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PACELC Theorem

This theorem expands the CAP Theorem: during a Network Partition (P), a distributed system chooses between Availability (A) and Consistency (C). Else (E), where there is no network partition, a distributed system chooses between either Latency (L) or Consistency (C).

Therefore an additional tradeoff is introduced when a distributed system is running normally, without network partitions, an operation can either have

  • low latency but possibly inconsistent data
  • consistent data by syncing between nodes at the cost of latency

source: https://www.scylladb.com/wp-content/uploads/PACELC-theorem-diagram-1.jpg

Databases with AP characteristics are usually also EL systems and are known as AP/EL systems.

Databases with CP characteristics are usually also EC systems and are known as CP/EC systems. These typically support ACID transactions and prioritize consistency during network partition.