CAP Theorem

Wikipedia says ..

the CAP theorem, also named Brewer's theorem after computer scientist Eric Brewer, states that it is impossible for a distributed data store to simultaneously provide more than two out 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
  -> It means A system that is partition-tolerant can sustain any amount of network failure that doesn't result in a failure of the entire network. Data records are sufficiently replicated across combinations of nodes and networks to keep the system up through intermittent outrages. when it comes to MODERN DISTRIBUTED SYSTEM, this is not an option. it is necessary.

When a network partition failure happens should we decide to

• Cancel the operation and thus decrease the availability but ensure consistency
• Proceed with the operation and thus provide availability but risk inconsistency

-> Thus, Both C and A can not be satisfied at the same time in network failure

 

The CAP theorem implies that in the presence of a network partition, one has to choose between consistency and availability. 

 

CAP is frequently misunderstood as if one has to choose to abandon one of the three guarantees at all times. In fact, the choice is really between consistency and availability only when a network partition or failure happens; at all other times, no trade-off has to be made.

 

Database systems designed with traditional ACID guarantees in mind such as RDBMS choose consistency over availability, whereas systems designed around the BASE philosophy, common in the NoSQL movement for example, choose availability over consistency

 

CA can be satisfied only in case the system is composed in single node.

 

NoSQL - Wikipedia

Reference

Reference: https://en.wikipedia.org/wiki/CAP_theorem