sequential


Sequential Consistency in Distributed Systems


I am learning Sequential Consistency in Distributed Systems but just could not understand the terms explained. I would appreciate if someone can shed some light in layman's term on why (a) and (c) below are sequentially consistent and (b) is not.
Thanks.
An execution e of operations is sequentially consistent if and if it can be permutated into a sequence s of these operations such that:
the sequence s respects the program order of each process. That is, for any two operations o1 and o2 of the same process, if o1 precedes o2 in e, then o1 should be placed before o2 in s;
in the sequence s, each read operation returns the value of the last preceding write operation over the same variable.
For (a), s can be:
W(x)b [P2], R(x)b [P3], R(x)b [P4], W(x)a [P1], R(x)a [P3], R(x)a [P4]
For (c), s can be:
W(x)a [P1], R(x)a [P2], R(x)a [P3], R(x)a [P4], W(x)b [P3], R(x)b [P1], R(x)b [P2], R(x)b [P4]
However, for (b):
the operations R(x)b, R(x)a from P3 require that W(x)b come before W(x)a
the operations R(x)a, R(x)b from P4 require that W(x)a come before W(x)b
It is impossible to construct such a sequence s.

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