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<h1><a name="kanchor64"></a><a name="kanchor65"></a>Messaging</h1>
<p>In client/server it possible to send messages between a client and the server. Possible use cases could be:</p>
<ul>
<li value="1">Shutting down and restarting the server.</li>
<li value="2">Triggering server backup.</li>
<li value="3">Using a customized login strategy to restrict the number of allowed client connections.</li>
<li value="4">Running special code on the server. For example batch updates.</li>
</ul>
<h2>Sending and Receiving Messages</h2>
<p>First you need to decide on a class that you want to use as the message. Any object that is storable in db4o can be used as a message. Of course you use multiple classes for representing different messages. Let's create a dedicated class.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">public class HelloMessage
{
private readonly string message;
public HelloMessage(string message)
{
this.message = message;
}
public override string ToString()
{
return message;
}
}
</pre>
<div class="codesnippet-meta">MessagingExample.cs: The message class
<div class="codedownload"><a href="../CodeExamples/clientserver/messaging/Example-clientserver-messaging-csharp.zip" class="codedownload" MadCap:conditions="Primary.Online">Download Code</a></div><div class="codedownload copylink-marker" MadCap:conditions="Primary.Online"><a href="#copy">Copy Code</a></div></div>
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Public Class HelloMessage
Private ReadOnly message As String
Public Sub New(ByVal message As String)
Me.message = message
End Sub
Public Overrides Function ToString() As String
Return message
End Function
End Class
</pre>
<div class="codesnippet-meta">MessagingExample.vb: The message class
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<p>Now you need to register a handler to handle arriving messages. This can be done by configuring a message recipient on the server. Let's simply print out the arriving message. Additionally we answer to the client with another message.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IServerConfiguration configuration = Db4oClientServer.NewServerConfiguration();
configuration.Networking.MessageRecipient = new ServerMessageReceiver();
IObjectServer server = Db4oClientServer.OpenServer(configuration, DatabaseFile, PortNumber);</pre>
<div class="codesnippet-meta">MessagingExample.cs: configure a message receiver for the server
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim configuration As IServerConfiguration = Db4oClientServer.NewServerConfiguration()
configuration.Networking.MessageRecipient = New ServerMessageReceiver()
Dim server As IObjectServer = Db4oClientServer.OpenServer(configuration, DatabaseFile, PortNumber)</pre>
<div class="codesnippet-meta">MessagingExample.vb: configure a message receiver for the server
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<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">class ServerMessageReceiver : IMessageRecipient
{
public void ProcessMessage(IMessageContext context, object message)
{
Console.WriteLine("The server received a '{0}' message", message);
// you can respond to the client
context.Sender.Send(new HelloMessage("Hi Client!"));
}
}</pre>
<div class="codesnippet-meta">MessagingExample.cs: The message receiver for the server
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Private Class ServerMessageReceiver
Implements IMessageRecipient
Public Sub ProcessMessage(ByVal context As IMessageContext, ByVal message As Object) _
Implements IMessageRecipient.ProcessMessage
Console.WriteLine("The server received a '{0}' message", message)
' you can respond to the client
context.Sender.Send(New HelloMessage("Hi Client!"))
End Sub
End Class</pre>
<div class="codesnippet-meta">MessagingExample.vb: The message receiver for the server
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<p>The same can be done on the client. Register a handler for the received messages. </p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IClientConfiguration configuration = Db4oClientServer.NewClientConfiguration();
configuration.Networking.MessageRecipient = new ClientMessageReceiver();</pre>
<div class="codesnippet-meta">MessagingExample.cs: configure a message receiver for a client
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim configuration As IClientConfiguration = Db4oClientServer.NewClientConfiguration()
configuration.Networking.MessageRecipient = New ClientMessageReceiver()</pre>
<div class="codesnippet-meta">MessagingExample.vb: configure a message receiver for a client
<div class="codedownload"><a href="../CodeExamples/clientserver/messaging/Example-clientserver-messaging-vb.zip" class="codedownload" MadCap:conditions="Primary.Online">Download Code</a></div><div class="codedownload copylink-marker" MadCap:conditions="Primary.Online"><a href="#copy">Copy Code</a></div></div>
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<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">class ClientMessageReceiver : IMessageRecipient
{
public void ProcessMessage(IMessageContext context, object message)
{
Console.WriteLine("The client received a '{0}' message",message);
}
}</pre>
<div class="codesnippet-meta">MessagingExample.cs: The message receiver for the client
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</div>
<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Private Class ClientMessageReceiver
Implements IMessageRecipient
Public Sub ProcessMessage(ByVal context As IMessageContext, ByVal message As Object) _
Implements IMessageRecipient.ProcessMessage
Console.WriteLine("The client received a '{0}' message", message)
End Sub
End Class</pre>
<div class="codesnippet-meta">MessagingExample.vb: The message receiver for the client
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<p>Now on the client we can get a message sender. The message sender allows you to send a message to the server. In this example we send a hello message.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IMessageSender sender = configuration.MessageSender;
using (IObjectContainer container = Db4oClientServer.OpenClient(configuration, "localhost",
PortNumber, UserAndPassword,
UserAndPassword))
{
sender.Send(new HelloMessage("Hi Server!"));
WaitForAWhile();
}</pre>
<div class="codesnippet-meta">MessagingExample.cs: Get the message sender and use it
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</div>
<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim sender As IMessageSender = configuration.MessageSender
Using container As IObjectContainer = Db4oClientServer.OpenClient(configuration, _
"localhost", PortNumber, UserAndPassword, UserAndPassword)
sender.Send(New HelloMessage("Hi Server!"))
WaitForAWhile()
End Using</pre>
<div class="codesnippet-meta">MessagingExample.vb: Get the message sender and use it
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