These four readings do a pretty good job of explaining what about building distributed systems is challenging. Collectively they outline a set of abstract but technical difficulties that the distributed systems engineer has to overcome, and set the stage for the more detailed investigation in later sections.
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Distributed Systems for Fun and Profit - a short book which tries to cover some of the basic issues in distributed systems including the role of time and different strategies for replication.
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Notes on distributed systems for young bloods - not theory, but a good practical counterbalance to keep the rest of your reading grounded.
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A Note on Distributed Systems - a classic paper on why you can’t just pretend all remote interactions are like local objects.
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The fallacies of distributed computing - 8 fallacies of distributed computing that set the stage for the kinds of things system designers forget. wikipedia
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The post continues with dozens more links organized into five topic areas.