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Consistency models with global operation sequencing and their composition (extended version)
Modern distributed systems often achieve availability and scalability by
providing consistency guarantees about the data they manage weaker than
linearizability. We consider a class of such consistency models that, despite
this weakening, guarantee that clients eventually agree on a global sequence of
operations, while seeing a subsequence of this final sequence at any given
point of time. Examples of such models include the classical Total Store Order
(TSO) and recently proposed dual TSO, Global Sequence Protocol (GSP) and
Ordered Sequential Consistency.
We define a unified model, called Global Sequence Consistency (GSC), that has
the above models as its special cases, and investigate its key properties.
First, we propose a condition under which multiple objects each satisfying GSC
can be composed so that the whole set of objects satisfies GSC. Second, we
prove an interesting relationship between special cases of GSC---GSP, TSO and
dual TSO: we show that clients that do not communicate out-of-band cannot tell
the difference between these models. To obtain these results, we propose a
novel axiomatic specification of GSC and prove its equivalence to the
operational definition of the model.Comment: Extended version of the paper from DISC'1