单选题 High availability (HA) is a combination of technologies and processes that work together to ensure that specific critical functions are always up and running at the necessary level. To provide this level of high availability, a company has to have a long list of technologies and processes that provide redundancy, fault tolerance, and failover capabilities. Which of the following best describes these characteristics?

A、 Redundancy is the duplication of noncritical components or functions of a system with the intention of decreasing reliability of the system. Fault tolerance is the capability of a technology to discontinue to operate as expected even if something unexpected takes place. If a technology has a failover capability, this means that if there is a failure that cannot be handled through normal means, then processing is "switched over" to a working system.
B、 Redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system. Fault tolerance is the capability of a technology to continue to operate as expected even if something unexpected takes place. If a technology has a failover capability, this means that if there is a failure that cannot be handled through normal means, then processing is "switched over" to a working system.
C、 Redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system. Fault tolerance is the capability of a technology to continue to operate as expected even if something unexpected tak
D、 Redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system. Fault tolerance is the capability of a technology to continue to operate as expected even if something unexpected tak

【正确答案】 D

【答案解析】解析：D正确。高可用性(High Availability，HA)是为了确保某些特定的关键功能始终处于良好的工作状态和运行状态而一起使用的技术和流程的组合。这些特定的技术和流程可以是数据库、网络、应用流程、电力供应等。为了提供这种级别的高可用性，公司必须拥有一个技术和流程的长列表，列举提供冗余、容错和失效备援能力的各种技术和流程。冗余、容错和失效备援能力可以增强系统或网络的可靠性。高可靠性也保证了高可用性。 A不正确。因为这种技术内的冗余包含系统的关键组件或功能的重复，达到增加系统可靠性的目的。冗余通常是内嵌到网络中的路由协议级。路由协议通常被配置为：如果某条链路出现故障，或遇到拥塞，那么流量将通过另一条不同的网络链路进行路由。冗余硬件也是可用的，因为如果主要设备出故障了，那么备份组件可以被交换出去，并被激活。 B不正确。因为容错指的是一种在有些意想不到的事情(错误)发生时，业务仍然能够按预期运营的技术能力。如果数据库遇到了一次意想不到的差错，它能够回滚到一个已知的好的状态，并像没人任何不好的事情发生过一样继续运行。如果在一个TCP会话中出现了包丢失或包损坏，那么TCP协议将重新发送该包，使得系统到系统的通信不会受到影响。如果RAID系统内的某个磁盘损坏了，系统会使用剩下的部分数据重新恢复损坏的数据，使得运行不会受到影响。 C不正确。因为如果说某个技术拥有失效备援能力，这意味着当一个通过一般方法无法处理的故障发生时，处理过程会“切换”到工作系统。