutable<T>.IWriter;
         procedure Lock; inline;
         procedure Release; inline;
     public
@@ -106,6 +110,11 @@ begin
     Result := Default(T);
 end;
 
+function TMycNullMutable<T>.GetWriter: TMutable<T>.IWriter;
+begin
+    Result := nil;
+end;
+
 { TMycMutableBase<T> }
 
 constructor TMycMutableBase<T>.Create(const AChanged: TSignal);
@@ -126,6 +135,11 @@ begin
     Result := FChanged.Signal;
 end;
 
+function TMycMutableBase<T>.GetWriter: TMutable<T>.IWriter;
+begin
+    Result := nil;
+end;
+
 { TMycWriteableMutable<T> }
 
 constructor TMycWriteableMutable<T>.Create(const AValue: T);
@@ -145,6 +159,11 @@ begin
     Result := FValue;
 end;
 
+function TMycWriteableMutable<T>.GetWriter: TMutable<T>.IWriter;
+begin
+    Result := Self;
+end;
+
 procedure TMycWriteableMutable<T>.SetValue(const Value: T);
 begin
     FValue := Value;
@@ -244,6 +263,11 @@ begin
     end;
 end;
 
+function TMycProtectedWriteableMutable<T>.GetWriter: TMutable<T>.IWriter;
+begin
+    Result := Self;
+end;
+
 procedure TMycProtectedWriteableMutable<T>.Lock;
 begin
     while AtomicExchange(FLock, 1) = 1 do
diff --git a/Src/Myc.Lazy.pas b/Src/Myc.Lazy.pas
index 55a2cfe..862c954 100644
--- a/Src/Myc.Lazy.pas
+++ b/Src/Myc.Lazy.pas
@@ -9,13 +9,19 @@ uses
 type
     TMutable<T> = record
         type
+            IWriter = interface
+                procedure SetValue(const Value: T);
+            end;
+
             IMutable = interface
                 {$REGION 'property access'}
                 function GetChanged: TSignal;
                 function GetValue: T;
+                function GetWriter: IWriter;
                 {$ENDREGION}
                 property Changed: TSignal read GetChanged;
                 property Value: T read GetValue;
+                property Writer: IWriter read GetWriter;
             end;
 
     {$REGION 'private'}
@@ -27,7 +33,9 @@ type
     private
         FMutable: IMutable;
         function GetChanged: TSignal; inline;
+        function GetIsWriteable: Boolean;
         function GetValue: T; inline;
+        procedure SetValue(const Value: T);
     {$ENDREGION}
     public
         constructor Create(const AMutable: IMutable);
@@ -38,35 +46,12 @@ type
         class property Null: IMutable read FNull;
 
         class function Construct(const Changing: TSignal; const Proc: TFunc<T>): TMutable<T>; overload; static;
-
-        property Value: T read GetValue;
-        property Changed: TSignal read GetChanged;
-    end;
-
-    TWriteable<T> = record
-        type
-            IWriteable = interface(TMutable<T>.IMutable)
-                procedure SetValue(const Value: T);
-                property Value: T read GetValue write SetValue;
-            end;
-
-    private
-        FWriteable: IWriteable;
-        function GetChanged: TSignal; inline;
-        function GetValue: T; inline;
-        procedure SetValue(const Value: T); inline;
-
-    public
-        constructor Create(const AWriteable: IWriteable);
-
-        class function CreateProtected: TWriteable<T>; overload; static;
-        class function CreateWriteable: TWriteable<T>; overload; static;
-
-        class operator Implicit(const A: IWriteable): TWriteable<T>; overload;
-        class operator Implicit(const A: TWriteable<T>): IWriteable; overload;
+        class function CreateProtected: TMutable<T>; overload; static;
+        class function CreateWriteable: TMutable<T>; overload; static;
 
         property Value: T read GetValue write SetValue;
         property Changed: TSignal read GetChanged;
+        property IsWriteable: Boolean read GetIsWriteable;
     end;
 
     TLazy<T> = record
@@ -127,16 +112,37 @@ begin
     Result := TMycFuncMutable<T>.Create(Changing, Proc);
 end;
 
+class function TMutable<T>.CreateProtected: TMutable<T>;
+begin
+    Result := TMycProtectedWriteableMutable<T>.Create(Default(T));
+end;
+
+class function TMutable<T>.CreateWriteable: TMutable<T>;
+begin
+    Result := TMycWriteableMutable<T>.Create(Default(T));
+end;
+
 function TMutable<T>.GetChanged: TSignal;
 begin
     Result := FMutable.Changed;
 end;
 
+function TMutable<T>.GetIsWriteable: Boolean;
+begin
+    Result := Assigned( FMutable.Writer );
+end;
+
 function TMutable<T>.GetValue: T;
 begin
     Result := FMutable.Value;
 end;
 
+procedure TMutable<T>.SetValue(const Value: T);
+begin
+    Assert( IsWriteable );
+    FMutable.Writer.SetValue( Value );
+end;
+
 class operator TMutable<T>.Implicit(const A: TMutable<T>): IMutable;
 begin
     Result := A.FMutable;
@@ -201,46 +207,4 @@ begin
     Dest.FLazy := FNull;
 end;
 
-{ TWriteable<T> }
-
-constructor TWriteable<T>.Create(const AWriteable: IWriteable);
-begin
-    FWriteable := AWriteable;
-end;
-
-class function TWriteable<T>.CreateProtected: TWriteable<T>;
-begin
-    Result := TMycProtectedWriteableMutable<T>.Create(Default(T));
-end;
-
-class function TWriteable<T>.CreateWriteable: TWriteable<T>;
-begin
-    Result := TMycWriteableMutable<T>.Create(Default(T));
-end;
-
-function TWriteable<T>.GetChanged: TSignal;
-begin
-    Result := FWriteable.Changed;
-end;
-
-function TWriteable<T>.GetValue: T;
-begin
-    Result := FWriteable.Value;
-end;
-
-procedure TWriteable<T>.SetValue(const Value: T);
-begin
-    FWriteable.Value := Value;
-end;
-
-class operator TWriteable<T>.Implicit(const A: IWriteable): TWriteable<T>;
-begin
-    Result.Create(A);
-end;
-
-class operator TWriteable<T>.Implicit(const A: TWriteable<T>): IWriteable;
-begin
-    Result := A.FWriteable;
-end;
-
 end.
diff --git a/Src/Myc.Trade.Core.DataPoint.pas b/Src/Myc.Trade.Core.DataPoint.pas
index 848e499..0c590ae 100644
--- a/Src/Myc.Trade.Core.DataPoint.pas
+++ b/Src/Myc.Trade.Core.DataPoint.pas
@@ -7,7 +7,7 @@ uses
 
 type
     // The implementation class for IDataSeries<T>.
-    TDataArray<T> = class(TInterfacedObject, IDataSeries<T>, IDataArray<T>)
+    TDataArray<T> = class(TInterfacedObject, IDataSeries<T>, IDataSeriesWriter<T>)
         const
             ChunkSize = 1024;
         type
@@ -15,7 +15,7 @@ type
     private
         FChunks: TArray<TChunk>;
         FCount: Int64;
-        FMaxLookback: Int64;
+        FLookback: Int64;
         FTotalCount: Int64;
 
         function LogicalToPhysicalIndex(LogicalIndex: Int64): Int64; inline;
@@ -25,15 +25,16 @@ type
         // IDataSeries<T> implementation
         function GetCount: Int64;
         function GetItems(Idx: Int64): TDataPoint<T>;
-        function GetMaxLookback: Int64;
+        function GetLookback: Int64;
         function GetTotalCount: Int64;
+        function GetWriter: IDataSeriesWriter<T>;
     public
-        constructor Create(AMaxLookback: Int64);
+        constructor Create(ALookback: Int64);
         procedure Add(const Data: TDataPoint<T>); overload;
         procedure Add(const Data: array of TDataPoint<T>; NumToAdd: Integer = -1); overload;
         procedure Clear;
-        function Copy: IDataSeries<T>;
-        property MaxLookback: Int64 read GetMaxLookback;
+        function Immutable: IDataSeries<T>;
+        property Lookback: Int64 read GetLookback;
     end;
 
     // Null object implementation for IDataSeries<T>
@@ -43,12 +44,13 @@ type
             FNull: IDataSeries<T>;
     private
         class constructor CreateClass;
+        function GetWriter: IDataSeriesWriter<T>;
     public
         function GetCount: Int64;
         function GetItems(Idx: Int64): TDataPoint<T>;
         function GetTotalCount: Int64;
         function IndexOf(TimeStamp: TDateTime): Int64;
-        function Copy: IDataSeries<T>;
+        function Immutable: IDataSeries<T>;
         class property Null: IDataSeries<T> read FNull;
     end;
 
@@ -57,7 +59,7 @@ type
     private
         FChunks: TArray<TDataArray<T>.TChunk>;
         FCount: Int64;
-        FMaxLookback: Int64;
+        FLookback: Int64;
         FTotalCount: Int64;
 
         function LogicalToPhysicalIndex(LogicalIndex: Int64): Int64; inline;
@@ -66,23 +68,24 @@ type
         // IDataSeries<T> implementation
         function GetCount: Int64;
         function GetItems(Idx: Int64): TDataPoint<T>;
-        function GetMaxLookback: Int64;
+        function GetLookback: Int64;
         function GetTotalCount: Int64;
+        function GetWriter: IDataSeriesWriter<T>;
 
     public
-        constructor Create(const AChunks: TArray<TDataArray<T>.TChunk>; ACount, AMaxLookback, ATotalCount: Int64);
-        function Copy: IDataSeries<T>;
-        property MaxLookback: Int64 read GetMaxLookback;
+        constructor Create(const AChunks: TArray<TDataArray<T>.TChunk>; ACount, ALookback, ATotalCount: Int64);
+        function Immutable: IDataSeries<T>;
+        property Lookback: Int64 read GetLookback;
     end;
 
 implementation
 
 { TDataArray<T> }
 
-constructor TDataArray<T>.Create(AMaxLookback: Int64);
+constructor TDataArray<T>.Create(ALookback: Int64);
 begin
     inherited Create;
-    FMaxLookback := AMaxLookback;
+    FLookback := ALookback;
     FCount := 0;
     FTotalCount := 0;
     FChunks := nil;
@@ -170,16 +173,16 @@ begin
     FTotalCount := 0;
 end;
 
-function TDataArray<T>.Copy: IDataSeries<T>;
+function TDataArray<T>.Immutable: IDataSeries<T>;
 begin
-    Result := TStaticDataSeries<T>.Create(FChunks, FCount, FMaxLookback, FTotalCount);
+    Result := TStaticDataSeries<T>.Create(FChunks, FCount, FLookback, FTotalCount);
 end;
 
 function TDataArray<T>.GetCount: Int64;
 begin
     Result := FCount;
-    if (FMaxLookback > 0) and (Result > FMaxLookback) then
-        Result := FMaxLookback;
+    if (FLookback > 0) and (Result > FLookback) then
+        Result := FLookback;
 end;
 
 function TDataArray<T>.GetItems(Idx: Int64): TDataPoint<T>;
@@ -191,9 +194,9 @@ begin
     Result := FChunks[physicalIndex div ChunkSize][physicalIndex mod ChunkSize];
 end;
 
-function TDataArray<T>.GetMaxLookback: Int64;
+function TDataArray<T>.GetLookback: Int64;
 begin
-    Result := FMaxLookback;
+    Result := FLookback;
 end;
 
 function TDataArray<T>.GetTotalCount: Int64;
@@ -201,9 +204,14 @@ begin
     Result := FTotalCount;
 end;
 
+function TDataArray<T>.GetWriter: IDataSeriesWriter<T>;
+begin
+    Result := Self;
+end;
+
 function TDataArray<T>.IsIndexInLimits(LogicalIndex: Int64): Boolean;
 begin
-    Result := (FMaxLookback <= 0) or (LogicalIndex < FMaxLookback);
+    Result := (FLookback <= 0) or (LogicalIndex < FLookback);
 end;
 
 function TDataArray<T>.LogicalToPhysicalIndex(LogicalIndex: Int64): Int64;
@@ -216,12 +224,12 @@ procedure TDataArray<T>.Trim;
 var
     itemsToRemove, chunksToRemove: Int64;
 begin
-    if (FCount = 0) or (FMaxLookback <= 0) then
+    if (FCount = 0) or (FLookback <= 0) then
         Exit;
 
     itemsToRemove := 0;
-    if FCount > FMaxLookback then
-        itemsToRemove := FCount - FMaxLookback;
+    if FCount > FLookback then
+        itemsToRemove := FCount - FLookback;
 
     if itemsToRemove <= 0 then
         Exit;
@@ -231,7 +239,7 @@ begin
     if chunksToRemove > 0 then
     begin
         var itemsInFreedChunks := chunksToRemove * ChunkSize;
-        FChunks := System.Copy(FChunks, chunksToRemove, Length(FChunks) - chunksToRemove);
+        FChunks := Copy(FChunks, chunksToRemove, Length(FChunks) - chunksToRemove);
         FCount := FCount - itemsInFreedChunks;
     end;
 end;
@@ -243,7 +251,7 @@ begin
     FNull := TNullDataSeries<T>.Create;
 end;
 
-function TNullDataSeries<T>.Copy: IDataSeries<T>;
+function TNullDataSeries<T>.Immutable: IDataSeries<T>;
 begin
     Result := FNull;
 end;
@@ -264,6 +272,11 @@ begin
     Result := 0;
 end;
 
+function TNullDataSeries<T>.GetWriter: IDataSeriesWriter<T>;
+begin
+    Result := nil;
+end;
+
 function TNullDataSeries<T>.IndexOf(TimeStamp: TDateTime): Int64;
 begin
     Result := -1;
@@ -271,16 +284,16 @@ end;
 
 { TStaticDataSeries<T> }
 
-constructor TStaticDataSeries<T>.Create(const AChunks: TArray<TDataArray<T>.TChunk>; ACount, AMaxLookback, ATotalCount: Int64);
+constructor TStaticDataSeries<T>.Create(const AChunks: TArray<TDataArray<T>.TChunk>; ACount, ALookback, ATotalCount: Int64);
 begin
     inherited Create;
     FChunks := AChunks;
     FCount := ACount;
-    FMaxLookback := AMaxLookback;
+    FLookback := ALookback;
     FTotalCount := ATotalCount;
 end;
 
-function TStaticDataSeries<T>.Copy: IDataSeries<T>;
+function TStaticDataSeries<T>.Immutable: IDataSeries<T>;
 begin
     Result := Self;
 end;
@@ -288,8 +301,8 @@ end;
 function TStaticDataSeries<T>.GetCount: Int64;
 begin
     Result := FCount;
-    if (FMaxLookback > 0) and (Result > FMaxLookback) then
-        Result := FMaxLookback;
+    if (FLookback > 0) and (Result > FLookback) then
+        Result := FLookback;
 end;
 
 function TStaticDataSeries<T>.GetItems(Idx: Int64): TDataPoint<T>;
@@ -301,9 +314,9 @@ begin
     Result := FChunks[physicalIndex div TDataArray<T>.ChunkSize][physicalIndex mod TDataArray<T>.ChunkSize];
 end;
 
-function TStaticDataSeries<T>.GetMaxLookback: Int64;
+function TStaticDataSeries<T>.GetLookback: Int64;
 begin
-    Result := FMaxLookback;
+    Result := FLookback;
 end;
 
 function TStaticDataSeries<T>.GetTotalCount: Int64;
@@ -311,9 +324,14 @@ begin
     Result := FTotalCount;
 end;
 
+function TStaticDataSeries<T>.GetWriter: IDataSeriesWriter<T>;
+begin
+    Result := nil;
+end;
+
 function TStaticDataSeries<T>.IsIndexInLimits(LogicalIndex: Int64): Boolean;
 begin
-    Result := (FMaxLookback <= 0) or (LogicalIndex < FMaxLookback);
+    Result := (FLookback <= 0) or (LogicalIndex < FLookback);
 end;
 
 function TStaticDataSeries<T>.LogicalToPhysicalIndex(LogicalIndex: Int64): Int64;
diff --git a/Src/Myc.Trade.DataPoint.pas b/Src/Myc.Trade.DataPoint.pas
index cf09b02..48ddc62 100644
--- a/Src/Myc.Trade.DataPoint.pas
+++ b/Src/Myc.Trade.DataPoint.pas
@@ -17,25 +17,27 @@ type
         constructor Create(ATime: TDateTime; const AData: T);
     end;
 
+    IDataSeriesWriter<T> = interface
+        procedure Add(const Data: TDataPoint<T>); overload;
+        procedure Add(const Data: array of TDataPoint<T>; NumToAdd: Integer = -1); overload;
+        procedure Clear;
+    end;
+
     // A time-ordered series of data points, optimized for chronological additions.
     // The most recently added element has the logical index 0.
     IDataSeries<T> = interface
         function GetCount: Int64;
         function GetItems(Idx: Int64): TDataPoint<T>;
         function GetTotalCount: Int64;
-        function Copy: IDataSeries<T>;
+        function Immutable: IDataSeries<T>;
+        function GetWriter: IDataSeriesWriter<T>;
         property Count: Int64 read GetCount;
         // Accesses data points by their logical index.
         // Index 0 is the newest element, Index (Count - 1) is the oldest.
         property Items[Idx: Int64]: TDataPoint<T> read GetItems; default;
         // The total number of items ever added to the series.
         property TotalCount: Int64 read GetTotalCount;
-    end;
-
-    IDataArray<T> = interface(IDataSeries<T>)
-        procedure Add(const Data: TDataPoint<T>); overload;
-        procedure Add(const Data: array of TDataPoint<T>; NumToAdd: Integer = -1); overload;
-        procedure Clear;
+        property Writer: IDataSeriesWriter<T> read GetWriter;
     end;
 
     // Interface Helper for IDataSeries<T>.
@@ -43,12 +45,11 @@ type
     TDataSeries<T> = record
     private
         FDataSeries: IDataSeries<T>;
-        FMaxLookback: Int64;
         function GetCount: Int64;
         function GetItems(Idx: Int64): TDataPoint<T>;
         function GetData(Idx: Int64): T;
+        function GetIsWriteable: Boolean;
         function GetTime(Idx: Int64): TDateTime;
-        function GetMaxLookback: Int64;
         class function GetNull: IDataSeries<T>; static;
         function GetTotalCount: Int64;
     public
@@ -60,10 +61,14 @@ type
         class operator Implicit(const A: TDataSeries<T>): IDataSeries<T>;
         class operator Implicit(const A: IDataSeries<T>): TDataSeries<T>;
 
-        class function CreateArray(MaxLookback: Int64): IDataArray<T>; static;
+        class function CreateWriteable(MaxLookback: Int64): TDataSeries<T>; static;
 
-        // Create a guaranteed immutable version of the given series.
-        function Copy: TDataSeries<T>;
+        // Create an immutable version of the given series.
+        function Immutable: TDataSeries<T>;
+
+        // Writing (only if IsWriteable=true)
+        procedure Add(const Data: array of TDataPoint<T>; NumToAdd: Integer = -1);
+        procedure Clear;
 
         // Searches for a data point by its timestamp.
         // Returns the logical index of the matching item.
@@ -75,8 +80,8 @@ type
         property Count: Int64 read GetCount;
         property Items[Idx: Int64]: TDataPoint<T> read GetItems; default;
         property Data[Idx: Int64]: T read GetData;
+        property IsWriteable: Boolean read GetIsWriteable;
         property Time[Idx: Int64]: TDateTime read GetTime;
-        property MaxLookback: Int64 read GetMaxLookback;
         property TotalCount: Int64 read GetTotalCount;
     end;
 
@@ -109,12 +114,24 @@ begin
         FDataSeries := Null;
 end;
 
-function TDataSeries<T>.Copy: TDataSeries<T>;
+procedure TDataSeries<T>.Add(const Data: array of TDataPoint<T>; NumToAdd: Integer = -1);
 begin
-    Result := FDataSeries.Copy;
+    Assert( IsWriteable );
+    FDataSeries.Writer.Add( Data, NumToAdd );
 end;
 
-class function TDataSeries<T>.CreateArray(MaxLookback: Int64): IDataArray<T>;
+procedure TDataSeries<T>.Clear;
+begin
+    Assert( IsWriteable );
+    FDataSeries.Writer.Clear;
+end;
+
+function TDataSeries<T>.Immutable: TDataSeries<T>;
+begin
+    Result := FDataSeries.Immutable;
+end;
+
+class function TDataSeries<T>.CreateWriteable(MaxLookback: Int64): TDataSeries<T>;
 begin
     Result := TDataArray<T>.Create(MaxLookback);
 end;
@@ -154,16 +171,16 @@ begin
     Result := FDataSeries[Idx].Data;
 end;
 
+function TDataSeries<T>.GetIsWriteable: Boolean;
+begin
+    Result := Assigned( FDataSeries.Writer );
+end;
+
 function TDataSeries<T>.GetTime(Idx: Int64): TDateTime;
 begin
     Result := FDataSeries[Idx].Time;
 end;
 
-function TDataSeries<T>.GetMaxLookback: Int64;
-begin
-    Result := FMaxLookback;
-end;
-
 class function TDataSeries<T>.GetNull: IDataSeries<T>;
 begin
     Result := TNullDataSeries<T>.Null;
diff --git a/Src/Myc.Trade.DataProvider.pas b/Src/Myc.Trade.DataProvider.pas
index 4fe60dc..ccf8fcc 100644
--- a/Src/Myc.Trade.DataProvider.pas
+++ b/Src/Myc.Trade.DataProvider.pas
@@ -12,12 +12,13 @@ type
     TDataStreamProvider<T> = class(TInterfacedObject, TMutable<TDataSeries<T>>.IMutable)
     private
         FStream: IDataStream<T>;
-        FDataArray: IDataArray<T>;
+        FDataSeries: TDataSeries<T>;
         FNewDataAvailable: TFlag;
         FChunk: TArray<TDataPoint<T>>;
         FLookback: Int64;
         function GetChanged: TSignal;
         function GetValue: TDataSeries<T>;
+        function GetWriter: TMutable<TDataSeries<T>>.IWriter;
     public
         constructor Create(ALookback, AMaxChunkSize: Int64; const AStream: IDataStream<T>);
         destructor Destroy; override;
@@ -38,7 +39,7 @@ begin
     SetLength(FChunk, AMaxChunkSize);
     FStream := AStream;
     FNewDataAvailable := TFlag.CreateObserver(FStream.HasData);
-    FDataArray := TDataSeries<T>.CreateArray(ALookback);
+    FDataSeries := TDataSeries<T>.CreateWriteable(ALookback);
 end;
 
 destructor TDataStreamProvider<T>.Destroy;
@@ -57,9 +58,14 @@ begin
     begin
         var cnt := FStream.GetChunk(FChunk);
         if cnt > 0 then
-            FDataArray.Add(FChunk, cnt);
+            FDataSeries.Add(FChunk, cnt);
     end;
-    Result := FDataArray;
+    Result := FDataSeries;
+end;
+
+function TDataStreamProvider<T>.GetWriter: TMutable<TDataSeries<T>>.IWriter;
+begin
+    Result := nil;
 end;
 
 { TDataStreamProvider }
diff --git a/Src/Myc.Trade.DataStream.pas b/Src/Myc.Trade.DataStream.pas
index 21579e2..049219a 100644
--- a/Src/Myc.Trade.DataStream.pas
+++ b/Src/Myc.Trade.DataStream.pas
@@ -101,7 +101,8 @@ type
         // Used by cache to actually load an uncached file.
         function DoLoad(const FileName: string): TFuture<TArray<TDataPoint<T>>>;
 
-        function LoadFile(DataFile: TAuraDataFile; Terminated: TState; Proc: TDataProc<T>): TState;
+        function ProcessFile(FileInfo: TAuraDataFile; const DataFile: TFuture<TArray<TDataPoint<T>>>; Terminated: TState; Proc: TDataProc<T>):
+            TState;
 
     strict private
         class var
@@ -392,27 +393,35 @@ begin
 
     Result :=
         FindFirstFile(Symbol)
-            .Chain(function(const FirstFile: TAuraDataFile): TState begin Result := LoadFile(FirstFile, terminated, capProc); end);
+            .Chain(
+            function(const FirstFileInfo: TAuraDataFile): TState
+            begin
+                Result := ProcessFile(FirstFileInfo, LoadDataFile(FirstFileInfo), terminated, capProc);
+            end);
 end;
 
 function TAuraDataServer<T>.LoadDataFile(const DataFile: TAuraDataFile): TFuture<TArray<TDataPoint<T>>>;
 begin
-    Result := FCachedFiles.GetOrAdd(DataFile.GetFullFileName);
+    if DataFile.IsValid then
+        Result := FCachedFiles.GetOrAdd(DataFile.GetFullFileName);
 end;
 
-function TAuraDataServer<T>.LoadFile(DataFile: TAuraDataFile; Terminated: TState; Proc: TDataProc<T>): TState;
+function TAuraDataServer<T>.ProcessFile(FileInfo: TAuraDataFile; const DataFile: TFuture<TArray<TDataPoint<T>>>; Terminated: TState; Proc:
+    TDataProc<T>): TState;
 begin
-    if not DataFile.IsValid or Terminated.IsSet then
+    if not FileInfo.IsValid or Terminated.IsSet then
         exit(TState.Null);
 
-    var data := FCachedFiles.GetOrAdd(DataFile.GetFullFileName);
+    // Read ahead the next file, while processing the current one
+    var nextFileInfo := FileInfo.GetNextFile;
+    var nextFile := LoadDataFile( nextFileInfo );
 
     Result :=
-        data.Chain(
+        DataFile.Chain(
             function(const Data: TArray<TDataPoint<T>>): TState
             begin
                 Proc(Data, Terminated);
-                Result := LoadFile(DataFile.GetNextFile, Terminated, Proc);
+                Result := ProcessFile(nextFileInfo, nextFile, Terminated, Proc);
             end
         );
 end;