期刊名称：Bulletin of the Technical Committee on Data Engineering
出版年度：2016
卷号：39
期号：1
页码：52
出版社：IEEE Computer Society
摘要：Geo-distributed web applications often favor high availability over strong consistency. In response tothis bias, modern-day replicated data stores often eschew sequential consistency in favor of weaker even-tual consistency (EC) data semantics. While most operations supported by a typical web application canbe engineered, with sufficient care, to function under EC, there are oftentimes critical operations thatrequire stronger consistency guarantees. A few off-the-shelf eventually consistent key-value stores offertunable consistency levels to address the need for varying consistency guarantees. However, these consis-tency levels often have poorly-defined ad hoc semantics that is usually too low-level from the perspectiveof an application to relate their guarantees to invariants that must be respected by the application. More-over, these guarantees are often defined in way that is strongly influenced by a specific implementationof the data store. While such low-level implementation-dependent solutions do not readily cater to thehigh-level requirements of an application, relying on ill-defined guarantees additionally complicates thealready hard task of reasoning about application semantics under eventual consistency.In this paper, we describe Quelea, a declarative programming model for eventually consistent datastores. A novel aspect of Quelea is that it abstracts the actual implementation of the data store via high-level programming and system-level models that are agnostic to a specific implementation of the datastore. By doing so, Quelea frees application programmers from having to reason about their applica-tion in terms of low-level implementation specific data store semantics. Instead, programmers can nowreason in terms of an abstract model of the data store, and develop applications by defining and com-posing high-level replicated data types. Quelea is equipped with a formal specification language that iscapable of expressing precise semantics of high-level consistency guarantees (e.g., causal consistency)in the abstract model. Any eventually consistent key-value store can support Quelea by implementinga thin shim layer and a choosen set of high-level consistency guarantees on top of its existing low-levelinterface. We describe one such instantiation on top of Cassandra, that includes support for causal andsequential consistency guarantees, and coordination-free transactions. We present a case study of alarge web application benchmark to demonstrate Quelea’s practical utility.
