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Receiving Messages in Azure Service Bus

 3 years ago
source link: https://www.pmichaels.net/2020/12/26/receiving-messages-in-azure-service-bus/?utm_campaign=receiving-messages-in-azure-service-bus
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In this post I covered the basics of setting up a queue and sending a message to it. Here, I’m going to cover the options around receiving that message.

Essentially, there are two possibilities here: you can either set-up an event listener, or you can poll the queue directly, and receive the messages one at a time.

Option 1 – Events

The events option seems to be the one that Microsoft now prefer – essentially, you register a handler and then as the messages come in, you simply handle them inside an event. The code here looks something like this:

var queueClient = new QueueClient(connectionString, "test-queue");
var messageHandlerOptions = new MessageHandlerOptions(ExceptionHandler);
queueClient.RegisterMessageHandler(handleMessage, messageHandlerOptions);

The event handlers:

private static Task ExceptionHandler(ExceptionReceivedEventArgs arg)
{
Console.WriteLine("Something bad happened!");
return Task.CompletedTask;
}
private static Task handleMessage(Message message, CancellationToken cancellation)
{
string messageBody = Encoding.UTF8.GetString(message.Body);
Console.WriteLine("Message received: {0}", messageBody);
return Task.CompletedTask;
}

Option 2 – Polling

With this option, you simply ask for a message. You’ll need to use the approach for things like deferred messages (which I hope to cover in a future post):

var messageReceiver = new MessageReceiver(connectionString, "test-queue", ReceiveMode.ReceiveAndDelete);           
var message = await messageReceiver.ReceiveAsync();
string messageBody = Encoding.UTF8.GetString(message.Body);           
Console.WriteLine("Message received: {0}", messageBody);

Option 3 – Option 1, but cruelly force it into option 2

I thought I’d include this, although I would strongly advise against using it in most cases. If you wish, you can register an event, but force the event into a procedural call, so that you can await it finishing. You can do this by using the TaskCompletionSource. First, declare a TaskCompletionSource in your code (somewhere accessible):

private static TaskCompletionSource<bool> _taskCompletionSource;

Then, in handleMessage (see above), when you’ve received the message you want, set the result:

if (message.CorrelationId == correlationId)
{
await client.CompleteAsync(message.SystemProperties.LockToken);
_taskCompletionSource.SetResult(true);
}

Finally, after you’ve registered the message handler, just await this task:

queueClient.RegisterMessageHandler(
(message, cancellationToken) => handleMessage(correlationId, queueClient, message, cancellationToken),
messageHandlerOptions);
await _taskCompletionSource.Task;

References

Advanced Features with Azure Service Bus


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