Durability and availability
When designing data storage systems, it is important to distinguish between the properties of availability and durability. Availability is a measure of the probability that the system is able to accept reads and writes at any given point in time. Durability is a measure of the probability that once a write has been accepted (in PostgreSQL, this means a successful transaction commit), that information will not be lost. All real systems must pick a compromise between availability, durability, performance, and cost.
For a simple example, we can consider a cluster consisting of two database servers. One is running as the primary server, accepting writes from our application. The other one is running as a standby, replicating ...
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