message queues | Brave New Geek
As I've described in the past, distributed systems are all about trade-offs. This is one of them. There are essentially three types of delivery semantics: at-most-once, at-least-once, and exactly-once. Of the three, the first two are feasible and widely used. If you want to be super anal, you might say at-least-once delivery is also impossible because, technically speaking, network partitions are not strictly time-bound. If the connection from you to the server is interrupted indefinitely, you can't deliver anything. Practically speaking, you have bigger fish to fry at that point—like calling your ISP—so we consider at-least-once delivery, for all intents and purposes, possible. With this model of thinking, network partitions are finitely bounded in time, however arbitrary this may be.
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