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Unformatted text preview: total order. For writes that is easy to see because they form a total order because there is only a single writer (2). For reads the argument is more complex because reads can happen truly concurrent, but it is never the case that a read on one processor observes a result that is inconsistent with an observation on another processor in a total order. This could only happen if a scenario like A or B above is possible, and the confirmation messages+locks ensure that never happens (3). Examples of systems that provide weaker consistency guarantees. DNS: multiple replicas, needed for throughput – a huge number of clients reading data from the DNS servers at the same time. E.g., for the root DNS, several hundred replicas spread around the globe. Caching to reduce the load on the server. How does update work? Update one replica, that replica replies, and in the background it propagates the change to the other replicas. Implication: may not read your own writes! Update DNS, read the value, and if you happen to connect to a...
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- Spring '14