Semantics of Concurrent Revisions

Abstract

Abstract. Enabling applications to execute various tasks in parallel is difficult if those tasks exhibit read and write conflicts. We recently developed a programming model based on concurrent revisions that addresses this challenge in a novel way: each forked task gets a conceptual copy of all the shared state, and state changes are integrated only when tasks are joined, at which time write-write conflicts are deterministically resolved.In this paper, we study the precise semantics of this model, in particular its guarantees for determinacy and consistency. First, we introduce a revision calculus that concisely captures the programming model. Despite allowing concurrent execution and locally nondeterministic scheduling, we prove that the calculus is confluent and guarantees determinacy. We show that the consistency guarantees of our calculus are a logical extension of snapshot isolation with support for conflict resolution and nesting. Moreover, we discuss how custom merge functions can provide stronger guarantees for particular data types that are tailored to the needs of the application.Finally, we show we can visualize the nonlinear history of state in our computations using revision diagrams that clarify the synchronization between tasks and allow local reasoning about state updates.

Eventually Consistent Transactions

“…By itself, it is however not very constructive, insofar that it does not give practical guidelines as to how a system can efficiently and correctly construct the necessary ordering (visibility and arbitration). We now proceed to describe a more specific implementation technique for eventually consistent systems, based on the notion of revision diagrams introduced in [6].…”

“…The join condition has some important, not immediately obvious consequences. For example, it implies that revision diagrams are always semilattices (for a proof of this nontrivial fact see [6]). Also, it ensures some diagram properties (Lemmas 2 and 3) that we need to prove our main result (Thm.…”

“…We call a path direct if all of its f-edges (if any) appear after all of its j-edges (if any). The following lemma (which appears as a theorem in [6], and for which we include a proof in [4] as well) shows that we can always choose direct paths:…”

“…Previous work on revisions [2,6,3,5] introduces revision diagrams and conflict resolution. In this paper we feature a simpler, more direct definition using graph construction rules.…”

“…We then proceed to describe a more concrete and operational implementation technique based on revision diagrams [6]. Revision diagrams provide implementors with a simple set of rules for managing updates and replicas.…”