At least once is generally the case in queuing systems unless you can
do a distributed transaction with your consumer.  What comes in handy
in an at least once case, is letting the consumer know that a message
may have 'potentially' already been processed.  That way he can double
check first before he goes off and processes the message again.  But
adding that info in ZK might be more expensive that doing the double
check every time in consumer anyways.

On Thu, Jan 8, 2009 at 11:42 AM, Benjamin Reed <> wrote:
> We should expand that section. the current queue recipe guarantees that 
> things are consumed at most once. to guarantee at least the consumer creates 
> an ephemeral node queue-X-inprocess to indicate that the node is being 
> processed. once the queue element has been processed the consumer deletes 
> queue-X and queue-X-inprocess (in that order).
> using an emphemeral node means that if a consumer crashes, the *-inprocess 
> node will be deleted allowing the queue elements it was working on to be 
> consumed by someone else. putting the *-inprocess nodes at the same level of 
> the queue-X nodes allows the consumer to get the list of queue elements and 
> the inprocess flags with the same getChildren call. the *-inprocess flag 
> ensures that only one consumer is processing a given item. by deleting 
> queue-X before queue-X-inprocess we make sure that no other consumer will see 
> queue-X as available for consumption after it is processed and before it is 
> deleted.
> this is at last once, because the consumer has a race condition. the consumer 
> may process the item and then crash before it can delete the corresponding 
> queue-X node.
> ben
> -----Original Message-----
> From: Stuart White []
> Sent: Thursday, January 08, 2009 7:15 AM
> To:
> Subject: Distributed queue: how to ensure no lost items?
> I'm interested in using ZooKeeper to provide a distributed
> producer/consumer queue for my distributed application.
> Of course I've been studying the recipes provided for queues, barriers, etc...
> My question is: how can I prevent packets of work from being lost if a
> process crashes?
> For example, following the distributed queue recipe, when a consumer
> takes an item from the queue, it removes the first "item" znode under
> the "queue" znode.  But, if the consumer immediately crashes after
> removing the item from the queue, that item is lost.
> Is there a recipe or recommended approach to ensure that no queue
> items are lost in the event of process failure?
> Thanks!



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