CAP
Under partition: CP systems (etcd, ZK) sacrifice availability. AP systems (Cassandra) sacrifice consistency. CA not real in distributed.
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PACELC (Abadi)
Under Partition: A or C. Else: Latency or Consistency. PA/EL Cassandra. PC/EC Google Spanner (uses TrueTime).
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Modern reality
Networks partition frequently (packet loss, slow links). Design must handle. CAP simplified but useful lens.