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Message volume shouldn't be a problem for services like Twitter. RabbitMQ and XMPP might help to explain the solution.
Cloud architecture isn't dramatically different than on-premise, though some inherent advantages make it a worthwhile deployment option.
AMQP is close to shaking up the MOM market. Get your head around it.
The two companies used to be the best of friends, but the need to control the data center increasingly puts them at odds.
Given the U.S. federal government's widespread adoption of open source, the amazing thing is that it took so long.