Using Azure Queues as a State Machine

Question

I'd like to use Azure Queues as a state machine for a high-load/high-scale web service.

The client would submit a request to a web service endpoint, at which point i'd return a request id.

I'd then submit the message to a queue so that a worker role can process it, but no database activity occurs during the submission process. Instead, I want to use the queue that the message lives in to represent it's current state.

My problem is that if a worker role grabs the message off the queue to process it, it becomes invisible on that queue. If I want to check the status of the processing of that message, I have an ambiguous message state. Either the message was lost/never received, or it's in the queue but invisible because it's being processed.

Ideally, I'd like to be able to peak at the invisible message. If I find one that matches the request id, I know it's being processed if it's invisible, or it's waiting to be processed if it's visible. Obviously, I know when it's completed processing because that operation will result in a database write.

So is this possible, or is the fact that I can't peek at invisible messages in an Azure queue make this a no?

Answer 1

Windows Azure Storage Queues are for message-passing. They're not going to help you for state-machine processing, especially since each message can be processed at least once (since an app can run into an unexpected exception case while processing a message, the vm instance could crash, etc., and then the queue message re-appears after timeout (and now potentially out of order with the rest of your messages.

You're better off using an Azure Table row (or SQL table row).

Answer 2

In this case, I'd recommend using a blob to store the status of the message. Whenever a worker picks up a message, the blobID could be included and the worker can update the status blob. Your out-of-band process/website/whatever can query the blob to gather status information.