Db4o's solution for the best collection performance and lowest memory consumption is to implement them directly on top of BTrees without an intermediate "stored-object-db4o" layer (P1Object, P1Collection, P2LinkedList).
This task is still under development, but already it makes sense to be ready to switch to the new fast collections seamlessly.
Current recommendation for collection usage with db4o is:
Please, avoid the following realizations, which will make the switching more difficult:
Let's look at application design, which will allow you to upgrade your application to fast collections with the least effort.
In our example we will save a list of pilots as members of one team. To make it simple let's use the following factory class to get the proper list implementation:
01/* Copyright (C) 2004 - 2007 db4objects Inc. http://www.db4o.com */ 02
using System; 03
using System.Collections; 04
05
namespace Db4objects.Db4odoc.Lists 06
{ 07
public class CollectionFactory 08
{ 09
public static IList newList() 10
{ 11
return new VerboseList(); 12
} 13
} 14
}
01' Copyright (C) 2004 - 2007 db4objects Inc. http://www.db4o.com 02
03
Imports System 04
Imports System.Collections 05
06
Namespace Db4objects.Db4odoc.Lists 07
Public Class CollectionFactory 08
Public Shared Function NewList() As IList 09
Return New VerboseList() 10
End Function 11
End Class 12
End Namespace
The concrete class returned by the CollectionFactory can be changed to any other collection implementation (fast collection) with the minimum coding effort.
We will use the following class as a team of pilots:
01/* Copyright (C) 2004 - 2007 db4objects Inc. http://www.db4o.com */ 02
using System; 03
using System.Collections; 04
05
namespace Db4objects.Db4odoc.Lists 06
{ 07
public class Team 08
{ 09
private IList _pilots; 10
private string _name; 11
12
public Team() 13
{ 14
_pilots = CollectionFactory.newList(); 15
} 16
17
public string Name 18
{ 19
get 20
{ 21
return _name; 22
} 23
24
set 25
{ 26
_name = value; 27
} 28
} 29
30
public void AddPilot(Pilot pilot) 31
{ 32
_pilots.Add(pilot); 33
} 34
35
public Pilot GetPilot(int index) 36
{ 37
return (Pilot)_pilots[index]; 38
} 39
40
public void RemovePilot(int index) 41
{ 42
_pilots.Remove(index); 43
} 44
45
public void UpdatePilot(int index, Pilot newPilot) 46
{ 47
_pilots[index] = newPilot; 48
} 49
} 50
}
01' Copyright (C) 2004 - 2007 db4objects Inc. http://www.db4o.com 02
Imports System.Collections 03
04
Namespace Db4objects.Db4odoc.Lists 05
Public Class Team 06
Private _pilots As IList 07
Private _name As String 08
09
Public Sub New() 10
_pilots = CollectionFactory.NewList() 11
End Sub 12
13
Public Property Name() As String 14
Get 15
Return _name 16
End Get 17
Set(ByVal Value As String) 18
_name = Value 19
End Set 20
End Property 21
22
Public Sub AddPilot(ByVal pilot As Pilot) 23
_pilots.Add(pilot) 24
End Sub 25
26
Public Function GetPilot(ByVal index As Integer) As evaluations.Pilot 27
Return CType(_pilots(index), evaluations.Pilot) 28
End Function 29
30
Public Sub RemovePilot(ByVal index As Integer) 31
_pilots.Remove(index) 32
End Sub 33
34
Public Sub UpdatePilot(ByVal index As Integer, ByVal NewPilot As evaluations.Pilot) 35
_pilots(index) = NewPilot 36
End Sub 37
End Class 38
End Namespace
Let's try to store our team:
01private static void SetTeam() 02
{ 03
File.Delete(Db4oFileName); 04
IObjectContainer db = Db4oFactory.OpenFile(Db4oFileName); 05
try 06
{ 07
Team ferrariTeam = new Team(); 08
ferrariTeam.Name = "Ferrari"; 09
10
Pilot pilot1 = new Pilot("Michael Schumacher", 100); 11
ferrariTeam.AddPilot(pilot1); 12
Pilot pilot2 = new Pilot("David Schumacher", 98); 13
ferrariTeam.AddPilot(pilot2); 14
15
db.Set(ferrariTeam); 16
IList protoList = CollectionFactory.newList(); 17
IObjectSet result = db.Get(protoList); 18
ListResult(result); 19
} 20
finally 21
{ 22
db.Close(); 23
} 24
}
01Public Shared Sub SetTeam() 02
File.Delete(Db4oFileName) 03
Dim db As IObjectContainer = Db4oFactory.OpenFile(Db4oFileName) 04
Try 05
Dim ferrariTeam As Team = New Team() 06
ferrariTeam.Name = "Ferrari" 07
08
Dim pilot1 As Pilot = New Pilot("Michael Schumacher", 100) 09
ferrariTeam.AddPilot(pilot1) 10
Dim pilot2 As Pilot = New Pilot("David Schumacher", 98) 11
ferrariTeam.AddPilot(pilot2) 12
13
db.Set(ferrariTeam) 14
Dim protoList As IList = CollectionFactory.NewList() 15
Dim result As IObjectSet = db.Get(protoList) 16
ListResult(result) 17
Finally 18
db.Close() 19
End Try 20
End Sub
If we want to update one of the pilots, we will have to retrieve the whole collection:
01private static void UpdateTeam() 02
{ 03
IObjectContainer db = Db4oFactory.OpenFile(Db4oFileName); 04
try 05
{ 06
IQuery query =db.Query(); 07
query.Constrain(typeof(Team)); 08
query.Descend("_name").Constrain("Ferrari"); 09
IObjectSet result = query.Execute(); 10
if (result.HasNext()) 11
{ 12
Team ferrariTeam = (Team)result.Next(); 13
14
Pilot pilot = new Pilot("David Schumacher", 100); 15
ferrariTeam.UpdatePilot(1,pilot); 16
17
db.Set(ferrariTeam); 18
} 19
IList protoList = CollectionFactory.newList(); 20
result = db.Get(protoList); 21
ListResult(result); 22
} 23
finally 24
{ 25
db.Close(); 26
} 27
}
01Public Shared Sub UpdateTeam() 02
Dim db As IObjectContainer = Db4oFactory.OpenFile(Db4oFileName) 03
Try 04
Dim query As IQuery = db.Query() 05
query.Constrain(GetType(Team)) 06
query.Descend("_name").Constrain("Ferrari") 07
Dim result As IObjectSet = query.Execute() 08
If result.HasNext() Then 09
Dim ferrariTeam As Team = CType(result.Next(), Team) 10
11
Dim pilot As Evaluations.Pilot = New Evaluations.Pilot("David Schumacher", 100) 12
ferrariTeam.UpdatePilot(1, pilot) 13
14
db.Set(ferrariTeam) 15
End If 16
Dim protoList As IList = CollectionFactory.NewList() 17
result = db.Get(protoList) 18
ListResult(result) 19
Finally 20
db.Close() 21
End Try 22
End Sub
The idea of the new fast collection implementation is to allow select/update of collection elements without an intermediate "stored-object-db4o" layer. This will allow random activation and fast querying, thus providing a considerable performance improvement especially on big collections holding deep object graphs.