延迟存在时间越长，服务器 1、2 和 3 就可能会随着应用程序服务器中越来越多的工作线程被阻塞而变得不可用。
The longer the delay persists, servers 1, 2, and 3 may become unavailable as more and more of the worker threads within the application servers become blocked.
没有单点故障问题：系统中不应该存在单点，就是说不能因为一个服务器的宕机或者故障而导致整个应用程序的不可用。
No single point of failure: there should be no one server which, if lost, causes downtime of the application.
memcached服务器不可用不应该是应用程序的终结，虽然这意味着在memcached服务器恢复正常之前性能会有所降低。
Losing your memcached server should not be the end of your application, although it might imply a decrease in performance until the memcached server comes back up.
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