Sequential consistency requires that all data operations appear to have executed atomically in some sequential order that is consistent with the order seen at every individual process.
If instead of individual data operations, we apply sequential consistency to transactions, the resultant condition is called serializability in database theory.
Linearizability imposes more constraints on an order decided by
sequential consistency - the order needs to make sense to an external
observer. Let us say that an external observer is running two threads
B. If thread
A performs action
a0, then syncs with
B following which
b0. A sequentially
consistent execution can order
a0, as long as both
B perceive the same order. On the other hand, such an execution
is not valid under linearizability condition. Linearizability dictates
that an operation should take effect instantaneously before the
perceived end of the operation.
Evidently, linearizability is a stronger constraint than sequential consistency (or serializability). Nevertheless, it is said that linearizability is a local property (i.e., composable), sc is not. (I don’t understand this. If you know what this means, please drop a comment.)