Composite Replicated Data Types

Abstract

Abstract. Modern large-scale distributed systems often rely on eventually consistent replicated stores, which achieve scalability in exchange for providing weak semantic guarantees. To compensate for this weakness, researchers have proposed various abstractions for programming on eventual consistency, such as replicated data types for resolving conflicting updates at different replicas and weak forms of transactions for maintaining relationships among objects. However, the subtle semantics of these abstractions makes using them correctly far from trivial. To address this challenge, we propose composite replicated data types, which formalise a common way of organising applications on top of eventually consistent stores. Similarly to an abstract data type, a composite data type encapsulates objects of replicated data types and operations used to access them, implemented using transactions. We develop a method for reasoning about programs with composite data types that reflects their modularity: the method allows abstracting away the internals of composite data type implementations when reasoning about their clients. We express the method as a denotational semantics for a programming language with composite data types. We demonstrate the effectiveness of our semantics by applying it to verify subtle data type examples and prove that it is sound and complete with respect to a standard non-compositional semantics.

A Denotational View of Replicated Data Types

“…Our contribution proposes a denotational view of replicated data types. While most of the traditional approaches are operational in flavour [4,7,8], we strived for a formalism for specifications which could exploit the classical tools of algebraic specification theory. More precisely, we associate to each configuration (i.e., visibility) a set of admissible arbitrations.…”

“…Note that P ∈ S(G) is a path over E G , and hence a total order of the events in G. However, we do not require P to be a topological ordering of G, i.e., ≺ G ⊆< P may not hold. Although some specification approaches consider only arbitrations that include visibility [7,5], our definition accommodates also presentations, such as [4,2], in which arbitrations may not preserve visibility. We focus later on in a few subclasses, such as coherent specifications, in order to establish a precise correspondence with replicated data types.…”

“…This strategy implicitly assumes that all events in a system can be arranged in a total order. Several recent approaches focus on the operational specification of replicated data types [12,4,2,3,7,5,14,8]. Usually, the specification describes the meaning of an operation in terms of two different relations among events: visibility, which explains the causes for each result, and arbitration, which totally orders events.…”

EPTL - A Temporal Logic for Weakly Consistent Systems (Short Paper)

“…A common way to specify properties about weakly consistent systems is to directly specify them based on the abstract execution. Gotsman and Yang [6] as well as Zeller and Poetzsch-Heffter [17] both use invariants about the execution which are extended by properties about the visibility between events. This makes the specification less readable because the temporal aspects of the specification are mixed with properties about the actual events.…”

“…Current approaches tend to base the behavior of the weakly consistent system on the specification of the conflict resolution mechanism used [2,6,17]. For many replicated data types like sets and maps, there are multiple possible implementations each with different semantics for concurrent modifications.…”