RabbitMq consume aysnc-CodePudding

I need to have an async consumer method to consume messages from RabbitMq. My problem is that the rabbitmq client for .net rely on an event handler. I tried to implement a blocking system with a Semaphore, which is working with a low volume. When I'm getting more volume, some messages are lost.

Here is my implementation :

    private long _lock;
    private string _message;
    private object _tag;
    private readonly SemaphoreSlim _signal;

    public void Configure()
    {
        Interlocked.Exchange(ref _lock, 0);

        var consumer = new EventingBasicConsumer(_channel);
        consumer.Received  = (sender, ea) =>
        {
            _message = Encoding.UTF8.GetString(ea.Body.ToArray());
            _tag = ea.DeliveryTag;
            Interlocked.Exchange(ref _lock, 1);
            _signal.Release();
        };

        _channel.BasicConsume(queue: _rabbitConfig.Queue, autoAck: true, consumer: consumer);
    }
    
    public async Task<string> Consume(CancellationToken cancellationToken)
    {
        while (0 == Interlocked.Read(ref _lock))
        {
            await _signal.WaitAsync();
        }
        Interlocked.Exchange(ref _lock, 0);
        return _message;
    }

I alse tried using BufferBlock but some messages are still lost. Is there any other way to implement a system keeping my Consume() method ?

CodePudding user response：

The issue with your code is that you are using a shared variable (_message) to store the received message, and you are not protecting this variable with a lock or other synchronization mechanism. This means that if multiple threads are calling the Consume() method at the same time, they could potentially access and modify the _message variable concurrently, leading to data races and possible data loss.

To fix this issue, you can use a lock statement to protect the shared variable and ensure that only one thread can access it at a time. This will prevent multiple threads from accessing and modifying the variable concurrently, and will ensure that the messages are consumed correctly.

Here is an example of how you could modify your code to use a lock statement to protect the shared variable:

private long _lock;
private string _message;
private object _tag;
private readonly SemaphoreSlim _signal;
private readonly object _syncRoot = new object();

public void Configure()
{
    Interlocked.Exchange(ref _lock, 0);

    var consumer = new EventingBasicConsumer(_channel);
    consumer.Received  = (sender, ea) =>
    {
        lock (_syncRoot)
        {
            _message = Encoding.UTF8.GetString(ea.Body.ToArray());
            _tag = ea.DeliveryTag;
        }
        Interlocked.Exchange(ref _lock, 1);
        _signal.Release();
    };

    _channel.BasicConsume(

CodePudding user response：

You're trying to create an asynchronous consumer for RabbitMQ using the .NET RabbitMQ client, but are running into issues with lost messages when the volume of messages increases. There are a few different ways you could approach this problem, but one option might be to use the BasicGet method of the RabbitMQ client instead of the BasicConsume method.

The BasicConsume method allows you to register an event handler for incoming messages, but it doesn't provide any built-in mechanism for ensuring that all messages are processed. In contrast, the BasicGet method allows you to retrieve a single message from a queue and process it without using an event handler. This means that you can use a while loop to repeatedly call BasicGet and process the messages one at a time, using a cancellation token to stop the loop when necessary.

Here's an example of how you might implement this using the BasicGet method:

public async Task<string> Consume(CancellationToken cancellationToken)
{
    while (!cancellationToken.IsCancellationRequested)
    {
        var result = _channel.BasicGet(_rabbitConfig.Queue, autoAck: true);
        if (result == null)
        {
            // No message was available in the queue, so we can wait a bit before checking again.
            await Task.Delay(TimeSpan.FromMilliseconds(100));
        }
        else
        {
            var message = Encoding.UTF8.GetString(result.Body.ToArray());
            return message;
        }
    }

    return null;
}

This approach should ensure that all messages are processed and should be more efficient than using an event handler and a SemaphoreSlim. Of course, there are many other ways to implement an asynchronous consumer for RabbitMQ, so you may want to experiment with different approaches to find one that works best for your specific use case.