The CAP Theorem is an important foundation for understanding distributed systems. Summarizing all the available literature on CAP and some modern interpretations of the theorem, we know that any distributed system must decide when dealing with latency or network failure ,whether it will choose Consistency or Availability.
Consistency - The result of a read at T1 is the same as T2 if there were no writes between T1 and T2. Basically, any read can see any previously completed write.
- Availability - The system can service all reads and writes.
Wikipedia defines Consistency as all nodes see the same data at the same time, but that is incorrect because CAP is defined by the observed rather than actual state of the system.
What is Optimistic Availability?
Optimistic availability is a characteristic of distributed systems that are coordinated by a master node which allows the system to tolerate arbitrary node failure while preserving availability as long as the master node and at least one relevant node agree on the state of the system. In laymen's terms: given sufficient replication, if a node fails, we're optimistic that the system will remain available.