The CAP Theorem
- Consistency - The same request returns the same data at T1 and T2 if there were no writes between T1 and T2. Basically, all reads see all previously completed writes. Wikipedia defines this as all nodes see the same data at the same time, but that is incorrect because the CAP Theorem is defined in terms of the observed state rather than the 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.