Running

When using Visual Studio 2015+ the NuGet packages should be restored automatically. If you find that they do not or if you are using an older version of Visual Studio please execute the following in a Visual Studio command prompt:

cd {extraction-folder}\Shuttle.Esb.Samples\Shuttle.Idempotence
nuget restore

Once you have opened the Shuttle.Idempotence.sln solution in Visual Studio set the following projects as startup projects:

• Shuttle.Idempotence.Client
• Shuttle.Idempotence.Server

Set Shuttle.Core.Host.exe as the Start external program option by navigating to the bin\debug folder of the server project for the Shuttle.Idempotence.Server project.

You will also need to create and configure a Sql Server database for this sample and remember to update the App.config connectionString settings to point to your database. Please reference the Database section below.

Implementation

When operations, or in our case messages, can be applied multiple with the same result they are said to be idempotent. Idempotence is something you should strive to implement directly on your endpoint by keeping track of some unique property of each message and whether the operation has been completed for that unique property.

An IIdempotenceService implementation can assist with this from a technical point-of-view by allowing a particular message id to be handled only once. This works fine for our at-least-once delivery mechanism where, in some edge case, we may receive the same message again. However, it will not aid us when two messages are going to be sent, each with its own message id, but they have contain the same data.

In this guide we’ll create the following projects:

• a Console Application called Shuttle.Idempotence.Client
• a Class Library called Shuttle.Idempotence.Server
• and another Class Library called Shuttle.Idempotence.Messages that will contain all our message classes

Messages

Create a new class library called Shuttle.Idempotence.Messages with a solution called Shuttle.Idempotence

Note: remember to change the Solution name.

RegisterMemberCommand

Rename the Class1 default file to RegisterMemberCommand and add a UserName property.

namespace Shuttle.Idempotence.Messages
{
	public class RegisterMemberCommand
	{
		public string UserName { get; set; }
	}
}

Client

Add a new Console Application to the solution called Shuttle.Idempotence.Client.

Install the Shuttle.Esb.Msmq nuget package.

This will provide access to the Msmq IQueue implementation and also include the required dependencies.

Add a reference to the Shuttle.Idempotence.Messages project.

Program

Implement the main client code as follows:

using System;
using Shuttle.Esb;
using Shuttle.Idempotence.Messages;

namespace Shuttle.Idempotence.Client
{
	class Program
	{
		static void Main(string[] args)
		{
			using (var bus = ServiceBus.Create().Start())
			{
				string userName;

				while (!string.IsNullOrEmpty(userName = Console.ReadLine()))
				{
					var command = new RegisterMemberCommand
					{
						UserName = userName
					};

					var transportMessage = bus.CreateTransportMessage(command, null);

					for (var i = 0; i < 5; i++)
					{
						bus.Dispatch(transportMessage); // will be processed once since message id is the same
					}

					bus.Send(command); // will be processed since it has a new message id
					bus.Send(command); // will also be processed since it also has a new message id
				}
			}
		}
	}
}

Keep in mind that the when you Send a message a TransportMessage envelope is created with a unique message id (Guid). In the above code we first manually create a TransportMessage so that we can send technically identical messages.

The next two Send operations do not use the TransportMessage but rather send individual messages. These will each have a TransportMessage envelope and, therefore, each have its own unique message id.

App.config

Create the shuttle configuration as follows:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
	<configSections>
		<section name='serviceBus' type="Shuttle.Esb.ServiceBusSection, Shuttle.Esb"/>
	</configSections>

	<serviceBus>
		<messageRoutes>
			<messageRoute uri="msmq://./shuttle-server-work">
				<add specification="StartsWith" value="Shuttle.Idempotence.Messages" />
			</messageRoute>
		</messageRoutes>		
	</serviceBus>
	
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>
</configuration>

This tells shuttle that all messages that are sent and have a type name starting with Shuttle.Idempotence.Messages should be sent to endpoint msmq://./shuttle-server-work.

Server

Add a new Class Library to the solution called Shuttle.Idempotence.Server.

Install noth the Shuttle.Esb.Msmq and Shuttle.Esb.SqlServer nuget packages.

This will provide access to the Msmq IQueue implementation and also include the required dependencies.

We will also have access to the Sql Server implementation of the IIdempotenceService.

Install the Shuttle.Core.Host nuget package.

The default mechanism used to host an endpoint is by using a Windows service. However, by using the Shuttle.Core.Host executable we are able to run the endpoint as a console application or register it as a Windows service for deployment.

Add a reference to the Shuttle.Idempotence.Messages project.

Host

Rename the default Class1 file to Host and implement the IHost and IDisposabe interfaces as follows:

using System;
using Shuttle.Core.Host;
using Shuttle.Esb;
using Shuttle.Esb.SqlServer.Idempotence;

namespace Shuttle.Idempotence.Server
{
	public class Host : IHost, IDisposable
	{
		private IServiceBus _bus;

		public void Start()
		{
			_bus = ServiceBus.Create(c=>c.IdempotenceService(IdempotenceService.Default())).Start();
		}

		public void Dispose()
		{
			_bus.Dispose();
		}
	}
}

Database

We need a store for our idempotence tracking. In this example we will be using Sql Server. If you use the express version remember to change the data source value to .\sqlexpress from the standard ..

When you reference the Shuttle.Esb.SqlServer package a number of scripts are included in the relevant package folder:

• .\Shuttle.PublishSubscribe\packages\Shuttle.Esb.SqlServer.{version}\scripts

The {version} bit will be in a semver format.

Create a new database called Shuttle and execute script IdempotenceServiceCreate.sql in the newly created database.

Whenever Publish is called the registered ISubscriptionManager instance is asked for the subscribers to the published message type. These are retrieved from the Sql Server database for the implementation we are using.

App.config

Add an Application Configuration File item to create the App.config and populate as follows:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
	<configSections>
		<section name='serviceBus' type="Shuttle.Esb.ServiceBusSection, Shuttle.Esb"/>
	</configSections>

	<connectionStrings>
		<add name="Idempotence"
			 connectionString="Data Source=.;Initial Catalog=shuttle;Integrated Security=SSPI;"
			 providerName="System.Data.SqlClient"/>
	</connectionStrings>

	<serviceBus>
		 <inbox
			workQueueUri="msmq://./shuttle-server-work"
			errorQueueUri="msmq://./shuttle-error" />
	</serviceBus>
</configuration>

RegisterMemberHandler

Add a new class called RegisterMemberHandler that implements the IMessageHandler<RegisterMemberCommand> interface as follows:

using System;
using Shuttle.Esb;
using Shuttle.Idempotence.Messages;

namespace Shuttle.Idempotence.Server
{
	public class RegisterMemberHandler : IMessageHandler<RegisterMemberCommand>
	{
		public void ProcessMessage(IHandlerContext<RegisterMemberCommand> context)
		{
			Console.WriteLine();
			Console.WriteLine("[MEMBER REGISTERED] : user name = '{0}' / message id = '{1}'",
				context.Message.UserName,
				context.TransportMessage.MessageId);
			Console.WriteLine();
		}
	}
}

This will write out some information to the console window.

Set Shuttle.Core.Host.exe as the Start external program option by navigating to the bin\debug folder of the server project.

Run

Set both the client and server projects as the startup.

Execute

Execute the application.

The client application will wait for you to input a user name. For this example enter my user name and press enter:

You have now implemented message idempotence.