PACELC Theorem — System Design

PACELC theorem is the extension of CAP theorem. The CAP theorem states in the presence of network partition, one has to choose between Availability and Consistency.

PACELC theorem extends the CAP theorem by introducing Latency ( L ) as an additional attribute in distributed systems. It states that ELSE ( E ), in the absence of network partition, one has to choose between Latency ( L ) and Consistency ( C ).

Example

Dynamo and Cassandra are PA/EL systems. They choose availability over consistency when a partition occurs; otherwise, they choose lower latency.

MongoDB can be considered PA/EC . In case of network partition, MongoDB choose availability, but otherwise guarantees consistency.