[rabbitmq-discuss] Exactly Once Delivery
matthias at rabbitmq.com
Sat Aug 7 20:46:50 BST 2010
John Apps wrote:
> That's really like the piece of string question, no? Of course it can
> fill up, as can the DB where things are persisted for those cases where
> messages cannot be delivered.
> Having an unique ID in every message is not something new and not
> restricted to messaging, of course. It is simply a very good idea!
I believe Matthew was simply trying to point out that many of the
supposed guarantees of messaging systems are a lot softer than most
people think. In reality a "guarantee" is little more than an increase
in the probability that the right thing will happen. Coming clean about
that is going to be important for cloud computing to succeed - improving
the probabilities does come at a price, and for systems at massive
scales the cost/benefit calculations look quite different.
So, for example, using publisher-supplied message ids for de-duping
simply does not scale. Think what a genuine cloud messaging system would
have to do to handle the case where a producer injects the same message
first in a node in Australia and then in New York.
> What has not been touched on in this little discussion so far is the
> question of transactions
Similar considerations apply here. XA in the cloud? Hmmm.
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