CAP Theorem
The CAP theorem is a concept in distributed systems that states a system can only guarantee two out of the following three properties: Consistency, Availability, and Partition Tolerance.
Consistency
Every read receives the most recent write or an error. In other words, all nodes see the same data at the same time. This means that any data written to the system will be immediately visible to all subsequent reads.
Availability
Every request receives a non-error response, without the guarantee that it contains the most recent write. This means that even if the system cannot find the most current data, it still responds to requests instead of failing.
Partition Tolerance
The system continues to operate despite arbitrary partitioning due to network failures. This means that the system remains operational even if some of the nodes are unable to communicate with others.
The CAP theorem states that in the event of a network partition, a system can only guarantee two of these three properties. For example, if a system is designed to prioritize availability and partition tolerance, it may serve stale data to ensure that every request receives a response, even if that data is not the most recent. On the other hand, if a system is designed to prioritize consistency and partition tolerance, it may refuse to respond to requests if it cannot verify the data's consistency across all nodes.
It's important to note that the CAP theorem does not imply that it's impossible to build a system that satisfies all three properties. Rather, it states that in the face of a network partition, a system can only guarantee two out of the three properties. Therefore, when designing a distributed system, developers must make a conscious decision about which two properties they want to prioritize.