Question

I’m working on an Azure based project for some research and have been running into some issues when deleting messages from a CloudQueue instance. The code is fairly straightforward, so I’m a bit baffled as to why an exception is being thrown when I try to delete a message from the queue.

Here is the code that produces data for the queue:

foreach (var cell in scheme(cells))
{
    string id = Guid.NewGuid().ToString();
    var blob = sweepItemContainer.GetBlobReference(id);
    using (BlobStream stream = blob.OpenWrite())
    {
        BinaryFormatter bf = new BinaryFormatter();
        bf.Serialize(stream, cell);
    }
    sweepItemQueue.AddMessage(new CloudQueueMessage(id), new TimeSpan(1, 0, 0));
}

Here is the code that consumes the data from the queue:

var msgs = sweepItemsQueue.GetMessages(MsgAmt);
foreach (var msg in msgs)
{
     _handleMessage(msg, sweepItemsContainer);
     sweepItemsQueue.DeleteMessage(msg);
     mergeItemsQueue.AddMessage(new CloudQueueMessage(msg.AsString), new TimeSpan(1, 0, 0));
}

I don’t see how the message cannot exist in the queue. Nothing else is mutating the queue besides other consumers. But I am assured that they cannot get the same message (so long as the timespan doesn’t run out), so how is this happening?

Answer 1

There are two timeouts that you need to worry about, how long the message lives in the queue (which you’ve specified in the your .AddMessage() call and the visibility timeout that is set when you call .GetMessages() (by default this is 30 seconds, there is an overload that allows you to specify the timeout). When you call .GetMessages() all of the messages returned are invisible to other consumers for the period ‘visibilityTimeout’. Once this period finishes all of the messages you haven’t already deleted become visible to all other consumers.

To check if this is the problem I would try using the overload of .GetMessages() with it’s maximum visibility timeout of 2 hours. If this is the problem you can fine tune this value down to a more sensible number. Another option would be to just retrieve one message at a time.