| # 7. Deep graphs We have already seen how db4o handles object associations, but our running example is still quite flat and simple, compared to real-world domain models. In particular we haven't seen how db4o behaves in the presence of recursive structures. We will emulate such a structure by replacing our history list with a linked list implicitely provided by the SensorReadout class.
`Imports System Namespace com.db4o.f1.chapter5 Public Class SensorReadout Private _time As DateTime Private _car As Car Private _description As String Private _next As SensorReadout Protected Sub New(ByVal time As DateTime, ByVal car As Car, ByVal description As String) _time = time _car = car _description = description _next = Nothing End Sub Public ReadOnly Property Car() As Car Get Return _car End Get End Property Public ReadOnly Property Time() As DateTime Get Return _time End Get End Property Public ReadOnly Property [Next]() As SensorReadout Get Return _next End Get End Property Public Sub Append(ByVal sensorReadout As SensorReadout) If _next Is Nothing Then _next = sensorReadout Else _next.Append(sensorReadout) End If End Sub Public Function CountElements() As Integer If _next Is Nothing Then Return 1 End If Return _next.CountElements() + 1 End Function Public Overloads Overrides Function ToString() As String Return String.Format("{0} : {1} : {2}", _car, _time, _description) End Function End Class End Namespace`
Our car only maintains an association to a 'head' sensor readout now.
`Imports System Namespace com.db4o.f1.chapter5 Public Class Car Private _model As String Private _pilot As Pilot Private _history As SensorReadout Public Sub New(ByVal model As String) _model = model _pilot = Nothing _history = Nothing End Sub Public Property Pilot() As Pilot Get Return _pilot End Get Set _pilot = value End Set End Property Public ReadOnly Property Model() As String Get Return _model End Get End Property Public Function GetHistory() As SensorReadout Return _history End Function Public Sub Snapshot() AppendToHistory(New TemperatureSensorReadout(DateTime.Now, Me, "oil", PollOilTemperature())) AppendToHistory(New TemperatureSensorReadout(DateTime.Now, Me, "water", PollWaterTemperature())) AppendToHistory(New PressureSensorReadout(DateTime.Now, Me, "oil", PollOilPressure())) End Sub Protected Function PollOilTemperature() As Double Return 0.1 * CountHistoryElements() End Function Protected Function PollWaterTemperature() As Double Return 0.2 * CountHistoryElements() End Function Protected Function PollOilPressure() As Double Return 0.3 * CountHistoryElements() End Function Public Overloads Overrides Function ToString() As String Return String.Format("{0}[{1}]/{2}", _model, _pilot, CountHistoryElements()) End Function Private Function CountHistoryElements() As Integer If _history Is Nothing Then Return 0 End If Return _history.CountElements() End Function Private Sub AppendToHistory(ByVal readout As SensorReadout) If _history Is Nothing Then _history = readout Else _history.Append(readout) End If End Sub End Class End Namespace`
## 7.1. Storing and updating No surprises here.
`[storeCar] Dim pilot As Pilot = New Pilot("Rubens Barrichello", 99) Dim car As Car = New Car("BMW") car.Pilot = pilot db.[Set](car)`
Now we would like to build a sensor readout chain. We already know about the update depth trap, so we configure this first.
`[setCascadeOnUpdate] Db4oFactory.Configure().ObjectClass(GetType(Car)).CascadeOnUpdate(True)`
Let's collect a few sensor readouts.
`[takeManySnapshots] Dim result As ObjectSet = db.[Get](GetType(Car)) Dim car As Car = DirectCast(result.[Next](), Car) Dim i As Integer = 0 While i < 5 car.Snapshot() System.Math.Max(System.Threading.Interlocked.Increment(i),i - 1) End While db.[Set](car)`
## 7.2. Retrieving Now that we have a sufficiently deep structure, we'll retrieve it from the database and traverse it. First let's verify that we indeed have taken lots of snapshots.
`[retrieveAllSnapshots] Dim result As ObjectSet = db.[Get](GetType(SensorReadout)) While result.HasNext() Console.WriteLine(result.[Next]()) End While`
OUTPUT: BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 4.2 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 2.6 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 1.2000000000000002 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 3.3 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 2.0 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.9 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 2.4 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.6000000000000001 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 0.6 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 1.4000000000000001 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 1.5 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 0.8 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.30000000000000004 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 0.2 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.0

All these readouts belong to one linked list, so we should be able to access them all by just traversing our list structure.

`[retrieveSnapshotsSequentially] Dim result As ObjectSet = db.[Get](GetType(Car)) Dim car As Car = DirectCast(result.[Next](), Car) Dim readout As SensorReadout = car.GetHistory() While Not readout Is Nothing Console.WriteLine(readout) readout = readout.[Next] End While`

OUTPUT: BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.0 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 0.2 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 0.6 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.30000000000000004 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 0.8 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 1.5 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.6000000000000001 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 1.4000000000000001 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 2.4 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.9 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 2.0 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 3.3 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 1.2000000000000002 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 2.6 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 4.2

Ouch! What's happening here?

## 7.2.1. Activation depth

Deja vu - this is just the other side of the update depth issue.
db4o cannot track when you are traversing references from objects retrieved from the database. So it would always have to return 'complete' object graphs on retrieval - in the worst case this would boil down to pulling the whole database content into memory for a single query.
This is absolutely undesirable in most situations, so db4o provides a mechanism to give the client fine-grained control over how much he wants to pull out of the database when asking for an object. This mechanism is called activation depthand works quite similar to our familiar update depth.
The default activation depth for any object is 5, so our example above runs into nulls after traversing 5 references.
We can dynamically ask objects to activate their member references. This allows us to retrieve each single sensor readout in the list from the database just as needed.

`[retrieveSnapshotsSequentiallyImproved] Dim result As ObjectSet = db.[Get](GetType(Car)) Dim car As Car = DirectCast(result.[Next](), Car) Dim readout As SensorReadout = car.GetHistory() While Not readout Is Nothing db.Activate(readout, 1) Console.WriteLine(readout) readout = readout.[Next] End While`

OUTPUT: BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.0 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 0.2 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 0.6 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.30000000000000004 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 0.8 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 1.5 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.6000000000000001 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 1.4000000000000001 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 2.4 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.9 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 2.0 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 3.3 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 1.2000000000000002 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 2.6 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 4.2

Note that 'cut' references may also influence the behavior of your objects: In this case the length of the list is calculated dynamically, and therefor constrained by activation depth.
Instead of dynamically activating subgraph elements, you can configure activation depth statically, too. We can tell our SensorReadout class objects to cascade activation automatically, for example.

`[setActivationDepth] Db4oFactory.Configure().ObjectClass(GetType(TemperatureSensorReadout)).CascadeOnActivate(True)`

OUTPUT:

`[retrieveSnapshotsSequentially] Dim result As ObjectSet = db.[Get](GetType(Car)) Dim car As Car = DirectCast(result.[Next](), Car) Dim readout As SensorReadout = car.GetHistory() While Not readout Is Nothing Console.WriteLine(readout) readout = readout.[Next] End While`

OUTPUT: BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.0 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 0.2 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 0.6 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.30000000000000004 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 0.8 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 1.5 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.6000000000000001 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 1.4000000000000001 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 2.4 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 0.9 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 2.0 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 3.3 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil temp : 1.2000000000000002 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : water temp : 2.6 BMW[Rubens Barrichello/99]/15 : Wed Jul 05 10:33:49 CEST 2006 : oil pressure : 4.2

You have to be very careful, though. Activation issues are tricky. Db4o provides a wide range of configuration features to control activation depth at a very fine-grained level. You'll find those triggers in com.db4o.config.Configuration and the associated ObjectClass and ObjectField classes.
Don't forget to clean up the database.

`[deleteAll] Dim result As ObjectSet = db.[Get](GetType(Object)) For Each item As Object In result db.Delete(item) Next`

## 7.3. Conclusion

Now we should have the tools at hand to work with arbitrarily complex object graphs. But so far we have only been working forward, hoping that the changes we apply to our precious data pool are correct. What if we have to roll back to a previous state due to some failure? In the next chapter we will introduce the db4o transaction concept.

## 7.4. Full source

`Imports System Imports System.IO Imports com.db4o Namespace com.db4o.f1.chapter5 Public Class DeepExample Inherits Util Public Shared Sub Main(ByVal args As String()) File.Delete(Util.YapFileName) Dim db As ObjectContainer = Db4oFactory.OpenFile(Util.YapFileName) Try StoreCar(db) db.Close() SetCascadeOnUpdate() db = Db4oFactory.OpenFile(Util.YapFileName) TakeManySnapshots(db) db.Close() db = Db4oFactory.OpenFile(Util.YapFileName) RetrieveAllSnapshots(db) db.Close() db = Db4oFactory.OpenFile(Util.YapFileName) RetrieveSnapshotsSequentially(db) RetrieveSnapshotsSequentiallyImproved(db) db.Close() SetActivationDepth() db = Db4oFactory.OpenFile(Util.YapFileName) RetrieveSnapshotsSequentially(db) Finally db.Close() End Try End Sub Public Shared Sub StoreCar(ByVal db As ObjectContainer) Dim pilot As Pilot = New Pilot("Rubens Barrichello", 99) Dim car As Car = New Car("BMW") car.Pilot = pilot db.[Set](car) End Sub Public Shared Sub SetCascadeOnUpdate() Db4oFactory.Configure().ObjectClass(GetType(Car)).CascadeOnUpdate(True) End Sub Public Shared Sub TakeManySnapshots(ByVal db As ObjectContainer) Dim result As ObjectSet = db.[Get](GetType(Car)) Dim car As Car = DirectCast(result.[Next](), Car) Dim i As Integer = 0 While i < 5 car.Snapshot() System.Math.Max(System.Threading.Interlocked.Increment(i),i - 1) End While db.[Set](car) End Sub Public Shared Sub RetrieveAllSnapshots(ByVal db As ObjectContainer) Dim result As ObjectSet = db.[Get](GetType(SensorReadout)) While result.HasNext() Console.WriteLine(result.[Next]()) End While End Sub Public Shared Sub RetrieveSnapshotsSequentially(ByVal db As ObjectContainer) Dim result As ObjectSet = db.[Get](GetType(Car)) Dim car As Car = DirectCast(result.[Next](), Car) Dim readout As SensorReadout = car.GetHistory() While Not readout Is Nothing Console.WriteLine(readout) readout = readout.[Next] End While End Sub Public Shared Sub RetrieveSnapshotsSequentiallyImproved(ByVal db As ObjectContainer) Dim result As ObjectSet = db.[Get](GetType(Car)) Dim car As Car = DirectCast(result.[Next](), Car) Dim readout As SensorReadout = car.GetHistory() While Not readout Is Nothing db.Activate(readout, 1) Console.WriteLine(readout) readout = readout.[Next] End While End Sub Public Shared Sub SetActivationDepth() Db4oFactory.Configure().ObjectClass(GetType(TemperatureSensorReadout)).CascadeOnActivate(True) End Sub End Class End Namespace`

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