What assumptions can I make about global time on A

Question

I want my Azure role to reprocess data in case of sudden failures. I consider the following option.

For every block of data to process I have a database table row and I could add a column meaning "time of last ping from a processing node". So when a node grabs a data block for processing it sets "processing" state and that time to "current time" and then it's the node responsibility to update that time say every one minute. Then periodically some node will ask for "all blocks that have processing state and ping time larger than ten minutes" and consider those blocks as abandoned and somehow queue them for reprocessing.

I have one very serious concern. The above approach requires that nodes have more or less the same time. Can I rely on all Azure nodes having the same time with some reasonable precision (say several seconds)?

Answer 1

Remote desktop into a role instance and check (a) the time zone (UTC, I think), and (b) that Internet Time is enabled in Date and Time settings. If so then you can rely on them being no more than a few ms apart. (This is not to say that the suggestions to use a message queue instead won't work, but perhaps they do not suit your needs.)

Answer 2

For processing times under 2 hrs, you can usually rely on queue semantics (visibility timeout). If you have the data stored in blob storage, you can have a worker pop a queue message containing the name of the blob to work on and set a reasonable visibility timeout on the message (up to 2 hrs today). Once it completes the work, it can delete the queue message. If it fails, the delete is never called and after the visibility timeout, it will reappear on the queue for reprocessing. This is why you want your work to be idempotent, btw.

For processing that lasts longer than two hours, I generally recommend a leasing strategy where the worker leases the underlying blob data (if possible or a dummy blob otherwise) using the intrisic lease functionality in Windows Azure blob storage. When a worker goes to retrieve a file, it tries to lease it. A file that is already leased is indicative of a worker role currently processing it. If failure occurs, the lease will be broken and it will become leasable by another instance. Leases must be renewed every min or so, but they can be held indefinitely.

Of course, you are keeping the data to be processed in blob storage, right? :)

As already indicated, you should not rely on synchronized times between VM nodes. If you store datetimes for any reason - use UTC or you will be sorry later.

Answer 3

The answer here isn't to use time based synchronization (if you would however, make sure you use UTCNow), but there is still no guarantee anywhere that the clocks are synced. Nor should there be.

For the problem you are describing a queue based system is the answer. I've been referencing a lot to it, and will do it again, but I've explained some benefits of queue based systems in my blog post.

The idea is the following:

1. You put a work item to the queue
2. Your worker role (one or many of them) peeks & locks the message
3. You try to process the message, if you succeed, you remove the message from the queue,
4. if not, you let it stay where it is

With your approach I would use AppFabric Queues because you can also have topics & subscriptions, which allows you to monitor the data items. The example in my blog post coveres this exact scenario, with the only difference being that instead of having a worker role I poll the queue from my web application. But the concept is the same.

Answer 4

I would try this a different way using queue storage. If you pop your block of data on a queue with a timeout then have your processing nodes (worker roles?) pull this data off the queue.

After the data is popped off the queue if the processing node does not delete the entry from the queue it will reappear on the queue for processing after the timeout period.