Why would someone want to do that? I have to unit-test exception handling mechanism in our application.
I presumed that dead letter queue is literally azure service bus queue, where I could publish messages using QueueClient
string dlQ = #"sb://**.servicebus.windows.net/**/Subscriptions/DefaultSubscription/$DeadLetterQueue";
string connectionString = CloudConfigurationManager.GetSetting("Microsoft.ServiceBus.ConnectionString");
NamespaceManager _namespaceManager = NamespaceManager.CreateFromConnectionString(connectionString);
QueueDescription qd = _namespaceManager.GetQueue(dataPromotionDLQ);
var queueClient = QueueClient.CreateFromConnectionString(connectionString, "DefaultSubscription/$DeadLetterQueue");
BrokeredMessage brokeredMessage = new BrokeredMessage("Message to PublishToDLQ");
try
{
queueClient.Send(brokeredMessage);
}
catch (Exception)
{
}
But I get MessagingEntityNotFoundException. What could be wrong?
You would never want to publish directly to a dead letter queue. It's where poisoned messages that can't be processed are placed.
There are two ways of placing messages onto the dead letter queue. The service bus itself dead-letters messages that have exceeded the maximum number of delivery attempts. You can also explicitly dead-letter a message that you have received using the DeadLetter() method.
Create your messages with a very short TTL via the BrokeredMessage.TimeToLive property.
The Subscription must have EnableDeadLetteringOnMessageExpiration set to true.
Though late here, adding to the answers of #Mikee and #Ben Morris may help someone. You can make use of #Mike's suggestion of making use of message.DeadLetter() or message.DeadLetterAsync() to dead-letter a message. Another suggestion can be to set very less or 0 second TimeToLive to move the messages to Dead letter.
After you perform any of these and try to view the messages in the Active end queue, you may still find that message is available sometimes (Which you are currently facing). The reason is that the messages that are dead-lettered due to TTLExpiredException, HeaderSizeExceeded or any system defined Errors, or manually Dead-Lettered messages like DeadLetter() methods are cleaned up by an asynchronous "garbage collection" program periodically. This doesn't occur immediately which we expect it to.
When you perform Peek operation, you can still see that the message is in the Active queue. You have to wait for the garbage collector to run or you can perform a Receive operation which forces the garbage collector to run first, thereby moving the messages to dead-letter before retrieval is done.
Related
I try to get 32 messages per request from Azure Queue.
queue.ApproximateMessageCount;
This code gives me the result of 1509. Telling me the connection is OK and it has records. Also I check in queue it really has 1509 records.
But when I try to retrieve records I don't get any record.
I do the following:
var messages = await queue.GetMessagesAsync(configuration.MessageBatchSize);
if (!messages.Any()) {
return;
}
It always goes in the if and returns.
What is going on here and what am I missing?
Do do that, receiving messages in batch mode, i use this kind of code :
var messages = await queueClient?.ReceiveBatchAsync(Max_Messages);
foreach (var message in messages)
{
await dispatcher.Dispatch(message); // do something with each message
}
But, for receiving messages with ReceiveBatchAsync, the queue have to be configured with the EnableBatchedOperations flag to true.
ApproximateMessageCount property represents the total number of messages available in queue at that particular moment. It does not represent that all messages (max #32 messages in a pull) are ready to be dequeued. You can use this property to infer that how many messages are in queue.
queue.ApproximateMessageCount;
If you could not retrieve the message by, GetMessagesAsync(numberOfMessages), then it says that all messages are not available or invisible for current QueueClient.
var cloudQueueMessages = await cloudQueue.GetMessagesAsync(numberOfMessages);
You could try polling the queue after sometime to see if messages came back to surface.
Note that, be advised of setting adequate visibility timeout for any message being dequeued to avoid indefinite starvation :)
I have created a queue in Azure Queue and enqueued two items in it. Using the nodejs sdk, i create a timer that executes every 5 secs and calls:
azure.createQueueService("precondevqueues", "<key>").getMessages(queueName, {numOfMessages : 1, visibilityTimeout: 1 }, callback)
I expect that the same message of the two in the queue to show up after every 5 secs but that does not seem to be the case. The output of this call alternates between the two messages.
This should not be the case since visibilityTimeout is set to 1 and hence, after 1 second, the message dequeued in the first call should be visible again before the next getMessage call is made.
As noted here, FIFO ordering is not guaranteed. So it may be the case, that most of the time messages are fetched in FIFO order, but that is not guaranteed and Azure can give you the messages in the order which is best for their implementation.
Messages are generally added to the end of the queue and retrieved
from the front of the queue, although first in, first out (FIFO)
behavior is not guaranteed.
Aha my mistake! I again read the getMessages documentation very carefully and realize that getMessages dequeues the message but retains a invisible copy outside of the queue. If the message processor does not delete the message before the visibility timeout expires, the copy is re-enqueued in the message and therefore they go to the end of the queue.
I'm beginning a project which will involve Azure Queue (not Service Bus).
I'm trying to figure out what is the best practice to find out whether there are messages waiting in the Queue.
AFAIK, there are two methods for that:
Using the ApproximateMessageCount property of the Queue object
Calling GetMessage, and if the returned value is null - there are no messages.
Which one is better performance-wise? Is there any difference?
From a billing POV, I understand there is a transaction cost for both of them, is that correct?
Thanks!
GetMessage is both faster and cheaper. GetMessage is also more correct from a logic perspective since the message count will return both messages that have already been retrieved by another reader as well as messages that have expired without being deleted.
I have also used this code in the past:
var cnnString = "the connection string";
var queueName = "the queue name";
var nsManager = NamespaceManager.CreateFromConnectionString(cnnString);
return nsManager.GetQueue(queueName).MessageCount;
That said - this was from about 4 months ago.
Any reason you need to do this (i.e. are you not just consuming messages off the queue?)
I'm using the Azure Service Bus SubscriptionClient.OnMessage method; configured to process up to 5 messages concurrently.
Within the code I need to wait for all messages to finish processing before I can continue (to properly shutdown an Azure Worker Role). How do I do this?
Will SubscriptionClient.Close() block until all messages have finished processing?
Calling Close on SubscriptionClient or QueueClient will not block. Calling Close closes off the entity immediately as far as I can tell. I tested quickly just using the Worker Role With Service Bus Queue project template that shipped with Windows Azure SDK 2.0. I added a thread sleep for many seconds in the message process action and then shut down the role while it was running. I saw the Close method get called while the messages were processing in their thread sleep but it certainly did not wait for the for message processing to complete, the role simple closed down.
To handle this gracefully you'll need to do the same thing we did when dealing with any worker role that was processing messages (Service Bus, Azure Storage queue or anything else): keep track of what is being worked on and shut down when it is complete. There are several ways to deal with that but all of them are manual and made messy in this case because of the multiple threads involved.
Given the way that OnMessage works you'll need to add something in the action that looks to see if the role has been told to shutdown, and if so, to not do any processing. The problem is, when the OnMessage action is executed it HAS a message already. You'd probably need to abandon the message but not exit the OnMessage action, otherwise it will keep getting a message if there are ones in the queue. You can't simply abandon the message and let the execution leave the action because then the system will be handed another message (possibly the same one) and several threads doing this may cause messages to get too many dequeue counts and get dead lettered. Also, you can't call Close on the SubscriptionClient or QueueClient, which would stop the receive loop internally, because once you call close any of the outstanding message processing will throw an exception when .Complete, .Abandon, etc. is called on the message because the message entity is now closed. This means you can't stop the incoming messages easily.
The main issue here is because you are using the OnMessage and setting up the concurrent message handling by setting the MaxConcurrentCalls on the OnMessageOptions, which means the code that starts and manages the threads is buried in the QueueClient and SubscriptionClient and you don't have control over that. You don't have a way to reduce the count of threads, or stop the threads individually, etc. You'll need to create a way to put the OnMessage action threads into a state where they are aware that the system is being told to shut down and then complete their message and not exit the action in order for them to not continuously be assigned new messages. This means you'll likely need to also set the MessageOptions to not use autocomplete and manually call complete in your OnMessage action.
Having to do all of this may severely reduce the actual benefit of using the OnMessage helper. Behind the scenes OnMessage is simply setting up a loop calling receive with the default timeout and handing of messages to another thread to do the action (loose description). So what you get by using the OnMessage approach is away from having to write that handler on your own, but then the problem you are having is because you didn't write that handler on your own you don't have control over those threads. Catch-22. If you really need to stop gracefully you may want to step away from the OnMessage approach, write your own Receive loop with threading and within the main loop stop receiving new messages and wait for all the workers to end.
One option, especially if the messages are idempotent (which means processing them more than once yields the same results... which you should be mindful of anyway) then if they are stopped in mid processing they will simply reappear on the queue to be processed by another instance later. If the work itself isn't resource intensive and the operations are idempotent then this really can be an option. No different than when an instance might fail due to hardware failure or other issues. Sure, it's not graceful or elegant, but it certainly removes all the complexity I've mentioned and is still something that can happen anyway due to other failures.
Note that the OnStop is called when an instance is told to shut down. You've got 5 minutes you can delay this until the fabric just shuts it off, so if your messages take longer than five minutes to process it won't really matter if you attempt to shut down gracefully or not, some will be cut off during processing.
You can tweak OnMessageAsync to wait for processing of messages to complete, and block new messages from beginning to be processed:
Here is the implementation:
_subscriptionClient.OnMessageAsync(async message =>
{
if (_stopRequested)
{
// Block processing of new messages. We want to wait for old messages to complete and exit.
await Task.Delay(_waitForExecutionCompletionTimeout);
}
else
{
try
{
// Track executing messages
_activeTaskCollection[message.MessageId] = message;
await messageHandler(message);
await message.CompleteAsync();
}
catch (Exception e)
{
// handle error by disposing or doing nothing to force a retry
}
finally
{
BrokeredMessage savedMessage;
if (!_activeTaskCollection.TryRemove(message.MessageId, out savedMessage))
{
_logger.LogWarning("Attempt to remove message id {0} failed.", savedMessage.MessageId);
}
}
}
}, onMessageOptions);
And an implementation of Stop that waits for completion:
public async Task Stop()
{
_stopRequested = true;
DateTime startWaitTime = DateTime.UtcNow;
while (DateTime.UtcNow - startWaitTime < _waitForExecutionCompletionTimeout && _activeTaskCollection.Count > 0)
{
await Task.Delay(_waitForExecutionCompletionSleepBetweenIterations);
}
await _subscriptionClient.CloseAsync();
}
Note that _activeTaskCollection is a ConcurrentDictionary (we can also use a counter with interlock to count the number of in progress messages, but using a dictionary allows you to investigate what happend easily in case of errors.
Problem:
I am reading messages from a Azure Storage Queue and then inserting them into a Storage Table using a Worker Role.
I want to read in messages but only process them if there are at least 100 (this is to optimize the Storage Table batch insert which is occurring). If there are less than 100 messages, then I want to cancel the message processing and make them immediately visible on the queue again for the next queue read.
Question:
Is it possible to mark a message which has just been read by CloudQueue.GetMessages(...) as visible without having to wait for the timeout to expire?
Code: (in WorkerRole.cs)
public override void Run()
{
while (true)
{
var messages = queue.GetMessages(100);
if (messages.Count() >= 100)
{
// This will process, insert into a table, and delete from the queue
ProcessMessages(messages);
}
else
{
//!!! MARK MESSAGES AS VISIBLE ON THE QUEUE
System.Threading.Thread.Sleep(1000);
}
}
}
Thanks
You can check the queue's `ApproximateMessageCount' property (details here), which will give you a rough idea how many messages are waiting in the queue.
Also: you can set a message's invisibility timeout to something small (maybe 5-10 seconds?). After that period, the message becomes visible again. You can also modify invisibility timeout to something shorter after you read it.
Just remember that reading from the queue counts as a transaction, as does updating messages (e.g. updating invisibility timeout).
Waiting for 100 messages may be a non-optimal optimization. Oh, and GetMessages()(details here) is limited to 32 messages, so it doesn't make sense to wait for 100. Also: Transactions are really, really cheap (a penny per 100K transactions). I don't necessarily see the value here.
Reset the expire time to 0.0. That will hopefully do the trick.