What is the difference between Receive and Peek in the context of Azure Service Bus Topics?
Peek: This method enables you to view messages without locking or receiving them.
Receive: Can work in 2 modes: PeekLock (receives the message but keeps it peek-locked until the receiver abandons the message. The maximum timeout is 5 minutes before message expiration) and ReceiveAndDelete (deletes the message after it is received).
So this means you'll use the Peek when your goal is to look at the messages without actually consuming them (maybe you're building a "Queue browser", ... or your process needs to decided if it wants/can consume the message). And you'll use Receive when you're really planning to consume the message and do whatever you need to do with it.
Here is documentation on the PeakLock and ReceiveAndDelete values for ReceiveMode. Note that PeakLock is the default.
Related
I am looking into setting up a web job trigger to read message from service bus queue. What would be the best practice to implement a retry logic in case of any errors handling the downstream systems.
Would we be able to throw an exception so that the message will not be deleted from the queue and will be retried after certain time period?
Appreciate your feedback.
You don't need to define retry logic explicitly. When the message is de-queued from service bus , it gets invisible from queue for certain time period (lock time default 30secs , you can configure it). You try to process the message , if it gets successful you simply call BrokeredMessage.CompleteAsync which means i am done and mark this message as completed. If you have some problem in down stream you can abandon the message by calling BrokeredMessage.AbandonAsync . This will unlock the message and the message appears back in the queue. The message will be picked up by the worker again and process it. Until you get successful or reach the max retry limit after which the message is send to dead letter queue.
I am working on Azure Service Bus. My service bus queue is processing one message 3 times. My lock time of message is 5 minutes. Every message is processing max of 2 mins but I don't know why the queue is picking same message and sending to processing and the duplicate messages are picking after 5 mins only.
How can I resolve this?
With Azure Service Bus messages will be re-processed when a message is not actioned by the receiving party. An action would be completing, deferring, dead-lettering. If you don't have any of those, once LockDuration on the broker side expires, the message will be re-delivered. Additional situation when a message would be re-delivered without waiting for LockDuration to time out would be to abandon a message. Then a message is picked up right away by the next request for new messages.
You should share your code to provide enough context. Messages can be received manually using MessageReceiver.ReceiveAsync() or using user-callback API. For the first option you have to action messages (complete for example). For the other option, there's a configuration API where you could opt-out of auto-completion and would be required manually complete message passed into user-callback.
According to this doc service bus supports two modes Receive-and-Delete and Peek-Lock.
If using Peek-Lock Mode if the consumer crashes/hangs/do a very long GC right after processing the message, but before the messageId is "Completed" and visibility time expires there's a chance that same message is delivered twice.
Then how does Microsoft says that Service Bus supports at most once delivery mode. Is it because of the Receive-and-Delete mode which sends messages only once.But then again, if something happens while consumers are processing the message then that valuable info is lost.
If yes then what is the best way to ensure exact once delivery using Azure Services Bus as Queue and Azure Functions as Consumers.
P.S. The one approach I can think of is storing MessageID's in blob but since in my case number of MessageID's could be very large storing and loading all of them is not right approach.
Azure Functions will always consume Service Bus messages in Peek-Lock mode. Exactly Once delivery is basically not possible in general case: there's always a chance that consuming application will crash at wrong time just before completing the message, and then the message will be re-delivered.
You should strive to implement Effectively Once processing. This is usually achieved with idempotent message processor.
Storing MessageID's (consumer-side de-duplication) is one option. You could have a policy to clean up old Message IDs to keep the size of such storage manageable. To make this 100% reliable you would have to store Message ID in the same transaction as other modifications done by processor.
Other options really depend on your processing scenario. Find a way to make it idempotent - so that processing the same message multiple times is functionally same as processing it just once.
I am using an Azure queue and have several different processes reading from the queue.
My system is built in a way that assumes each message is read only once.
This Microsoft article claims Azure queues have an at least once delivery guarantee which potentially means two processes can read the same message from the queue.
This StackOverflow thread claims that if I use GetMessage then the message becomes invisible to all other processes for the invisibility timeout.
Assuming I use GetMessage() and never exceed the message invisibility time before I DeleteMessage, can I assume I will get each message only once?
I think there is a property in queue message named DequeueCount, which is the number of times this message has been dequeued. And it's maintained by queue service. I think you can use this property to identify whether your message had been read before.
https://learn.microsoft.com/en-us/dotnet/api/azure.storage.queues.models.queuemessage.dequeuecount?view=azure-dotnet
No. The following can happen:
GetMessage()
Add some records in a database...
Generate some files...
DeleteMessage() -> Unexpected failure (process that crashes, instance that reboots, network connectivity issues, ...)
In this case your logic was executed without calling DeleteMessage. This means, once the invisibility timeout expires, the message will appear in the queue and be processed once again. You will need to make sure that your process is idempotent:
Idempotence is the property of certain operations in mathematics and
computer science, that they can be applied multiple times without
changing the result beyond the initial application.
An alternative solution would be to use Service Bus Queues with the ReceiveAndDelete mode (see this page under How to Receive Messages from a Queue). If you receive the message it will be marked as consumed and never appear again. This way you can be sure it is delivered At-Most-Once (see the comparison with Storage Queues here). But then again, if something happens while your are processing the message (ie: server crashes, ...), you could loose valuable information.
Update:
This will simulate an At-Most-Once in storage queues. The message can arrive multiple times via GetMessage, but will only be processed once by your business logic (with the risk that some of your business logic will never execute).
GetMessage()
DeleteMessage()
AddRecordsToDatabase()
GenerateFiles()
I'm just getting started with Windows Azure Service Bus (Topics & Queues) and I'm trying to implement a Competing-Consumers messaging pattern.
Essentially, I want to have a set of message Producers and a set of message Consumers. Once a message is produced, I want the first available Consumer to process the message. No other Consumers should get the message.
Is there a way to do this in Azure?
Simple. Just make two (or more) receivers that concurrently receive from a single queue and you're done. Any retrieved message goes to exactly one of those receivers since the cursor over the mesasage log is advanced as a message is taken. Competing consumers are an inherent capability of a networked queue so there's really nothing special needed.
If you need the opposite - each message goes to each consumer - you make a subscrioption per consumer which gives you an isolated cusor over the message log that can move independent of other receivers. For kicks, you can obviously also have competing consumers on a subscription.
Clemens
Topics are a feature of brokered messaging, but are a one-to-many "publish/subscribe" pattern. Queues are one-to-one message communication. So yes, it sounds like you should simply use queues. Also see http://msdn.microsoft.com/en-us/library/hh689723(VS.103).aspx.
You probably don't want Topics then, but rather Brokered Messaging.
You can emulate Topic-like functionality in Brokered Messaging by using the message's Label and/or Content Type properties along with the PeekLock receive mode.