CAP Theorem

Distributed Data Store can only provide two of the following three guarantees.

Consistency

Every read receives the most recent write or an error.

Availability

Every request receives a (non-error) response, without the guarantee that it contains the most recent write.

Partition tolerance

The system continues to operate despite an arbitrary number of messages being dropped (or delayed) by the network between nodes.

In Practice

No distributed system is safe from network failures, thus network partitioning generally has to be tolerated

When a network partition failure happens, it must be decided whether to do one of the following:

• cancel the operation and thus decrease the availability but ensure consistency

• proceed with the operation and thus provide availability but risk inconsistency.

CAP theorem indicates that we can’t have availability and consistency under network partitions.