CoVer-ability：并发对象的一致性版本控制
CoVer-ability: Consistent Versioning for Concurrent Objects
摘要：
对象类型表征域空间和可以在该类型的对象上调用的操作。在本文中，我们引入了一个新的属性并发对象，我们称之为可覆盖性，其目的是提供精确的保证对象的一致性演化。这个新属性适用于各种分布式对象，包括需要操作最新版本对象的并发文件对象。我们提出了两个层次的覆盖性：（i）强覆盖性和（ii）弱覆盖性。强可覆盖性要求只有一个操作可以修改对象的最新版本，即用新版本“覆盖”最新版本，对对象修改施加总顺序。弱可覆盖性放宽了强可覆盖性的强要求，并允许多个操作修改对象的同一版本，其中每个修改都会导致不同的版本。弱可覆盖性通过要求任何后续操作仅修改新引入的版本之一来保持对象的一致演化。可覆盖性与原子保证相结合产生可覆盖的原子读/写寄存器。我们还表明，强覆盖的原子寄存器是等效的权力的共识。因此，我们专注于弱覆盖寄存器，我们证明了它们的重要性，表明它们不能使用类似类型的寄存器，如排名寄存器。此外，我们弱覆盖寄存器可以用来实现基本的（弱）读修改写和文件对象。最后，我们通过修改现有的MWMR原子寄存器实现弱覆盖寄存器。
英文摘要：
An object type characterizes the domain space and the operations that can be invoked on an object of that type. In this paper we introduce a new property for concurrent objects, we call coverability, that aims to provide precise guarantees on the consistent evolution of an object. This new property is suitable for a variety of distributed objects including concurrent file objects that demand operations to manipulate the latest version of the object. We propose two levels of coverability: (i) strong coverability and (ii) weak coverability. Strong coverability requires that only a single operation can modify the latest version of the object, i.e. "covers" the latest version with a new version, imposing a total order on object modifications. Weak coverability relaxes the strong requirements of strong coverability and allows multiple operations to modify the same version of an object, where each modification leads to a different version. Weak coverability preserves consistent evolution of the object, by demanding any subsequent operation to only modify one of the newly introduced versions. Coverability combined with atomic guarantees yield to coverable atomic read/write registers. We also show that strongly coverable atomic registers are equivalent in power to consensus. Thus, we focus on weakly coverable registers, and we demonstrate their importance by showing that they cannot be implemented using similar types of registers, like ranked-registers. Furthermore we show that weakly coverable registers may be used to implement basic (weak) read-modify-write and file objects. Finally, we implement weakly coverable registers by modifying an existing MWMR atomic register implementation.
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