The document discusses lessons learned from building scalable, highly concurrent, and fault-tolerant systems, noting that shared mutable state and threads can lead to non-deterministic code, and recommending approaches like dataflow concurrency, actors, and software transactional memory to avoid these issues. It also advocates designing systems to be reactive by using asynchronous messaging and non-blocking I/O, and to be fault-tolerant using failure recovery mechanisms like those enabled by actor models.
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