如果主服务器不可用，但客户机尚具有网络连接，就需要故障转换到备用服务器，而不是使用系统时间。
If the primary server is unavailable, but the client still has network connectivity, you want to fail over to a secondary server as opposed to using system time.
要确保主服务器发生故障和辅助服务器实施接管之间的延迟时间最小。
Just make sure the lag time between the master host failing and the slave host taking over is minimal.
如果主服务器失败，备用服务器就非常快速地接管工作负载(在大多数情况下，转换时间少于30秒)。
If the primary server fails, the standby can take over the workload very quickly (most cases in under 30 seconds).
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