Cassandra -去中心化的结构化存储系统 (3)


2. 相关工作 Related work

文件系统和数据库社区,对于通过分布数据方式来实现性能,可用性,可靠性,进行了广泛的研究。不同于P2P存储系统只能支持扁平的命名空间,分布式文件系统支持层级式的命名空间。像Ficus和Coda,通过复制文件来达到高可用性,但是与此同时,系统牺牲了一致性。对于更新冲突,一般通过专门的冲突解决程序来处理。Farsite 在未采用任何中心化服务器的同时,实现了分布式文件系统。Farsite通过复制技术,来实现高可用性和可扩展性。 GFS是另外一个分布式存储系统,用来存储Google内部程序数据。 GFS采用了一个简单的设计,通过一个Master服务器来存储全部的metadata. 客户的数据被切分成数据块,存储在块存储服务器上。当然现在Google通过Chubby实现了Master服务器的容灾。Bayou 是一个分布式的关系型数据库,允许节点离线,提供实现最终一致性保证。 在上面这些系统中,Bayou,Coda,Ficus 允许节点离线操作,系统能够弹性的处理网络分区和运行中断。 这些系统在冲突处理的方式上存在差别。比如,Coda 和 Ficus实现系统层面的冲突解决。Bayou允许应用程序层面进行冲突解决,所有这些系统,都能够实现最终一致性。类似这些系统, Dynamo 在发生网络分区的时候,仍旧允许读写操作,然后通过不同的冲突处理机制解决冲突,有一些机制,是客户端驱动的。传统的关系型数据库的复制技术,关注于确保复制数据的强一致性。虽然强一致性对于应用编写者来说,是个方便的编程模型,但是这些系统因为强一致性的保证,而不能应付网络分区的情况。

Dynamo是Amazon用来存取购物车的存储系统。 Dynamo 基于成员算法的 GOSSIP协议,帮助系统中每个节点维护其他全部节点的信息。 可以把Dynamo理解成一个结构化的层,请求最多通过1跳路由达到目的。Dynamo通过时钟向量图来检测更新冲突,优先采用客户端来解决冲突的机制。Dynamo中的写操作执行前,需要一个读操作来获取时间戳向量,这个特点,在系统需要高的写吞吐量的时候,会成为瓶颈。 Bigtable提供结构化和数据的分布式,但需要依赖于一个分布式文件系统来实现持续服务。

5 thoughts on “Cassandra -去中心化的结构化存储系统 (3)

  1. LED display

    Dynamo通过时钟向量图来检测更新冲突,优先采用客户端来解决冲突的机制。Dynamo中的写操作执行前,需要一个读操作来获取时间戳向量,这个特点,在系统需要高的写吞吐量的时候,会成为瓶颈。 Bigtable提供结构化和数据的分布式,但需要依赖于一个分布式文件系统来实现持续服务。

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