NoSql Tutorial 3 : CAP Theorm

In theoretical computer science , the CAP theorem, also known as Brewer's theorem. In order to talk about NoSQL databases or when designing any distributed system. CAP theorem states that there are three basic requirements which exist in a special relation when designing applications for a distributed architecture.

Consistency - This means that the data in the database remains consistent after the execution of an operation. For example after an update operation all clients see the same data.
Availability - This means that the system is always on (service guarantee availability), no downtime.
Partition Tolerance - This means that the system continues to function even the communication among the servers is unreliable, i.e. the servers may be partitioned into multiple groups that cannot communicate with one another.

In theoretically it is impossible to fulfill all 3 requirements. CAP provides the basic requirements for a distributed system to follow 2 of the 3 requirements. Therefore all the current NoSQL database follow the different combinations of the C, A, P from the CAP theorem. Here is the brief description of three combinations CA, CP, AP :

CA - Single site cluster, therefore all nodes are always in contact. When a partition occurs, the system blocks.
CP - Some data may not be accessible, but the rest is still consistent/accurate. 
AP - System is still available under partitioning, but some of the data returned may be inaccurate.


The BASE
A BASE system gives up on consistency.
·         Basically Available indicates that the system does guarantee availability, in terms of the CAP theorem.
·         Soft state indicates that the state of the system may change over time, even without input. This is because of the eventual consistency model.
Eventual consistency indicates that the system will become consistent over time, given that the system doesn't receive input during that time

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