Heatbeat Message

Heartbeat Message：是由消息源发送到目标对象的一种消息，使得目标对象能检测消息源是否以及何时失败或不可用。HeartMessage是从消息源启动后发出的周期性不间断消息，一直到消息源关闭。当目标对象在预知的消息到达时间内如果未收到消息，那么目标对象就会认为消息源失败、关闭或者不可用。Heatbeat Message一般用于保障高可靠性和容错性。

--------------------------------
参考：A heartbeat message is a message sent from an originator to a destination that enables the destination to identify if and when the originator fails or is no longer available.[1] Heartbeat messages are typically sent non-stop on a periodic or recurring basis from the originator's start-up until the originator's shutdown. When the destination identifies a lack of heartbeat messages during an anticipated arrival period, the destination may determine that the originator has failed, shutdown, or is generally no longer available. Heartbeat messages may be used for high-availability and fault tolerance purposes.
from： http://en.wikipedia.org/wiki/Heartbeat_message