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Each Member periodically sends HeartbeatMesage (UDP) to all the coordinator and two other members in the distributed system, including the coordinator. The two members selected are most likely monitoring the sender of the HeartbeatMessage. Upon receiving the HearbeatMessage, the receiving member updates its record of receiving timestamp associated with the sender. The receiver does not reply to such HeartbeatMessages with requestID equals -1. The requestID of HeartbeatMessage is set to -1 so that the receiver does not look for a thread waiting for the message. HeartbeatMessages are themselves used as replies to HeartbeatRequestMessages(see next section). 

title This diagram shows a member (M) sending HeartbeatMessage to all the other members (N, C) in the distributed system
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entity M
entity N
entity C
M -> N: HeartbeatMessage(-1)
note right : update the record of timestamp
M -> C : HeartbeatMessage(-1)
note right : update the record of timestamp

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