A Commutative Replicated Data Type (CRDT) is one where all concurrent operations commute. The replicas of a CRDT converge automatically, without complex concurrency control. This paper describes Treedoc, a novel CRDT design for cooperative text editing. An essential property is that the identifiers of Treedoc atoms are selected from a dense space. We discuss practical alternatives for implementing the identifier space based on an extended binary tree. We also discuss storage alternatives for data and meta-data, and mechanisms for compacting the tree. In the best case, Treedoc incurs no overhead with respect to a linear text buffer. We validate the results with traces from existing edit histories. 1. Our happened-before and concurrency relations are identical to the formal definition of Lamport [1].
International audienceReplicas of a commutative replicated data type (CRDT) eventually converge without any complex concurrency control. We validate the design of a non-trivial CRDT, a replicated sequence, with performance measurements in the context of Wikipedia. Furthermore, we discuss how to eliminate a remaining scalability bottleneck: Whereas garbage collection previously required a system-wide consensus, here we propose a flexible two-tier architecture and a protocol for migrating between tiers. We also discuss how the CRDT concept can be generalised, and its limitations
Abstract-As the classic transactional abstraction is sometimes considered too restrictive in leveraging parallelism, a lot of work has been devoted to devising relaxed transactional models with the goal of improving concurrency. Nevertheless, the quest for improving concurrency has somehow led to neglect one of the most appealing aspects of transactions: software composition, namely, the ability to develop pieces of software independently and compose them into applications that behave correctly in the face of concurrency. Indeed, a closer look at relaxed transactional models reveals that they do jeopardize composition, raising the fundamental question whether it is at all possible to devise such models while preserving composition. This paper shows that the answer is positive.We present outheritance, a necessary and sufficient condition for a (potentially relaxed) transactional memory to support composition. Basically, outheritance requires child transactions to pass their conflict information to their parent transaction, which in turn maintains this information until commit time. Concrete instantiations of this idea have been used before, classic transactions being the most prevalent example, but we believe to be the first to capture this as a general principle as well as to prove that it is, strictly speaking, equivalent to ensuring composition.We illustrate the benefits of outheritance using elastic transactions and show how they can satisfy outheritance and provide composition without hampering concurrency. We leverage this to present a new (transactional) Java package, a composable alternative to the concurrency package of the JDK, and evaluate efficiency through an implementation that speeds up state of the art software transactional memory implementations (TL2, LSA, SwissTM) by almost a factor of 3.
Abstract. Disjoint Access Parallelism (DAP) stipulates that operations involving disjoint sets of memory words must be able to progress independently, without interfering with each other. In this work we argue towards revising the two decade old wisdom saying that DAP is a binary condition that splits concurrent programs into scalable and non-scalable. We first present situations where DAP algorithms scale poorly, thus showing that not even algorithms that achieve this property provide scalability under all circumstances. Next, we show that algorithms which violate DAP can sometimes achieve the same scalability and performance as their DAP counterparts. We continue to show how by violating DAP and without sacrificing scalability we are able to circumvent three theoretical results showing that DAP is incompatible with other desirable properties of concurrent programs. Finally we introduce a new property called generalized disjoint-access parallelism (GDAP) which estimates how much of an algorithm is DAP. Algorithms having a large DAP part scale similar to DAP algorithms while not being subject to the same impossibility results.
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