Publish/Subscribe

This tutorial will introduce you to the fundamentals of the Solace API by connecting a client, adding a topic subscription and sending a message matching this topic subscription. This forms the basis for any publish / subscribe message exchange illustrated here:

Assumptions

This tutorial assumes the following:

• You are familiar with Solace core concepts.

• You have access to Solace messaging with the following configuration details:

  • Connectivity information for a Solace message-VPN
  • Enabled client username and password

One simple way to get access to Solace messaging quickly is to create a messaging service in Solace Cloud as outlined here. You can find other ways to get access to Solace messaging below.

Goals

The goal of this tutorial is to demonstrate the most basic messaging interaction using Solace. This tutorial will show you:

1. How to build and send a message on a topic
2. How to subscribe to a topic and receive a message

For the purposes of this tutorial, you will connect to the default message VPN of a Solace VMR so the only required information to proceed is the Solace VMR host string which this tutorial accepts as an argument.

Connecting to the Solace message router

In order to send or receive messages, an application must connect a Solace session. The Solace session is the basis for all client communication with the Solace message router.

In the Solace messaging API for C# (CSCSMP) before using the API, the ContextFactory must be initialized. This allows the API to initialize required state, threads, and logging. The ContextFactory is initialized with a set of ContextFactoryProperties.

ContextFactoryProperties cfp = new ContextFactoryProperties()
{
SolClientLogLevel = SolLogLevel.Warning
};
cfp.LogToConsoleError();
ContextFactory.Instance.Init(cfp);

Then the ContextFactory instance can be used to create the context IContext (see API concepts that is used to create Solace sessions (ISession) from a set of SessionProperties.

Notice the optional HandleMessage parameter in the CreateSession call. This is the message consumer. It needs to be present only for receiving a message (see details on how to receive a message in the next section of this tutorial).

SessionProperties sessionProps = new SessionProperties()
{
    Host = host,
    VPNName = VPNName,
    UserName = UserName,
    Password = Password,
    ReconnectRetries = DefaultReconnectRetries
};

Console.WriteLine("Connecting as {0}@{1} on {2}...", UserName, VPNName, host);

using (IContext context =
                ContextFactory.Instance.CreateContext(new ContextProperties(), null))
using (ISession session = context.CreateSession(sessionProps, HandleMessage, null))
{
    ReturnCode returnCode = session.Connect();
    if (returnCode == ReturnCode.SOLCLIENT_OK)
    {
        // connected to the Solace message router
    }
}

At this point your client is connected to the Solace message router. You can use PubSub+ Manager to view the client connection and related details.

Receiving a message

This tutorial uses “Direct” messages which are at most once delivery messages. So first, let’s express interest in the messages by subscribing to a Solace topic. Then you can look at publishing a matching message and see it received.

With a session connected in the previous step, the next step is to create a message consumer. Message consumers enable the asynchronous receipt of messages through callbacks. These callbacks are defined in CSCSMP by the message receive delegate.

private void HandleMessage(object source, MessageEventArgs args)
{
    Console.WriteLine("Received published message.");
    // Received a message
    using (IMessage message = args.Message)
    {
        // Expecting the message content as a binary attachment
        Console.WriteLine("Message content: {0}", Encoding.ASCII.GetString(message.BinaryAttachment));
        // finish the program
        WaitEventWaitHandle.Set();
    }
}

The message consumer code uses an event wait handle in this example to release the blocked main thread when a single message has been received.

You must subscribe to a topic in order to express interest in receiving messages. This tutorial uses the topic “tutorial/topic”.

ReturnCode returnCode = Session.Connect();
if (returnCode == ReturnCode.SOLCLIENT_OK)
{
    Console.WriteLine("Session successfully connected.");

    Session.Subscribe(ContextFactory.Instance.CreateTopic("tutorial/topic"), true);

    Console.WriteLine("Waiting for a message to be published...");
    WaitEventWaitHandle.WaitOne();
}

Then after the subscription is added, the consumer is started. At this point the consumer is ready to receive messages and the main thread is blocked until a message is received.

WaitEventWaitHandle.WaitOne();

Sending a message

Now it is time to send a message to the waiting consumer.

Creating and sending the message

To send a message, you must create a message and a topic. Both of these are created from the ContextFactory. This tutorial will send a Solace message with Text contents “Sample Message”.

using (IMessage message = ContextFactory.Instance.CreateMessage())
{
    message.Destination = ContextFactory.Instance.CreateTopic("tutorial/topic");
    message.BinaryAttachment = Encoding.ASCII.GetBytes("Sample Message");

    Console.WriteLine("Publishing message...");
    ReturnCode returnCode = session.Send(message);
    if (returnCode == ReturnCode.SOLCLIENT_OK)
    {
        Console.WriteLine("Done.");
    }
    else
    {
        Console.WriteLine("Publishing failed, return code: {0}", returnCode);
    }
}

At this point a message to the Solace message router has been sent and your waiting consumer will have received the message and printed its contents to the screen.

Summarizing

Combining the example source code shown above results in the following source code files:

• TopicPublisher.cs
• TopicSubscriber.cs

Building

Build it from Microsoft Visual Studio or command line:

> csc /reference:SolaceSystems.Solclient.Messaging_64.dll /optimize /out:TopicPublisher.exe  TopicPublisher.cs
> csc /reference:SolaceSystems.Solclient.Messaging_64.dll /optimize /out:TopicSubscriber.exe  TopicSubscriber.cs

You need SolaceSystems.Solclient.Messaging_64.dll (or SolaceSystems.Solclient.Messaging.dll) at compile and runtime time and libsolclient.dll at runtime in the same directory where your source and executables are.

Both DLLs are part of the Solace C#/.NET API distribution and located in solclient-dotnet\lib directory of that distribution.

Sample Output

First start the TopicSubscriber.exe so that it is up and waiting for published messages. Then you can use the TopicPublisher.exe sample to publish a message. Pass your Solace messaging router connection properties as parameters.

$ ./TopicSubscriber <host> <username>@<vpnname> <password>
Connecting as <username>@<vpnname> on <host>...
Session successfully connected.
Waiting for a message to be published...
Received published message.
Message content: Sample Message
Finished.

$ ./TopicPublisher <host> <username>@<vpnname> <password>
Connecting as <username>@<vpnname> on <host>...
Session successfully connected.
Publishing message...
Done.
Finished.

With that you now know how to successfully implement publish-subscribe message exchange pattern using Direct messages.