The CAP theorem

Most distributed systems are generally analyzed using the CAP theorem, which states that a distributed system cannot ensure all of the following:

• Consistency: A read operation is guaranteed to return the most recent write

• Availability: Any operation is guaranteed to receive a response saying whether it has succeeded or failed

• Partition tolerance: The system continues to operate when a network partition occurs

Since Redis Sentinel and Redis Cluster are distributed systems, it is fair to analyze them using the CAP theorem. Network partitions are unavoidable in a distributed system, so it should ensure either consistency or availability; that is, it should be either CP or AP.

Theoretically, Redis Sentinel and Redis Cluster are neither consistent nor available under network partitions. However, there are some configurations that can minimize the consistency and availability problems.

They cannot provide availability because there is a quorum that needs to agree on a master election...