Designing in a Distributed World - The Practice of Cloud System Administration - page 19

Figure 1.9: The CAP Principle
SQLandotherrelationaldatabasesusetheterm ACID todescribetheirsideoftheCAP
triangle. ACID stands for Atomicity (transactions are “all or nothing”), Consistency (after
each transaction the database is in a valid state), Isolation (concurrent transactions give
the same results as if they were executed serially), and Durability (a committed transac-
tion's data will not be lost in the event of a crash or other problem). Databases that provide
weaker consistency models often refer to themselves as NoSQL and describe themselves
as BASE : Basically Available Soft-state services with Eventual consistency.
1.6 Loosely Coupled Systems
Distributed systems are expected to be highly available, to last a long time, and to evolve
andchangewithoutdisruption.Entiresubsystemsareoftenreplacedwhilethesystemisup
and running.